-<Chapter id="using-GHC">
-<Title>Using GHC
-</Title>
-
-<Para>
-<IndexTerm><Primary>GHC, using</Primary></IndexTerm>
-<IndexTerm><Primary>using GHC</Primary></IndexTerm>
-GHC is a command-line compiler: in order to compile a Haskell program,
-GHC must be invoked on the source file(s) by typing a command to the
-shell. The steps involved in compiling a program can be automated
-using the <Command>make</Command> tool (this is especially useful if the program
-consists of multiple source files which depend on each other). This
-section describes how to use GHC from the command-line.
-</Para>
-
-<Sect1 id="command-line-structure">
-<Title>Overall command-line structure
-</Title>
-
-<Para>
-<IndexTerm><Primary>structure, command-line</Primary></IndexTerm>
-<IndexTerm><Primary>command-line structure</Primary></IndexTerm>
-</Para>
-
-<Para>
-An invocation of GHC takes the following form:
-</Para>
-
-<Para>
-
-<Screen>
-ghc [argument...]
-</Screen>
-
-</Para>
-
-<Para>
-Command-line arguments are either options or file names.
-</Para>
-
-<Para>
-Command-line options begin with <Literal>-</Literal>. They may <Emphasis>not</Emphasis> be
-grouped: <Option>-vO</Option> is different from <Option>-v -O</Option>. Options need not
-precede filenames: e.g., <Command>ghc *.o -o foo</Command>. All options are
-processed and then applied to all files; you cannot, for example, invoke
-<Command>ghc -c -O1 Foo.hs -O2 Bar.hs</Command> to apply different optimisation
-levels to the files <Filename>Foo.hs</Filename> and <Filename>Bar.hs</Filename>. For conflicting
-options, e.g., <Option>-c -S</Option>, we reserve the right to do anything we
-want. (Usually, the last one applies.)
-</Para>
-
-</Sect1>
-
-<Sect1 id="file-suffixes">
-<Title>Meaningful file suffixes
-</Title>
-
-<Para>
-<IndexTerm><Primary>suffixes, file</Primary></IndexTerm>
-<IndexTerm><Primary>file suffixes for GHC</Primary></IndexTerm>
-</Para>
-
-<Para>
-File names with “meaningful” suffixes (e.g., <Filename>.lhs</Filename> or <Filename>.o</Filename>)
-cause the “right thing” to happen to those files.
-</Para>
-
-<Para>
-<VariableList>
-
-<VarListEntry>
-<Term><Filename>.lhs</Filename>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>lhs suffix</Primary></IndexTerm>
-A “literate Haskell” module.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Filename>.hs</Filename>:</Term>
-<ListItem>
-<Para>
-A not-so-literate Haskell module.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Filename>.hi</Filename>:</Term>
-<ListItem>
-<Para>
-A Haskell interface file, probably compiler-generated.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Filename>.hc</Filename>:</Term>
-<ListItem>
-<Para>
-Intermediate C file produced by the Haskell compiler.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Filename>.c</Filename>:</Term>
-<ListItem>
-<Para>
-A C file not produced by the Haskell compiler.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Filename>.s</Filename>:</Term>
-<ListItem>
-<Para>
-An assembly-language source file, usually
-produced by the compiler.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Filename>.o</Filename>:</Term>
-<ListItem>
-<Para>
-An object file, produced by an assembler.
-</Para>
-</ListItem>
-</VarListEntry>
-</VariableList>
-</Para>
-
-<Para>
-Files with other suffixes (or without suffixes) are passed straight
-to the linker.
-</Para>
-
-</Sect1>
-
-<Sect1 id="options-help">
-<Title>Help and verbosity options
-</Title>
-
-<Para>
-<IndexTerm><Primary>help options (GHC)</Primary></IndexTerm>
-<IndexTerm><Primary>verbose option (GHC)</Primary></IndexTerm>
-</Para>
-
-<Para>
-A good option to start with is the <Option>-help</Option> (or <Option>-?</Option>) option.
-<IndexTerm><Primary>-help option</Primary></IndexTerm>
-<IndexTerm><Primary>-? option</Primary></IndexTerm>
-GHC spews a long message to standard output and then exits.
-</Para>
-
-<Para>
-The <Option>-v</Option><IndexTerm><Primary>-v option</Primary></IndexTerm> option makes GHC <Emphasis>verbose</Emphasis>: it
-reports its version number and shows (on stderr) exactly how it invokes each
-phase of the compilation system. Moreover, it passes
-the <Option>-v</Option> flag to most phases; each reports
-its version number (and possibly some other information).
-</Para>
-
-<Para>
-Please, oh please, use the <Option>-v</Option> option when reporting bugs!
-Knowing that you ran the right bits in the right order is always the
-first thing we want to verify.
-</Para>
-
-<Para>
-If you're just interested in the compiler version number, the
-<Option>--version</Option><IndexTerm><Primary>--version option</Primary></IndexTerm> option prints out a
-one-line string containing the requested info.
-</Para>
-
-</Sect1>
-
-<Sect1 id="options-order">
-<Title>Running the right phases in the right order
-</Title>
-
-<Para>
-<IndexTerm><Primary>order of passes in GHC</Primary></IndexTerm>
-<IndexTerm><Primary>pass ordering in GHC</Primary></IndexTerm>
-The basic task of the <Command>ghc</Command> driver is to run each input file
-through the right phases (compiling, linking, etc.).
-</Para>
-
-<Para>
-The first phase to run is determined by the input-file suffix, and the
-last phase is determined by a flag. If no relevant flag is present,
-then go all the way through linking. This table summarises:
-</Para>
-
-<Para>
-<InformalTable>
-<TGroup Cols="4">
-<ColSpec Align="Left">
-<ColSpec Align="Left">
-<ColSpec Align="Left">
-<ColSpec Align="Left">
-<TBody>
-
-<Row>
-<Entry>Phase of the compilation system</Entry>
-<Entry>Suffix saying “start here”</Entry>
-<Entry>Flag saying “stop after”</Entry>
-<Entry>(suffix of) output file</Entry>
-</Row>
-
-<Row>
-<Entry>
-literate pre-processor </Entry>
-<Entry> .lhs </Entry>
-<Entry> - </Entry>
-<Entry> - </Entry>
-</Row>
-<Row>
-<Entry>
-C pre-processor (opt.) </Entry>
-<Entry> - </Entry>
-<Entry> - </Entry>
-<Entry> - </Entry>
-</Row>
-<Row>
-<Entry>
-Haskell compiler </Entry>
-<Entry> .hs </Entry>
-<Entry> -C, -S </Entry>
-<Entry> .hc, .s </Entry>
-</Row>
-<Row>
-<Entry>
-C compiler (opt.) </Entry>
-<Entry> .hc or .c </Entry>
-<Entry> -S </Entry>
-<Entry> .s </Entry>
-</Row>
-<Row>
-<Entry>
-assembler </Entry>
-<Entry> .s </Entry>
-<Entry> -c </Entry>
-<Entry> .o </Entry>
-</Row>
-<Row>
-<Entry>
-linker </Entry>
-<Entry> other </Entry>
-<Entry> - </Entry>
-<Entry> a.out </Entry>
-</Row>
-</TBody>
-</TGroup>
-</InformalTable>
-
-<IndexTerm><Primary>-C option</Primary></IndexTerm>
-<IndexTerm><Primary>-S option</Primary></IndexTerm>
-<IndexTerm><Primary>-c option</Primary></IndexTerm>
-</Para>
-
-<Para>
-Thus, a common invocation would be: <Command>ghc -c Foo.hs</Command>
-</Para>
-
-<Para>
-Note: What the Haskell compiler proper produces depends on whether a
-native-code generator is used (producing assembly language) or not
-(producing C).
-</Para>
-
-<Para>
-The option <Option>-cpp</Option><IndexTerm><Primary>-cpp option</Primary></IndexTerm> must be given for the C
-pre-processor phase to be run, that is, the pre-processor will be run
-over your Haskell source file before continuing.
-</Para>
-
-<Para>
-The option <Option>-E</Option><IndexTerm><Primary>-E option</Primary></IndexTerm> runs just the pre-processing
-passes of the compiler, outputting the result on stdout before
-stopping. If used in conjunction with -cpp, the output is the
-code blocks of the original (literal) source after having put it
-through the grinder that is the C pre-processor. Sans <Option>-cpp</Option>, the
-output is the de-litted version of the original source.
-</Para>
-
-<Para>
-The option <Option>-optcpp-E</Option><IndexTerm><Primary>-optcpp-E option</Primary></IndexTerm> runs just the
-pre-processing stage of the C-compiling phase, sending the result to
-stdout. (For debugging or obfuscation contests, usually.)
-</Para>
-
-</Sect1>
-
-<Sect1 id="options-output">
-<Title>Re-directing the compilation output(s)
-</Title>
-
-<Para>
-<IndexTerm><Primary>output-directing options</Primary></IndexTerm>
-</Para>
-
-<Para>
-GHC's compiled output normally goes into a <Filename>.hc</Filename>, <Filename>.o</Filename>, etc., file,
-depending on the last-run compilation phase. The option <Option>-o
-foo</Option><IndexTerm><Primary>-o option</Primary></IndexTerm> re-directs the output of that last-run
-phase to file <Filename>foo</Filename>.
-</Para>
-
-<Para>
-Note: this “feature” can be counterintuitive:
-<Command>ghc -C -o foo.o foo.hs</Command> will put the intermediate C code in the
-file <Filename>foo.o</Filename>, name notwithstanding!
-</Para>
-
-<Para>
-EXOTICA: But the <Option>-o</Option> option isn't of much use if you have
-<Emphasis>several</Emphasis> input files… Non-interface output files are
-normally put in the same directory as their corresponding input file
-came from. You may specify that they be put in another directory
-using the <Option>-odir <dir></Option><IndexTerm><Primary>-odir <dir> option</Primary></IndexTerm> (the
-“Oh, dear” option). For example:
-</Para>
-
-<Para>
-
-<Screen>
-% ghc -c parse/Foo.hs parse/Bar.hs gurgle/Bumble.hs -odir `arch`
-</Screen>
-
-</Para>
-
-<Para>
-The output files, <Filename>Foo.o</Filename>, <Filename>Bar.o</Filename>, and <Filename>Bumble.o</Filename> would be
-put into a subdirectory named after the architecture of the executing
-machine (<Filename>sun4</Filename>, <Filename>mips</Filename>, etc). The directory must already
-exist; it won't be created.
-</Para>
-
-<Para>
-Note that the <Option>-odir</Option> option does <Emphasis>not</Emphasis> affect where the
-interface files are put. In the above example, they would still be
-put in <Filename>parse/Foo.hi</Filename>, <Filename>parse/Bar.hi</Filename>, and <Filename>gurgle/Bumble.hi</Filename>.
-</Para>
-
-<Para>
-MORE EXOTICA: The <Option>-osuf <suffix></Option><IndexTerm><Primary>-osuf <suffix>
-option</Primary></IndexTerm> will change the <Filename>.o</Filename> file suffix for object files to
-whatever you specify. (We use this in compiling the prelude.).
-Similarly, the <Option>-hisuf <suffix></Option><IndexTerm><Primary>-hisuf <suffix>
-option</Primary></IndexTerm> will change the <Filename>.hi</Filename> file suffix for non-system
-interface files (see <XRef LinkEnd="hi-options">).
-</Para>
-
-<Para>
-The <Option>-hisuf</Option>/<Option>-osuf</Option> game is useful if you want to compile a program
-with both GHC and HBC (say) in the same directory. Let HBC use the
-standard <Filename>.hi</Filename>/<Filename>.o</Filename> suffixes; add <Option>-hisuf g_hi -osuf g_o</Option> to your
-<Command>make</Command> rule for GHC compiling…
-</Para>
-
-<Para>
-FURTHER EXOTICA: If you are doing a normal <Filename>.hs</Filename>-to-<Filename>.o</Filename> compilation
-but would like to hang onto the intermediate <Filename>.hc</Filename> C file, just
-throw in a <Option>-keep-hc-file-too</Option> option<IndexTerm><Primary>-keep-hc-file-too option</Primary></IndexTerm>.
-If you would like to look at the assembler output, toss in a
-<Option>-keep-s-file-too</Option>,<IndexTerm><Primary>-keep-s-file-too option</Primary></IndexTerm> too.
-</Para>
-
-<Sect2 id="saving-ghc-stderr">
-<Title>Saving GHC's standard error output
-</Title>
-
-<Para>
-<IndexTerm><Primary>standard error, saving</Primary></IndexTerm>
-</Para>
-
-<Para>
-Sometimes, you may cause GHC to be rather chatty on standard error;
-with <Option>-v</Option>, for example. You can instruct GHC to <Emphasis>append</Emphasis> this
-output to a particular log file with a <Option>-odump <blah></Option><IndexTerm><Primary>-odump
-<blah> option</Primary></IndexTerm> option.
-</Para>
-
-</Sect2>
-
-<Sect2 id="temp-files">
-<Title>Redirecting temporary files
-</Title>
-
-<Para>
-<IndexTerm><Primary>temporary files, redirecting</Primary></IndexTerm>
-</Para>
-
-<Para>
-If you have trouble because of running out of space in <Filename>/tmp</Filename> (or
-wherever your installation thinks temporary files should go), you may
-use the <Option>-tmpdir <dir></Option><IndexTerm><Primary>-tmpdir <dir> option</Primary></IndexTerm> option
-to specify an alternate directory. For example, <Option>-tmpdir .</Option> says to
-put temporary files in the current working directory.
-</Para>
-
-<Para>
-Alternatively, use your <Constant>TMPDIR</Constant> environment variable.<IndexTerm><Primary>TMPDIR
-environment variable</Primary></IndexTerm> Set it to the name of the directory where
-temporary files should be put. GCC and other programs will honour the
-<Constant>TMPDIR</Constant> variable as well.
-</Para>
-
-<Para>
-Even better idea: Set the <Constant>TMPDIR</Constant> variable when building GHC, and
-never worry about <Constant>TMPDIR</Constant> again. (see the build documentation).
-</Para>
-
-</Sect2>
-
-</Sect1>
-
-<Sect1 id="options-sanity">
-<Title>Warnings and sanity-checking
-</Title>
-
-<Para>
-<IndexTerm><Primary>sanity-checking options</Primary></IndexTerm>
-<IndexTerm><Primary>warnings</Primary></IndexTerm>
-GHC has a number of options that select which types of non-fatal error
-messages, otherwise known as warnings, can be generated during
-compilation. By default, you get a standard set of warnings which are
-generally likely to indicate bugs in your program. These are:
-<Option>-fwarn-overlpapping-patterns</Option>, <Option>-fwarn-duplicate-exports</Option>, and
-<Option>-fwarn-missing-methods</Option>. The following flags are simple ways to
-select standard “packages” of warnings:
-</Para>
-
-<Para>
-<VariableList>
-
-<VarListEntry>
-<Term><Option>-Wnot</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-Wnot option</Primary></IndexTerm>
-Turns off all warnings, including the standard ones.
-</Para>
-</ListItem>
-</VarListEntry>
-
-<VarListEntry>
-<Term><Option>-w</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-w option</Primary></IndexTerm>
-Synonym for <Option>-Wnot</Option>.
-</Para>
-</ListItem>
-</VarListEntry>
-
-<VarListEntry>
-<Term><Option>-W</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-W option</Primary></IndexTerm>
-Provides the standard warnings plus <Option>-fwarn-incomplete-patterns</Option>,
-<Option>-fwarn-unused-imports</Option> and <Option>-fwarn-unused-binds</Option>.
-</Para>
-</ListItem>
-</VarListEntry>
-
-<VarListEntry>
-<Term><Option>-Wall</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-Wall option</Primary></IndexTerm>
-Turns on all warning options.
-</Para>
-</ListItem>
-</VarListEntry>
-
-</VariableList>
-</Para>
-
-<Para>
-The full set of warning options is described below. To turn off any
-warning, simply give the corresponding <Option>-fno-warn-...</Option> option on
-the command line.
-</Para>
-
-<Para>
-<VariableList>
-
-<VarListEntry>
-<Term><Option>-fwarn-name-shadowing</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-fwarn-name-shadowing option</Primary></IndexTerm>
-<IndexTerm><Primary>shadowing, warning</Primary></IndexTerm>This option causes a warning to be emitted whenever an inner-scope
-value has the same name as an outer-scope value, i.e. the inner value
-shadows the outer one. This can catch typographical errors that turn
-into hard-to-find bugs, e.g., in the inadvertent cyclic definition
-<Literal>let x = ... x ... in</Literal>.
-</Para>
-
-<Para>
-Consequently, this option does <Emphasis>not</Emphasis> allow cyclic recursive
-definitions.
-</Para>
-</ListItem>
-</VarListEntry>
-
-<VarListEntry>
-<Term><Option>-fwarn-overlapping-patterns</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-fwarn-overlapping-patterns option</Primary></IndexTerm>
-<IndexTerm><Primary>overlapping patterns, warning</Primary></IndexTerm>
-<IndexTerm><Primary>patterns, overlapping</Primary></IndexTerm>
-By default, the compiler will warn you if a set of patterns are
-overlapping, i.e.,
-</Para>
-
-<Para>
-<ProgramListing>
-f :: String -> Int
-f [] = 0
-f (_:xs) = 1
-f "2" = 2
-</ProgramListing>
-</Para>
-
-<Para>
-where the last pattern match in <Function>f</Function> won't ever be reached, as the
-second pattern overlaps it. More often than not, redundant patterns
-is a programmer mistake/error, so this option is enabled by default.
-</Para>
-</ListItem>
-</VarListEntry>
-
-<VarListEntry>
-<Term><Option>-fwarn-incomplete-patterns</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-fwarn-incomplete-patterns option</Primary></IndexTerm>
-<IndexTerm><Primary>incomplete patterns, warning</Primary></IndexTerm>
-<IndexTerm><Primary>patterns, incomplete</Primary></IndexTerm>
-Similarly for incomplete patterns, the function <Function>g</Function> below will fail
-when applied to non-empty lists, so the compiler will emit a warning
-about this when <Option>-fwarn-incomplete-patterns</Option> is enabled.
-</Para>
-
-<Para>
-<ProgramListing>
-g [] = 2
-</ProgramListing>
-</Para>
-
-<Para>
-This option isn't enabled be default because it can be a bit noisy,
-and it doesn't always indicate a bug in the program. However, it's
-generally considered good practice to cover all the cases in your
-functions.
-</Para>
-</ListItem>
-</VarListEntry>
-
-<VarListEntry>
-<Term><Option>-fwarn-missing-methods</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-fwarn-missing-methods option</Primary></IndexTerm>
-<IndexTerm><Primary>missing methods, warning</Primary></IndexTerm>
-<IndexTerm><Primary>methods, missing</Primary></IndexTerm>
-This option is on by default, and warns you whenever an instance
-declaration is missing one or more methods, and the corresponding
-class declaration has no default declaration for them.
-</Para>
-</ListItem>
-</VarListEntry>
-
-<VarListEntry>
-<Term><Option>-fwarn-missing-fields</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-fwarn-missing-fields option</Primary></IndexTerm>
-<IndexTerm><Primary>missing fields, warning</Primary></IndexTerm>
-<IndexTerm><Primary>fields, missing</Primary></IndexTerm>
-This option is on by default, and warns you whenever the construction
-of a labelled field constructor isn't complete, missing initializers
-for one or more fields. While not an error (the missing fields are
-initialised with bottoms), it is often an indication of a programmer
-error.
-</Para>
-</ListItem>
-</VarListEntry>
-
-<VarListEntry>
-<Term><Option>-fwarn-unused-imports</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-fwarn-unused-imports option</Primary></IndexTerm>
-<IndexTerm><Primary>unused imports, warning</Primary></IndexTerm>
-<IndexTerm><Primary>imports, unused</Primary></IndexTerm>
-Report any objects that are explicitly imported but never used.
-</Para>
-</ListItem>
-</VarListEntry>
-
-<VarListEntry>
-<Term><Option>-fwarn-unused-binds</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-fwarn-unused-binds option</Primary></IndexTerm>
-<IndexTerm><Primary>unused binds, warning</Primary></IndexTerm>
-<IndexTerm><Primary>binds, unused</Primary></IndexTerm>
-Report any function definitions (and local bindings) which are unused.
-For top-level functions, the warning is only given if the binding is
-not exported.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-fwarn-unused-matches</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-fwarn-unused-matches option</Primary></IndexTerm>
-<IndexTerm><Primary>unused matches, warning</Primary></IndexTerm>
-<IndexTerm><Primary>matches, unused</Primary></IndexTerm>
-Report all unused variables which arise from pattern matches,
-including patterns consisting of a single variable. For instance <Literal>f x
-y = []</Literal> would report <VarName>x</VarName> and <VarName>y</VarName> as unused. To eliminate the warning,
-all unused variables can be replaced with wildcards.
-</Para>
-</ListItem>
-</VarListEntry>
-
-<VarListEntry>
-<Term><Option>-fwarn-duplicate-exports</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-fwarn-duplicate-exports option</Primary></IndexTerm>
-<IndexTerm><Primary>duplicate exports, warning</Primary></IndexTerm>
-<IndexTerm><Primary>export lists, duplicates</Primary></IndexTerm>
-Have the compiler warn about duplicate entries in export lists. This
-is useful information if you maintain large export lists, and want to
-avoid the continued export of a definition after you've deleted (one)
-mention of it in the export list.
-</Para>
-
-<Para>
-This option is on by default.
-</Para>
-</ListItem>
-</VarListEntry>
-
-<VarListEntry>
-<Term><Option>-fwarn-type-defaults</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-fwarn-type-defaults option</Primary></IndexTerm>
-<IndexTerm><Primary>defaulting mechanism, warning</Primary></IndexTerm>
-Have the compiler warn/inform you where in your source the Haskell
-defaulting mechanism for numeric types kicks in. This is useful
-information when converting code from a context that assumed one
-default into one with another, e.g., the `default default' for Haskell
-1.4 caused the otherwise unconstrained value <Constant>1</Constant> to be given
-the type <Literal>Int</Literal>, whereas Haskell 98 defaults it to
-<Literal>Integer</Literal>. This may lead to differences in performance and
-behaviour, hence the usefulness of being non-silent about this.
-</Para>
-
-<Para>
-This warning is off by default.
-</Para>
-</ListItem>
-</VarListEntry>
-
-<VarListEntry>
-<Term><Option>-fwarn-missing-signatures</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-fwarn-missing-signatures option</Primary></IndexTerm>
-<IndexTerm><Primary>type signatures, missing</Primary></IndexTerm>
-If you would like GHC to check that every top-level function/value has
-a type signature, use the <Option>-fwarn-missing-signatures</Option> option. This
-option is off by default.
-</Para>
-</ListItem>
-</VarListEntry>
-</VariableList>
-</Para>
-
-<Para>
-If you're feeling really paranoid, the <Option>-dcore-lint</Option>
-option<IndexTerm><Primary>-dcore-lint option</Primary></IndexTerm> is a good choice. It turns on
-heavyweight intra-pass sanity-checking within GHC. (It checks GHC's
-sanity, not yours.)
-</Para>
-
-</Sect1>
-
-<Sect1 id="separate-compilation">
-<Title>Separate compilation
-</Title>
-
-<Para>
-<IndexTerm><Primary>separate compilation</Primary></IndexTerm>
-<IndexTerm><Primary>recompilation checker</Primary></IndexTerm>
-<IndexTerm><Primary>make and recompilation</Primary></IndexTerm>
-</Para>
-
-<Para>
-This section describes how GHC supports separate compilation.
-</Para>
-
-<Sect2 id="hi-files">
-<Title>Interface files
-</Title>
-
-<Para>
-<IndexTerm><Primary>interface files</Primary></IndexTerm>
-<IndexTerm><Primary>.hi files</Primary></IndexTerm>
-</Para>
-
-<Para>
-When GHC compiles a source file <Filename>F</Filename> which contains a module <Literal>A</Literal>, say,
-it generates an object <Filename>F.o</Filename>, <Emphasis>and</Emphasis> a companion <Emphasis>interface
-file</Emphasis> <Filename>A.hi</Filename>. The interface file is not intended for human
-consumption, as you'll see if you take a look at one. It's merely
-there to help the compiler compile other modules in the same program.
-</Para>
-
-<Para>
-NOTE: Having the name of the interface file follow the module name and
-not the file name, means that working with tools such as <Command>make</Command>
-become harder. <Command>make</Command> implicitly assumes that any output files
-produced by processing a translation unit will have file names that
-can be derived from the file name of the translation unit. For
-instance, pattern rules becomes unusable. For this reason, we
-recommend you stick to using the same file name as the module name.
-</Para>
-
-<Para>
-The interface file for <Literal>A</Literal> contains information needed by the compiler
-when it compiles any module <Literal>B</Literal> that imports <Literal>A</Literal>, whether directly or
-indirectly. When compiling <Literal>B</Literal>, GHC will read <Filename>A.hi</Filename> to find the
-details that it needs to know about things defined in <Literal>A</Literal>.
-</Para>
-
-<Para>
-Furthermore, when compiling module <Literal>C</Literal> which imports <Literal>B</Literal>, GHC may
-decide that it needs to know something about <Literal>A</Literal>—for example, <Literal>B</Literal>
-might export a function that involves a type defined in <Literal>A</Literal>. In this
-case, GHC will go and read <Command>A.hi</Command> even though <Literal>C</Literal> does not explicitly
-import <Literal>A</Literal> at all.
-</Para>
-
-<Para>
-The interface file may contain all sorts of things that aren't
-explicitly exported from <Literal>A</Literal> by the programmer. For example, even
-though a data type is exported abstractly, <Filename>A.hi</Filename> will contain the
-full data type definition. For small function definitions, <Filename>A.hi</Filename>
-will contain the complete definition of the function. For bigger
-functions, <Filename>A.hi</Filename> will contain strictness information about the
-function. And so on. GHC puts much more information into <Filename>.hi</Filename> files
-when optimisation is turned on with the <Option>-O</Option> flag. Without <Option>-O</Option> it
-puts in just the minimum; with <Option>-O</Option> it lobs in a whole pile of stuff.
-<IndexTerm><Primary>optimsation, effect on .hi files</Primary></IndexTerm>
-</Para>
-
-<Para>
-<Filename>A.hi</Filename> should really be thought of as a compiler-readable version of
-<Filename>A.o</Filename>. If you use a <Filename>.hi</Filename> file that wasn't generated by the same
-compilation run that generates the <Filename>.o</Filename> file the compiler may assume
-all sorts of incorrect things about <Literal>A</Literal>, resulting in core dumps and
-other unpleasant happenings.
-</Para>
-
-</Sect2>
-
-<Sect2 id="options-finding-imports">
-<Title>Finding interface files
-</Title>
-
-<Para>
-<IndexTerm><Primary>interface files, finding them</Primary></IndexTerm>
-<IndexTerm><Primary>finding interface files</Primary></IndexTerm>
-</Para>
-
-<Para>
-In your program, you import a module <Literal>Foo</Literal> by saying
-<Literal>import Foo</Literal>. GHC goes looking for an interface file, <Filename>Foo.hi</Filename>.
-It has a builtin list of directories (notably including <Filename>.</Filename>) where
-it looks.
-</Para>
-
-<Para>
-<VariableList>
-
-<VarListEntry>
-<Term><Option>-i<dirs></Option></Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-i<dirs> option</Primary></IndexTerm>This flag
-prepends a colon-separated list of <Filename>dirs</Filename> to the “import
-directories” list.
-See also <XRef LinkEnd="recomp"> for the significance of using
-relative and absolute pathnames in the <Option>-i</Option> list.
-</Para>
-</ListItem>
-</VarListEntry>
-
-<VarListEntry>
-<Term><Option>-i</Option></Term>
-<ListItem>
-<Para>
-resets the “import directories” list back to nothing.
-</Para>
-</ListItem>
-</VarListEntry>
-
-<VarListEntry>
-<Term><Option>-fno-implicit-prelude</Option></Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-fno-implicit-prelude option</Primary></IndexTerm>
-GHC normally imports <Filename>Prelude.hi</Filename> files for you. If you'd rather it
-didn't, then give it a <Option>-fno-implicit-prelude</Option> option. You are
-unlikely to get very far without a Prelude, but, hey, it's a free
-country.
-</Para>
-</ListItem>
-</VarListEntry>
-
-<VarListEntry>
-<Term><Option>-syslib <lib></Option></Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-syslib <lib> option</Primary></IndexTerm>
-If you are using a system-supplied non-Prelude library (e.g., the
-POSIX library), just use a <Option>-syslib posix</Option> option (for example). The
-right interface files should then be available. <XRef LinkEnd="ghc-prelude"> lists the
-libraries available by this mechanism.
-</Para>
-</ListItem>
-</VarListEntry>
-
-<VarListEntry>
-<Term><Option>-I<dir></Option></Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-I<dir> option</Primary></IndexTerm>
-Once a Haskell module has been compiled to C (<Filename>.hc</Filename> file), you may
-wish to specify where GHC tells the C compiler to look for <Filename>.h</Filename> files.
-(Or, if you are using the <Option>-cpp</Option> option<IndexTerm><Primary>-cpp option</Primary></IndexTerm>, where
-it tells the C pre-processor to look…) For this purpose, use a <Option>-I</Option>
-option in the usual C-ish way.
-</Para>
-</ListItem>
-</VarListEntry>
-
-</VariableList>
-</Para>
-
-</Sect2>
-
-<Sect2 id="hi-options">
-<Title>Other options related to interface files
-</Title>
-
-<Para>
-<IndexTerm><Primary>interface files, options</Primary></IndexTerm>
-The interface output may be directed to another file
-<Filename>bar2/Wurble.iface</Filename> with the option <Option>-ohi bar2/Wurble.iface</Option><IndexTerm><Primary>-ohi
-<file> option</Primary></IndexTerm> (not recommended).
-</Para>
-
-<Para>
-To avoid generating an interface file at all, use a <Option>-nohi</Option>
-option.<IndexTerm><Primary>-nohi option</Primary></IndexTerm>
-</Para>
-
-<Para>
-The compiler does not overwrite an existing <Filename>.hi</Filename> interface file if
-the new one is byte-for-byte the same as the old one; this is friendly
-to <Command>make</Command>. When an interface does change, it is often enlightening to
-be informed. The <Option>-hi-diffs</Option><IndexTerm><Primary>-hi-diffs option</Primary></IndexTerm> option will
-make GHC run <Command>diff</Command> on the old and new <Filename>.hi</Filename> files. You can also
-record the difference in the interface file itself, the
-<Option>-keep-hi-diffs</Option><IndexTerm><Primary>-keep-hi-diffs</Primary></IndexTerm> option takes care of that.
-</Para>
-
-<Para>
-The <Filename>.hi</Filename> files from GHC contain “usage” information which changes
-often and uninterestingly. If you really want to see these changes
-reported, you need to use the
-<Option>-hi-diffs-with-usages</Option><IndexTerm><Primary>-hi-diffs-with-usages option</Primary></IndexTerm>
-option.
-</Para>
-
-<Para>
-Interface files are normally jammed full of compiler-produced
-<Emphasis>pragmas</Emphasis>, which record arities, strictness info, etc. If you
-think these pragmas are messing you up (or you are doing some kind of
-weird experiment), you can tell GHC to ignore them with the
-<Option>-fignore-interface-pragmas</Option><IndexTerm><Primary>-fignore-interface-pragmas
-option</Primary></IndexTerm> option.
-</Para>
-
-<Para>
-When compiling without optimisations on, the compiler is extra-careful
-about not slurping in data constructors and instance declarations that
-it will not need. If you believe it is getting it wrong and not
-importing stuff which you think it should, this optimisation can be
-turned off with <Option>-fno-prune-tydecls</Option> and <Option>-fno-prune-instdecls</Option>.
-<IndexTerm><Primary>-fno-prune-tydecls option</Primary></IndexTerm><IndexTerm><Primary>-fno-prune-instdecls
-option</Primary></IndexTerm>
-</Para>
-
-<Para>
-See also <XRef LinkEnd="options-linker">, which describes how the linker finds standard
-Haskell libraries.
-</Para>
-
-</Sect2>
-
-<Sect2 id="recomp">
-<Title>The recompilation checker
-</Title>
-
-<IndexTerm><Primary>recompilation checker</Primary></IndexTerm>
-
-<Para>
-<variablelist>
-<VarListEntry>
-<Term><Option>-recomp</Option></Term>
-<IndexTerm><Primary><option>-recomp</option> option</Primary></IndexTerm>
-<ListItem>
-<Para>
-Turn on recompilation checking. This will stop compilation early,
-leaving an existing <filename>.o</filename> file in place, if it can
-be determined that the module does not need to be recompiled.
-</Para>
-</ListItem>
-</VarListEntry>
-</VariableList>
-</Para>
-
-<Para>
-In the olden days, GHC compared the newly-generated
-<Filename>.hi</Filename> file with the previous version; if they were
-identical, it left the old one alone and didn't change its
-modification date. In consequence, importers of a module with an
-unchanged output <Filename>.hi</Filename> file were not recompiled.
-</Para>
-
-<Para>
-This doesn't work any more. In our earlier example, module
-<Literal>C</Literal> does not import module <Literal>A</Literal>
-directly, yet changes to <Filename>A.hi</Filename> should force a
-recompilation of <Literal>C</Literal>. And some changes to
-<Literal>A</Literal> (changing the definition of a function that
-appears in an inlining of a function exported by <Literal>B</Literal>,
-say) may conceivably not change <Filename>B.hi</Filename> one jot. So
-now…
-</Para>
-
-<Para>
-GHC keeps a version number on each interface file, and on each type
-signature within the interface file. It also keeps in every interface
-file a list of the version numbers of everything it used when it last
-compiled the file. If the source file's modification date is earlier
-than the <Filename>.o</Filename> file's date (i.e. the source hasn't
-changed since the file was last compiled), and the
-<option>-recomp</option> is given on the command line, GHC will be
-clever. It compares the version numbers on the things it needs this
-time with the version numbers on the things it needed last time
-(gleaned from the interface file of the module being compiled); if
-they are all the same it stops compiling rather early in the process
-saying “Compilation IS NOT required”. What a beautiful
-sight!
-</Para>
-
-<Para>
-GHC <Emphasis>only</Emphasis> keeps detailed dependency information
-for “user” modules, not for “library” modules.
-It distinguishes the two by a hack: a module whose
-<Filename>.hi</Filename> file has an absolute path name is considered
-a library module, while a relative path name indicates a user module.
-So if you have a multi-directory application, use
-<Emphasis>relative</Emphasis> path names in your <Option>-i</Option>
-path, to force GHC to record detailed dependency information. Use
-absolute path names only for directories containing slowly-changing
-library modules.
-</Para>
-
-<Para>
-A path is considered “absolute” if it starts with
-“<Filename>/</Filename>”, or
-“<Filename>A:/</Filename>”, or
-“<Filename>A:\</Filename>” (or
-“<Filename>B:/</Filename>”,
-“<Filename>B:\</Filename>” etc).
-</Para>
-
-<Para>
-Patrick Sansom had a workshop paper about how all this is done (though
-the details have changed quite a bit). <ULink URL="mailto:sansom@dcs.gla.ac.uk">Ask him</ULink>) if you want a copy.
-</Para>
-
-</Sect2>
-
-<Sect2 id="using-make">
-<Title>Using <Command>make</Command>
-</Title>
-
-<Para>
-<IndexTerm><Primary><literal>make</literal></Primary></IndexTerm>
-</Para>
-
-<Para>
-It is reasonably straightforward to set up a <Filename>Makefile</Filename> to use with
-GHC, assuming you name your source files the same as your modules.
-Thus:
-</Para>
-
-<Para>
-
-<ProgramListing>
-HC = ghc
-HC_OPTS = -cpp $(EXTRA_HC_OPTS)
-
-SRCS = Main.lhs Foo.lhs Bar.lhs
-OBJS = Main.o Foo.o Bar.o
-
-.SUFFIXES : .o .hi .lhs .hc .s
-
-cool_pgm : $(OBJS)
- rm $@
- $(HC) -o $@ $(HC_OPTS) $(OBJS)
-
-# Standard suffix rules
-.o.hi:
- @:
-
-.lhs.o:
- $(HC) -c $< $(HC_OPTS)
-
-.hs.o:
- $(HC) -c $< $(HC_OPTS)
-
-# Inter-module dependencies
-Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
-Main.o Main.hc Main.s : Foo.hi Baz.hi # Main imports Foo and Baz
-</ProgramListing>
-
-</Para>
-
-<Para>
-(Sophisticated <Command>make</Command> variants may achieve some of the above more
-elegantly. Notably, <Command>gmake</Command>'s pattern rules let you write the more
-comprehensible:
-</Para>
-
-<Para>
-
-<ProgramListing>
-%.o : %.lhs
- $(HC) -c $< $(HC_OPTS)
-</ProgramListing>
-
-</Para>
-
-<Para>
-What we've shown should work with any <Command>make</Command>.)
-</Para>
-
-<Para>
-Note the cheesy <Literal>.o.hi</Literal> rule: It records the dependency of the
-interface (<Filename>.hi</Filename>) file on the source. The rule says a <Filename>.hi</Filename> file can
-be made from a <Filename>.o</Filename> file by doing…nothing. Which is true.
-</Para>
-
-<Para>
-Note the inter-module dependencies at the end of the Makefile, which
-take the form
-</Para>
-
-<Para>
-
-<ProgramListing>
-Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
-</ProgramListing>
-
-</Para>
-
-<Para>
-They tell <Command>make</Command> that if any of <Literal>Foo.o</Literal>, <Literal>Foo.hc</Literal> or <Literal>Foo.s</Literal> have an
-earlier modification date than <Literal>Baz.hi</Literal>, then the out-of-date file
-must be brought up to date. To bring it up to date, <Literal>make</Literal> looks for
-a rule to do so; one of the preceding suffix rules does the job
-nicely.
-</Para>
-
-<Para>
-Putting inter-dependencies of the form <Literal>Foo.o : Bar.hi</Literal> into your
-<Filename>Makefile</Filename> by hand is rather error-prone. Don't worry—never fear,
-<Command>mkdependHS</Command> is here! (and is distributed as part of GHC) Add the
-following to your <Filename>Makefile</Filename>:
-</Para>
-
-<Para>
-
-<ProgramListing>
-depend :
- mkdependHS -- $(HC_OPTS) -- $(SRCS)
-</ProgramListing>
-
-</Para>
-
-<Para>
-Now, before you start compiling, and any time you change the <Literal>imports</Literal>
-in your program, do <Command>make depend</Command> before you do <Command>make cool_pgm</Command>.
-<Command>mkdependHS</Command> will append the needed dependencies to your <Filename>Makefile</Filename>.
-<Command>mkdependHS</Command> is fully described in <XRef LinkEnd="mkdependHS">.
-</Para>
-
-<Para>
-A few caveats about this simple scheme:
-</Para>
-
-<Para>
-
-<ItemizedList>
-<ListItem>
-
-<Para>
- You may need to compile some modules explicitly to create their
-interfaces in the first place (e.g., <Command>make Bar.o</Command> to create <Filename>Bar.hi</Filename>).
-
-</Para>
-</ListItem>
-<ListItem>
-
-<Para>
- You may have to type <Command>make</Command> more than once for the dependencies
-to have full effect. However, a <Command>make</Command> run that does nothing
-<Emphasis>does</Emphasis> mean “everything's up-to-date.”
-
-</Para>
-</ListItem>
-<ListItem>
-
-<Para>
- This scheme will work with mutually-recursive modules but,
-again, it may take multiple iterations to “settle.”
-
-</Para>
-</ListItem>
-
-</ItemizedList>
-
-</Para>
-
-</Sect2>
-
-<Sect2 id="mutual-recursion">
-<Title>How to compile mutually recursive modules
-</Title>
-
-<Para>
-<IndexTerm><Primary>module system, recursion</Primary></IndexTerm>
-<IndexTerm><Primary>recursion, between modules</Primary></IndexTerm>
-</Para>
-
-<Para>
-Currently, the compiler does not have proper support for dealing with
-mutually recursive modules:
-</Para>
-
-<Para>
-
-<ProgramListing>
-module A where
-
-import B
-
-newtype TA = MkTA Int
-
-f :: TB -> TA
-f (MkTB x) = MkTA x
---------
-module B where
-
-import A
-
-data TB = MkTB !Int
-
-g :: TA -> TB
-g (MkTA x) = MkTB x
-</ProgramListing>
-
-</Para>
-
-<Para>
-When compiling either module A and B, the compiler will try (in vain)
-to look for the interface file of the other. So, to get mutually
-recursive modules off the ground, you need to hand write an interface
-file for A or B, so as to break the loop. These hand-written
-interface files are called <Literal>hi-boot</Literal> files, and are placed in a file
-called <Filename><module>.hi-boot</Filename>. To import from an <Literal>hi-boot</Literal> file instead
-of the standard <Filename>.hi</Filename> file, use the following syntax in the importing module:
-<IndexTerm><Primary>hi-boot files</Primary></IndexTerm>
-<IndexTerm><Primary>importing, hi-boot files</Primary></IndexTerm>
-</Para>
-
-<Para>
-
-<ProgramListing>
-import {-# SOURCE #-} A
-</ProgramListing>
-
-</Para>
-
-<Para>
-The hand-written interface need only contain the bare minimum of
-information needed to get the bootstrapping process started. For
-example, it doesn't need to contain declarations for <Emphasis>everything</Emphasis>
-that module <Literal>A</Literal> exports, only the things required by the module that
-imports <Literal>A</Literal> recursively.
-</Para>
-
-<Para>
-For the example at hand, the boot interface file for A would look like
-the following:
-</Para>
-
-<Para>
-
-<ProgramListing>
-__interface A 1 404 where
-__export A TA{MkTA} ;
-1 newtype TA = MkTA PrelBase.Int ;
-</ProgramListing>
-
-</Para>
-
-<Para>
-The syntax is essentially the same as a normal <Filename>.hi</Filename> file
-(unfortunately), but you can usually tailor an existing <Filename>.hi</Filename> file to
-make a <Filename>.hi-boot</Filename> file.
-</Para>
-
-<Para>
-Notice that we only put the declaration for the newtype <Literal>TA</Literal> in the
-<Literal>hi-boot</Literal> file, not the signature for <Function>f</Function>, since <Function>f</Function> isn't used by
-<Literal>B</Literal>.
-</Para>
-
-<Para>
-The number “1” after “__interface A” gives the version number of module A;
-it is incremented whenever anything in A's interface file changes. The “404” is
-the version number of the interface file <Emphasis>syntax</Emphasis>; we change it when
-we change the syntax of interface files so that you get a better error message when
-you try to read an old-format file with a new-format compiler.
-</Para>
-
-<Para>
-The number “1” at the beginning of a declaration is the <Emphasis>version
-number</Emphasis> of that declaration: for the purposes of <Filename>.hi-boot</Filename> files
-these can all be set to 1. All names must be fully qualified with the
-<Emphasis>original</Emphasis> module that an object comes from: for example, the
-reference to <Literal>Int</Literal> in the interface for <Literal>A</Literal> comes from <Literal>PrelBase</Literal>,
-which is a module internal to GHC's prelude. It's a pain, but that's
-the way it is.
-</Para>
-
-<Para>
-If you want an hi-boot file to export a data type, but you don't want to give its constructors
-(because the constructors aren't used by the SOURCE-importing module), you can write simply:
-</Para>
-
-<Para>
-
-<ProgramListing>
-__interface A 1 404 where
-__export A TA;
-1 data TA
-</ProgramListing>
-
-</Para>
-
-<Para>
-(You must write all the type parameters, but leave out the '=' and everything that follows it.)
-</Para>
-
-<Para>
-<Emphasis>Note:</Emphasis> This is all a temporary solution, a version of the
-compiler that handles mutually recursive modules properly without the manual
-construction of interface files, is (allegedly) in the works.
-</Para>
-
-</Sect2>
-
-</Sect1>
-
-<Sect1 id="options-optimise">
-<Title>Optimisation (code improvement)
-</Title>
-
-<Para>
-<IndexTerm><Primary>optimisation (GHC)</Primary></IndexTerm>
-<IndexTerm><Primary>improvement, code (GHC)</Primary></IndexTerm>
-</Para>
-
-<Para>
-The <Option>-O*</Option> options specify convenient “packages” of optimisation
-flags; the <Option>-f*</Option> options described later on specify
-<Emphasis>individual</Emphasis> optimisations to be turned on/off; the <Option>-m*</Option>
-options specify <Emphasis>machine-specific</Emphasis> optimisations to be turned
-on/off.
-</Para>
-
-<Sect2 id="optimise-pkgs">
-<Title><Option>-O*</Option>: convenient “packages” of optimisation flags.
-</Title>
-
-<Para>
-<IndexTerm><Primary>-O options</Primary></IndexTerm>
-</Para>
-
-<Para>
-There are <Emphasis>many</Emphasis> options that affect the quality of code
-produced by GHC. Most people only have a general goal, something like
-“Compile quickly” or “Make my program run like greased lightning.”
-The following “packages” of optimisations (or lack thereof) should
-suffice.
-</Para>
-
-<Para>
-Once you choose a <Option>-O*</Option> “package,” stick with it—don't chop and
-change. Modules' interfaces <Emphasis>will</Emphasis> change with a shift to a new
-<Option>-O*</Option> option, and you may have to recompile a large chunk of all
-importing modules before your program can again be run
-safely (see <XRef LinkEnd="recomp">).
-</Para>
-
-<Para>
-<VariableList>
-
-<VarListEntry>
-<Term>No <Option>-O*</Option>-type option specified:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-O* not specified</Primary></IndexTerm>
-This is taken to mean: “Please compile quickly; I'm not over-bothered
-about compiled-code quality.” So, for example: <Command>ghc -c Foo.hs</Command>
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-O</Option> or <Option>-O1</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-O option</Primary></IndexTerm>
-<IndexTerm><Primary>-O1 option</Primary></IndexTerm>
-<IndexTerm><Primary>optimise normally</Primary></IndexTerm>
-Means: “Generate good-quality code without taking too long about it.”
-Thus, for example: <Command>ghc -c -O Main.lhs</Command>
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-O2</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-O2 option</Primary></IndexTerm>
-<IndexTerm><Primary>optimise aggressively</Primary></IndexTerm>
-Means: “Apply every non-dangerous optimisation, even if it means
-significantly longer compile times.”
-</Para>
-
-<Para>
-The avoided “dangerous” optimisations are those that can make
-runtime or space <Emphasis>worse</Emphasis> if you're unlucky. They are
-normally turned on or off individually.
-</Para>
-
-<Para>
-At the moment, <Option>-O2</Option> is <Emphasis>unlikely</Emphasis> to produce
-better code than <Option>-O</Option>.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-O2-for-C</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-O2-for-C option</Primary></IndexTerm>
-<IndexTerm><Primary>gcc, invoking with -O2</Primary></IndexTerm>
-</Para>
-
-<Para>
-Says to run GCC with <Option>-O2</Option>, which may be worth a few percent in
-execution speed. Don't forget <Option>-fvia-C</Option>, lest you use the native-code
-generator and bypass GCC altogether!
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-Onot</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-Onot option</Primary></IndexTerm>
-<IndexTerm><Primary>optimising, reset</Primary></IndexTerm>
-</Para>
-
-<Para>
-This option will make GHC “forget” any <Option>-O</Option>ish options it has seen so
-far. Sometimes useful; for example: <Command>make all EXTRA_HC_OPTS=-Onot</Command>.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-Ofile <file></Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-Ofile <file> option</Primary></IndexTerm>
-<IndexTerm><Primary>optimising, customised</Primary></IndexTerm>
-</Para>
-
-<Para>
-For those who need <Emphasis>absolute</Emphasis> control over <Emphasis>exactly</Emphasis>
-what options are used (e.g., compiler writers, sometimes :-), a list
-of options can be put in a file and then slurped in with <Option>-Ofile</Option>.
-</Para>
-
-<Para>
-In that file, comments are of the <Literal>#</Literal>-to-end-of-line variety; blank
-lines and most whitespace is ignored.
-</Para>
-
-<Para>
-Please ask if you are baffled and would like an example of <Option>-Ofile</Option>!
-</Para>
-</ListItem>
-</VarListEntry>
-</VariableList>
-</Para>
-
-<Para>
-At Glasgow, we don't use a <Option>-O*</Option> flag for day-to-day work. We use
-<Option>-O</Option> to get respectable speed; e.g., when we want to measure
-something. When we want to go for broke, we tend to use <Option>-O -fvia-C
--O2-for-C</Option> (and we go for lots of coffee breaks).
-</Para>
-
-<Para>
-The easiest way to see what <Option>-O</Option> (etc.) “really mean” is to run with
-<Option>-v</Option>, then stand back in amazement. Alternatively, just look at the
-<Literal>HsC_minus<blah></Literal> lists in the GHC driver script.
-</Para>
-
-</Sect2>
-
-<Sect2>
-<Title><Option>-f*</Option>: platform-independent flags</Title>
-
-<Para>
-<IndexTerm><Primary>-f* options (GHC)</Primary></IndexTerm>
-<IndexTerm><Primary>-fno-* options (GHC)</Primary></IndexTerm>
-</Para>
-
-<Para>
-Flags can be turned <Emphasis>off</Emphasis> individually. (NB: I hope you have a
-good reason for doing this…) To turn off the <Option>-ffoo</Option> flag, just use
-the <Option>-fno-foo</Option> flag.<IndexTerm><Primary>-fno-<opt> anti-option</Primary></IndexTerm> So, for
-example, you can say <Option>-O2 -fno-strictness</Option>, which will then drop out
-any running of the strictness analyser.
-</Para>
-
-<Para>
-The options you are most likely to want to turn off are:
-
-<ItemizedList>
-<ListItem>
-
-<Para>
-<Option>-fno-strictness</Option><IndexTerm><Primary>-fno-strictness option</Primary></IndexTerm> (strictness
-analyser, because it is sometimes slow),
-</Para>
-</ListItem>
-<ListItem>
-
-<Para>
-<Option>-fno-specialise</Option><IndexTerm><Primary>-fno-specialise option</Primary></IndexTerm> (automatic
-specialisation of overloaded functions, because it can make your code
-bigger) (US spelling also accepted), and
-</Para>
-</ListItem>
-<ListItem>
-
-<Para>
-<Option>-fno-cpr-analyse</Option><IndexTerm><Primary>-fno-cpr-analyse option</Primary></IndexTerm> switches off the CPR (constructed product
-result) analyser.
-</Para>
-</ListItem>
-
-</ItemizedList>
-
-</Para>
-
-<Para>
-Should you wish to turn individual flags <Emphasis>on</Emphasis>, you are advised
-to use the <Option>-Ofile</Option> option, described above. Because the order in
-which optimisation passes are run is sometimes crucial, it's quite
-hard to do with command-line options.
-</Para>
-
-<Para>
-Here are some “dangerous” optimisations you <Emphasis>might</Emphasis> want to try:
-<VariableList>
-
-<VarListEntry>
-<Term><Option>-fvia-C</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-fvia-C option</Primary></IndexTerm>
-<IndexTerm><Primary>native code generator, turning off</Primary></IndexTerm>
-</Para>
-
-<Para>
-Compile via C, and don't use the native-code generator. (There are
-many cases when GHC does this on its own.) You might pick up a little
-bit of speed by compiling via C. If you use <Function>_ccall_gc_</Function>s or
-<Function>_casm_</Function>s, you probably <Emphasis>have</Emphasis> to use <Option>-fvia-C</Option>.
-</Para>
-
-<Para>
-The lower-case incantation, <Option>-fvia-c</Option>, is synonymous.
-</Para>
-
-<Para>
-Compiling via C will probably be slower (in compilation time) than
-using GHC's native code generator.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-funfolding-interface-threshold<n></Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-funfolding-interface-threshold option</Primary></IndexTerm>
-<IndexTerm><Primary>inlining, controlling</Primary></IndexTerm>
-<IndexTerm><Primary>unfolding, controlling</Primary></IndexTerm>
-(Default: 30) By raising or lowering this number, you can raise or
-lower the amount of pragmatic junk that gets spewed into interface
-files. (An unfolding has a “size” that reflects the cost in terms
-of “code bloat” of expanding that unfolding in another module. A
-bigger function would be assigned a bigger cost.)
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-funfolding-creation-threshold<n></Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-funfolding-creation-threshold option</Primary></IndexTerm>
-<IndexTerm><Primary>inlining, controlling</Primary></IndexTerm>
-<IndexTerm><Primary>unfolding, controlling</Primary></IndexTerm>
-(Default: 30) This option is similar to
-<Option>-funfolding-interface-threshold</Option>, except that it governs unfoldings
-within a single module. Increasing this figure is more likely to
-result in longer compile times than faster code. The next option is
-more useful:
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-funfolding-use-threshold<n></Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-funfolding-use-threshold option</Primary></IndexTerm>
-<IndexTerm><Primary>inlining, controlling</Primary></IndexTerm>
-<IndexTerm><Primary>unfolding, controlling</Primary></IndexTerm>
-(Default: 8) This is the magic cut-off figure for unfolding: below
-this size, a function definition will be unfolded at the call-site,
-any bigger and it won't. The size computed for a function depends on
-two things: the actual size of the expression minus any discounts that
-apply (see <Option>-funfolding-con-discount</Option>).
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-funfolding-con-discount<n></Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-funfolding-con-discount option</Primary></IndexTerm>
-<IndexTerm><Primary>inlining, controlling</Primary></IndexTerm>
-<IndexTerm><Primary>unfolding, controlling</Primary></IndexTerm>
-(Default: 2) If the compiler decides that it can eliminate some
-computation by performing an unfolding, then this is a discount factor
-that it applies to the funciton size before deciding whether to unfold
-it or not.
-</Para>
-
-<Para>
-OK, folks, these magic numbers `30', `8', and '2' are mildly
-arbitrary; they are of the “seem to be OK” variety. The `8' is the
-more critical one; it's what determines how eager GHC is about
-expanding unfoldings.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-funbox-strict-fields</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-funbox-strict-fields option</Primary></IndexTerm>
-<IndexTerm><Primary>strict constructor fields</Primary></IndexTerm>
-<IndexTerm><Primary>constructor fields, strict</Primary></IndexTerm>
-</Para>
-
-<Para>
-This option causes all constructor fields which are marked strict
-(i.e. “!”) to be unboxed or unpacked if possible. For example:
-</Para>
-
-<Para>
-
-<ProgramListing>
-data T = T !Float !Float
-</ProgramListing>
-
-</Para>
-
-<Para>
-will create a constructor <Literal>T</Literal> containing two unboxed floats if the
-<Option>-funbox-strict-fields</Option> flag is given. This may not always be an
-optimisation: if the <Function>T</Function> constructor is scrutinised and the floats
-passed to a non-strict function for example, they will have to be
-reboxed (this is done automatically by the compiler).
-</Para>
-
-<Para>
-This option should only be used in conjunction with <Option>-O</Option>, in order to
-expose unfoldings to the compiler so the reboxing can be removed as
-often as possible. For example:
-</Para>
-
-<Para>
-
-<ProgramListing>
-f :: T -> Float
-f (T f1 f2) = f1 + f2
-</ProgramListing>
-
-</Para>
-
-<Para>
-The compiler will avoid reboxing <Function>f1</Function> and <Function>f2</Function> by inlining <Function>+</Function> on
-floats, but only when <Option>-O</Option> is on.
-</Para>
-
-<Para>
-Any single-constructor data is eligible for unpacking; for example
-</Para>
-
-<Para>
-
-<ProgramListing>
-data T = T !(Int,Int)
-</ProgramListing>
-
-</Para>
-
-<Para>
-will store the two <Literal>Int</Literal>s directly in the <Function>T</Function> constructor, by flattening
-the pair. Multi-level unpacking is also supported:
-</Para>
-
-<Para>
-
-<ProgramListing>
-data T = T !S
-data S = S !Int !Int
-</ProgramListing>
-
-</Para>
-
-<Para>
-will store two unboxed <Literal>Int#</Literal>s directly in the <Function>T</Function> constructor.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-fsemi-tagging</Option>:</Term>
-<ListItem>
-<Para>
-This option (which <Emphasis>does not work</Emphasis> with the native-code generator)
-tells the compiler to add extra code to test for already-evaluated
-values. You win if you have lots of such values during a run of your
-program, you lose otherwise. (And you pay in extra code space.)
-</Para>
-
-<Para>
-We have not played with <Option>-fsemi-tagging</Option> enough to recommend it.
-(For all we know, it doesn't even work anymore… Sigh.)
-</Para>
-</ListItem>
-</VarListEntry>
-</VariableList>
-</Para>
-
-</Sect2>
-
-<Sect2>
-<Title><Option>-m*</Option>: platform-specific flags</Title>
-
-<Para>
-<IndexTerm><Primary>-m* options (GHC)</Primary></IndexTerm>
-<IndexTerm><Primary>platform-specific options</Primary></IndexTerm>
-<IndexTerm><Primary>machine-specific options</Primary></IndexTerm>
-</Para>
-
-<Para>
-Some flags only make sense for particular target platforms.
-</Para>
-
-<Para>
-<VariableList>
-
-<VarListEntry>
-<Term><Option>-mv8</Option>:</Term>
-<ListItem>
-<Para>
-(SPARC machines)<IndexTerm><Primary>-mv8 option (SPARC only)</Primary></IndexTerm>
-Means to pass the like-named option to GCC; it says to use the
-Version 8 SPARC instructions, notably integer multiply and divide.
-The similiar <Option>-m*</Option> GCC options for SPARC also work, actually.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-mlong-calls</Option>:</Term>
-<ListItem>
-<Para>
-(HPPA machines)<IndexTerm><Primary>-mlong-calls option (HPPA only)</Primary></IndexTerm>
-Means to pass the like-named option to GCC. Required for Very Big
-modules, maybe. (Probably means you're in trouble…)
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-monly-[32]-regs</Option>:</Term>
-<ListItem>
-<Para>
-(iX86 machines)<IndexTerm><Primary>-monly-N-regs option (iX86 only)</Primary></IndexTerm>
-GHC tries to “steal” four registers from GCC, for performance
-reasons; it almost always works. However, when GCC is compiling some
-modules with four stolen registers, it will crash, probably saying:
-
-<Screen>
-Foo.hc:533: fixed or forbidden register was spilled.
-This may be due to a compiler bug or to impossible asm
-statements or clauses.
-</Screen>
-
-Just give some registers back with <Option>-monly-N-regs</Option>. Try `3' first,
-then `2'. If `2' doesn't work, please report the bug to us.
-</Para>
-</ListItem>
-</VarListEntry>
-</VariableList>
-</Para>
-
-</Sect2>
-
-<Sect2 id="optimise-C-compiler">
-<Title>Code improvement by the C compiler.
-</Title>
-
-<Para>
-<IndexTerm><Primary>optimisation by GCC</Primary></IndexTerm>
-<IndexTerm><Primary>GCC optimisation</Primary></IndexTerm>
-</Para>
-
-<Para>
-The C compiler (GCC) is run with <Option>-O</Option> turned on. (It has
-to be, actually).
-</Para>
-
-<Para>
-If you want to run GCC with <Option>-O2</Option>—which may be worth a few
-percent in execution speed—you can give a
-<Option>-O2-for-C</Option><IndexTerm><Primary>-O2-for-C option</Primary></IndexTerm> option.
-</Para>
-
-</Sect2>
-
-</Sect1>
-
-<Sect1 id="options-phases">
-<Title>Options related to a particular phase
-</Title>
-
-<Sect2 id="c-pre-processor">
-<Title>The C pre-processor
-</Title>
-
-<Para>
-<IndexTerm><Primary>pre-processing: cpp</Primary></IndexTerm>
-<IndexTerm><Primary>C pre-processor options</Primary></IndexTerm>
-<IndexTerm><Primary>cpp, pre-processing with</Primary></IndexTerm>
-</Para>
-
-<Para>
-The C pre-processor <Command>cpp</Command> is run over your Haskell code only if the
-<Option>-cpp</Option> option <IndexTerm><Primary>-cpp option</Primary></IndexTerm> is given. Unless you are
-building a large system with significant doses of conditional
-compilation, you really shouldn't need it.
-<VariableList>
-
-<VarListEntry>
-<Term><Option>-D<foo></Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-D<name> option</Primary></IndexTerm>
-Define macro <Constant><foo></Constant> in the usual way. NB: does <Emphasis>not</Emphasis> affect
-<Option>-D</Option> macros passed to the C compiler when compiling via C! For those,
-use the <Option>-optc-Dfoo</Option> hack… (see <XRef LinkEnd="forcing-options-through">).
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-U<foo></Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-U<name> option</Primary></IndexTerm>
-Undefine macro <Command><foo></Command> in the usual way.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-I<dir></Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-I<dir> option</Primary></IndexTerm>
-Specify a directory in which to look for <Literal>#include</Literal> files, in
-the usual C way.
-</Para>
-</ListItem>
-</VarListEntry>
-</VariableList>
-</Para>
-
-<Para>
-The GHC driver pre-defines several macros when processing Haskell
-source code (<Filename>.hs</Filename> or <Filename>.lhs</Filename> files):
-</Para>
-
-<Para>
-<VariableList>
-
-<VarListEntry>
-<Term><Constant>__HASKELL98__</Constant>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>__HASKELL98__</Primary></IndexTerm>
-If defined, this means that GHC supports the language defined by the
-Haskell 98 report.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Constant>__HASKELL__=98</Constant>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>__HASKELL__</Primary></IndexTerm>
-In GHC 4.04 and later, the <Constant>__HASKELL__</Constant> macro is defined as having
-the value <Constant>98</Constant>.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Constant>__HASKELL1__</Constant>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>__HASKELL1__ macro</Primary></IndexTerm>
-If defined to <Emphasis>n</Emphasis>, that means GHC supports the Haskell language
-defined in the Haskell report version <Emphasis>1.n</Emphasis>. Currently 5. This
-macro is deprecated, and will probably disappear in future versions.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Constant>__GLASGOW_HASKELL__</Constant>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>__GLASGOW_HASKELL__ macro</Primary></IndexTerm>
-For version <Emphasis>n</Emphasis> of the GHC system, this will be <Literal>#define</Literal>d to
-<Emphasis>100n</Emphasis>. So, for version 4.00, it is 400.
-</Para>
-
-<Para>
-With any luck, <Constant>__GLASGOW_HASKELL__</Constant> will be undefined in all other
-implementations that support C-style pre-processing.
-</Para>
-
-<Para>
-(For reference: the comparable symbols for other systems are:
-<Constant>__HUGS__</Constant> for Hugs and <Constant>__HBC__</Constant> for Chalmers.)
-</Para>
-
-<Para>
-NB. This macro is set when pre-processing both Haskell source and C
-source, including the C source generated from a Haskell module
-(i.e. <Filename>.hs</Filename>, <Filename>.lhs</Filename>, <Filename>.c</Filename> and <Filename>.hc</Filename> files).
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Constant>__CONCURRENT_HASKELL__</Constant>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>__CONCURRENT_HASKELL__ macro</Primary></IndexTerm>
-This symbol is defined when pre-processing Haskell (input) and
-pre-processing C (GHC output). Since GHC from verion 4.00 now
-supports concurrent haskell by default, this symbol is always defined.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Constant>__PARALLEL_HASKELL__</Constant>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>__PARALLEL_HASKELL__ macro</Primary></IndexTerm>
-Only defined when <Option>-parallel</Option> is in use! This symbol is defined when
-pre-processing Haskell (input) and pre-processing C (GHC output).
-</Para>
-</ListItem>
-</VarListEntry>
-</VariableList>
-</Para>
-
-<Para>
-Options other than the above can be forced through to the C
-pre-processor with the <Option>-opt</Option> flags (see
-<XRef LinkEnd="forcing-options-through">).
-</Para>
-
-<Para>
-A small word of warning: <Option>-cpp</Option> is not friendly to “string
-gaps”.<IndexTerm><Primary>-cpp vs string gaps</Primary></IndexTerm><IndexTerm><Primary>string gaps vs
--cpp</Primary></IndexTerm>. In other words, strings such as the following:
-</Para>
-
-<Para>
-
-<ProgramListing>
-strmod = "\
-\ p \
-\ "
-</ProgramListing>
-
-</Para>
-
-<Para>
-don't work with <Option>-cpp</Option>; <Filename>/usr/bin/cpp</Filename> elides the
-backslash-newline pairs.
-</Para>
-
-<Para>
-However, it appears that if you add a space at the end of the line,
-then <Command>cpp</Command> (at least GNU <Command>cpp</Command> and possibly other <Command>cpp</Command>s)
-leaves the backslash-space pairs alone and the string gap works as
-expected.
-</Para>
-
-</Sect2>
-
-<Sect2 id="options-C-compiler">
-<Title>Options affecting the C compiler (if applicable)
-</Title>
-
-<Para>
-<IndexTerm><Primary>include-file options</Primary></IndexTerm>
-<IndexTerm><Primary>C compiler options</Primary></IndexTerm>
-<IndexTerm><Primary>GCC options</Primary></IndexTerm>
-</Para>
-
-<Para>
-At the moment, quite a few common C-compiler options are passed on
-quietly to the C compilation of Haskell-compiler-generated C files.
-THIS MAY CHANGE. Meanwhile, options so sent are:
-</Para>
-
-<Para>
-
-<InformalTable>
-<TGroup Cols="2">
-<ColSpec Align="Left" Colsep="0">
-<ColSpec Align="Left" Colsep="0">
-<TBody>
-<Row>
-<Entry><Option>-ansi</Option> </Entry>
-<Entry> do ANSI C (not K&R) </Entry>
-</Row>
-<Row>
-<Entry>
-<Option>-pedantic</Option> </Entry>
-<Entry> be so</Entry>
-</Row>
-<Row>
-<Entry>
-<Option>-dgcc-lint</Option> </Entry>
-<Entry> (hack) short for “make GCC very paranoid”</Entry>
-</Row>
-
-</TBody>
-
-</TGroup>
-</InformalTable>
-
-<IndexTerm><Primary>-ansi option (for GCC)</Primary></IndexTerm>
-<IndexTerm><Primary>-pedantic option (for GCC)</Primary></IndexTerm>
-<IndexTerm><Primary>-dgcc-lint option (GCC paranoia)</Primary></IndexTerm>
-</Para>
-
-<Para>
-If you are compiling with lots of <Literal>ccalls</Literal>, etc., you may need to
-tell the C compiler about some <Literal>#include</Literal> files. There is no real
-pretty way to do this, but you can use this hack from the
-command-line:
-</Para>
-
-<Para>
+<chapter id="using-ghc">
+ <title>Using GHC</title>
+
+ <indexterm><primary>GHC, using</primary></indexterm>
+ <indexterm><primary>using GHC</primary></indexterm>
+
+ <para>GHC can work in one of three “modes”:</para>
+
+ <variablelist>
+ <varlistentry>
+ <term><cmdsynopsis><command>ghc</command>
+ <arg choice=plain>––interactive</arg>
+ </cmdsynopsis></term>
+ <indexterm><primary>interactive mode</primary>
+ </indexterm>
+ <indexterm><primary>ghci</primary>
+ </indexterm>
+ <listitem>
+ <para>Interactive mode, which is also available as
+ <command>ghci</command>. Interactive mode is described in
+ more detail in <xref linkend="ghci">.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><cmdsynopsis><command>ghc</command>
+ <arg choice=plain>––make</arg>
+ </cmdsynopsis></term>
+ <indexterm><primary>make mode</primary>
+ </indexterm>
+ <indexterm><primary><option>––make</option></primary>
+ </indexterm>
+ <listitem>
+ <para>In this mode, GHC will build a multi-module Haskell
+ program automatically, figuring out dependencies for itself.
+ If you have a straightforward Haskell program, this is likely
+ to be much easier, and faster, than using
+ <command>make</command>.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><cmdsynopsis>
+ <command>ghc</command>
+ <group>
+ <arg>-E</arg>
+ <arg>-C</arg>
+ <arg>-S</arg>
+ <arg>-c</arg>
+ </group>
+ </cmdsynopsis></term>
+ <indexterm><primary><option>-E</option></primary></indexterm>
+ <indexterm><primary><option>-C</option></primary></indexterm>
+ <indexterm><primary><option>-S</option></primary></indexterm>
+ <indexterm><primary><option>-c</option></primary></indexterm>
+ <listitem>
+ <para>This is the traditional batch-compiler mode, in which
+ GHC can compile source files one at a time, or link objects
+ together into an executable.</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+
+ <sect1>
+ <title>Options overview</title>
+
+ <para>GHC's behaviour is controlled by
+ <firstterm>options</firstterm>, which for historical reasons are
+ also sometimes referred to as command-line flags or arguments.
+ Options can be specified in three ways:</para>
+
+ <sect2>
+ <title>Command-line arguments</title>
+
+ <indexterm><primary>structure, command-line</primary></indexterm>
+ <indexterm><primary>command-line</primary><secondary>arguments</secondary></indexterm>
+ <indexterm><primary>arguments</primary><secondary>command-line</secondary></indexterm>
+
+ <para>An invocation of GHC takes the following form:</para>
<Screen>
-% ghc -c '-#include <X/Xlib.h>' Xstuff.lhs
+ghc [argument...]
</Screen>
-</Para>
-
-</Sect2>
-
-<Sect2 id="options-linker">
-<Title>Linking and consistency-checking
-</Title>
-
-<Para>
-<IndexTerm><Primary>linker options</Primary></IndexTerm>
-<IndexTerm><Primary>ld options</Primary></IndexTerm>
-</Para>
-
-<Para>
-GHC has to link your code with various libraries, possibly including:
-user-supplied, GHC-supplied, and system-supplied (<Option>-lm</Option> math
-library, for example).
-</Para>
-
-<Para>
-<VariableList>
-
-<VarListEntry>
-<Term><Option>-l<FOO></Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-l<lib> option</Primary></IndexTerm>
-Link in a library named <Filename>lib<FOO>.a</Filename> which resides somewhere on the
-library directories path.
-</Para>
-
-<Para>
-Because of the sad state of most UNIX linkers, the order of such
-options does matter. Thus: <Command>ghc -lbar *.o</Command> is almost certainly
-wrong, because it will search <Filename>libbar.a</Filename> <Emphasis>before</Emphasis> it has
-collected unresolved symbols from the <Filename>*.o</Filename> files.
-<Command>ghc *.o -lbar</Command> is probably better.
-</Para>
-
-<Para>
-The linker will of course be informed about some GHC-supplied
-libraries automatically; these are:
-</Para>
-
-<Para>
-
-<InformalTable>
-<TGroup Cols="2">
-<ColSpec Align="Left" Colsep="0">
-<ColSpec Align="Left" Colsep="0">
-<TBody>
-<Row>
-<Entry><Emphasis>-l equivalent</Emphasis> </Entry>
-<Entry> <Emphasis>description</Emphasis> </Entry>
-</Row>
-
-<Row>
-<Entry>
-<Option>-lHSrts,-lHSclib</Option> </Entry>
-<Entry> basic runtime libraries </Entry>
-</Row>
-<Row>
-<Entry>
-<Option>-lHS</Option> </Entry>
-<Entry> standard Prelude library </Entry>
-</Row>
-<Row>
-<Entry>
-<Option>-lHS_cbits</Option> </Entry>
-<Entry> C support code for standard Prelude library </Entry>
-</Row>
-<Row>
-<Entry>
-<Option>-lgmp</Option> </Entry>
-<Entry> GNU multi-precision library (for Integers)</Entry>
-</Row>
-
-</TBody>
-
-</TGroup>
-</InformalTable>
-
-</Para>
+ <para>Command-line arguments are either options or file names.</para>
+
+ <para>Command-line options begin with <literal>-</literal>.
+ They may <emphasis>not</emphasis> be grouped:
+ <option>-vO</option> is different from <option>-v -O</option>.
+ Options need not precede filenames: e.g., <literal>ghc *.o -o
+ foo</literal>. All options are processed and then applied to
+ all files; you cannot, for example, invoke <literal>ghc -c -O1
+ Foo.hs -O2 Bar.hs</literal> to apply different optimisation
+ levels to the files <filename>Foo.hs</filename> and
+ <filename>Bar.hs</filename>.</para>
+ </sect2>
+
+ <Sect2 id="source-file-options">
+ <title>Command line options in source files</title>
+
+ <indexterm><primary>source-file options</primary></indexterm>
+
+ <para>Sometimes it is useful to make the connection between a
+ source file and the command-line options it requires quite
+ tight. For instance, if a Haskell source file uses GHC
+ extensions, it will always need to be compiled with the
+ <option>-fglasgow-exts</option> option. Rather than maintaining
+ the list of per-file options in a <filename>Makefile</filename>,
+ it is possible to do this directly in the source file using the
+ <literal>OPTIONS</literal> pragma <indexterm><primary>OPTIONS
+ pragma</primary></indexterm>:</para>
-<Para>
-<IndexTerm><Primary>-lHS library</Primary></IndexTerm>
-<IndexTerm><Primary>-lHS_cbits library</Primary></IndexTerm>
-<IndexTerm><Primary>-lHSrts library</Primary></IndexTerm>
-<IndexTerm><Primary>-lgmp library</Primary></IndexTerm>
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-syslib <name></Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-syslib <name> option</Primary></IndexTerm>
-</Para>
+<ProgramListing>
+{-# OPTIONS -fglasgow-exts #-}
+module X where
+...
+</ProgramListing>
+
+ <para><literal>OPTIONS</literal> pragmas are only looked for at
+ the top of your source files, upto the first
+ (non-literate,non-empty) line not containing
+ <literal>OPTIONS</literal>. Multiple <literal>OPTIONS</literal>
+ pragmas are recognised. Note that your command shell does not
+ get to the source file options, they are just included literally
+ in the array of command-line arguments the compiler driver
+ maintains internally, so you'll be desperately disappointed if
+ you try to glob etc. inside <literal>OPTIONS</literal>.</para>
+
+ <para>NOTE: the contents of OPTIONS are prepended to the
+ command-line options, so you <emphasis>do</emphasis> have the
+ ability to override OPTIONS settings via the command
+ line.</para>
+
+ <para>It is not recommended to move all the contents of your
+ Makefiles into your source files, but in some circumstances, the
+ <literal>OPTIONS</literal> pragma is the Right Thing. (If you
+ use <option>-keep-hc-file-too</option> and have OPTION flags in
+ your module, the OPTIONS will get put into the generated .hc
+ file).</para>
+ </sect2>
+
+ <sect2>
+ <title>Setting options in GHCi</title>
+
+ <para>Options may also be modified from within GHCi, using the
+ <literal>:set</literal> command. See <xref linkend="ghci-set">
+ for more details.</para>
+ </sect2>
+ </sect1>
+
+ <sect1 id="static-dynamic-flags">
+ <title>Static vs. Dynamic options</title>
+ <indexterm><primary>static</primary><secondary>options</secondary>
+ </indexterm>
+ <indexterm><primary>dynamic</primary><secondary>options</secondary>
+ </indexterm>
+
+ <para>Each of GHC's command line options is classified as either
+ <firstterm>static</firstterm> or <firstterm>dynamic</firstterm>.
+ A static flag may only be specified on the command line, whereas a
+ dynamic flag may also be given in an <literal>OPTIONS</literal>
+ pragma in a source file or set from the GHCi command-line with
+ <literal>:set</literal>.</para>
+
+ <para>As a rule of thumb, all the language options are dynamic, as
+ are the warning options and the debugging options. The rest are
+ static, with the notable exceptions of <option>-v</option>,
+ <option>-cpp</option>, <option>-fasm</option>,
+ <option>-fvia-C</option>, and <option>-#include</option>.
+
+ The flag reference tables (<xref linkend="flag-reference">) lists
+ the status of each flag.</para>
+ </sect1>
+
+ <sect1 id="file-suffixes">
+ <title>Meaningful file suffixes</title>
+
+ <indexterm><primary>suffixes, file</primary></indexterm>
+ <indexterm><primary>file suffixes for GHC</primary></indexterm>
+
+ <para>File names with “meaningful” suffixes (e.g.,
+ <filename>.lhs</filename> or <filename>.o</filename>) cause the
+ “right thing” to happen to those files.</para>
+
+ <variablelist>
+
+ <varlistentry>
+ <term><filename>.lhs</filename></term>
+ <indexterm><primary><literal>lhs</literal> suffix</primary></indexterm>
+ <listitem>
+ <para>A “literate Haskell” module.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><filename>.hs</filename></term>
+ <listitem>
+ <para>A not-so-literate Haskell module.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><filename>.hi</filename></term>
+ <listitem>
+ <para>A Haskell interface file, probably
+ compiler-generated.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><filename>.hc</filename></term>
+ <listitem>
+ <para>Intermediate C file produced by the Haskell
+ compiler.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><filename>.c</filename></term>
+ <listitem>
+ <para>A C file not produced by the Haskell
+ compiler.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><filename>.s</filename></term>
+ <listitem>
+ <para>An assembly-language source file, usually produced by
+ the compiler.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><filename>.o</filename></term>
+ <listitem>
+ <para>An object file, produced by an assembler.</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+
+ <para>Files with other suffixes (or without suffixes) are passed
+ straight to the linker.</para>
+
+ </sect1>
+
+ <sect1 id="options-help">
+ <title>Help and verbosity options</title>
+
+ <IndexTerm><Primary>help options</Primary></IndexTerm>
+ <IndexTerm><Primary>verbosity options</Primary></IndexTerm>
+
+ <variablelist>
+ <varlistentry>
+ <term><option>––help</option></term>
+ <term><option>-?</option></term>
+ <indexterm><primary><option>-?</option></primary></indexterm>
+ <indexterm><primary><option>––help</option></primary></indexterm>
+ <listitem>
+ <para>Cause GHC to spew a long usage message to standard
+ output and then exit.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-v</option></term>
+ <indexterm><primary><option>-v</option></primary></indexterm>
+ <listitem>
+ <para>The <option>-v</option> option makes GHC
+ <emphasis>verbose</emphasis>: it reports its version number
+ and shows (on stderr) exactly how it invokes each phase of
+ the compilation system. Moreover, it passes the
+ <option>-v</option> flag to most phases; each reports its
+ version number (and possibly some other information).</para>
+
+ <para>Please, oh please, use the <option>-v</option> option
+ when reporting bugs! Knowing that you ran the right bits in
+ the right order is always the first thing we want to
+ verify.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-v</option><replaceable>n</replaceable></term>
+ <indexterm><primary><option>-v</option></primary></indexterm>
+ <listitem>
+ <para>To provide more control over the compiler's verbosity,
+ the <option>-v</option> flag takes an optional numeric
+ argument. Specifying <option>-v</option> on its own is
+ equivalent to <option>-v3</option>, and the other levels
+ have the following meanings:</para>
+
+ <variablelist>
+ <varlistentry>
+ <term><option>-v0</option></term>
+ <listitem>
+ <para>Disable all non-essential messages (this is the
+ default).</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-v1</option></term>
+ <listitem>
+ <para>Minimal verbosity: print one line per
+ compilation (this is the default when
+ <option>––make</option> or
+ <option>––interactive</option> is on).</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-v2</option></term>
+ <listitem>
+ <para>Print the name of each compilation phase as it
+ is executed. (equivalent to
+ <option>-dshow-passes</option>).</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-v3</option></term>
+ <listitem>
+ <para>The same as <option>-v2</option>, except that in
+ addition the full command line (if appropriate) for
+ each compilation phase is also printed.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-v4</option></term>
+ <listitem>
+ <para>The same as <option>-v3</option> except that the
+ intermediate program representation after each
+ compilation phase is also printed (excluding
+ preprocessed and C/assembly files).</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-V</option></term>
+ <term><option>––version</option></term>
+ <indexterm><primary><option>-V</option></primary></indexterm>
+ <indexterm><primary><option>––version</option></primary></indexterm>
+ <listitem>
+ <para>Print a one-line string including GHC's version number.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>––numeric-version</option></term>
+ <indexterm><primary><option>––numeric-version</option></primary></indexterm>
+ <listitem>
+ <para>Print GHC's numeric version number only.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>––print-libdir</option></term>
+ <indexterm><primary><option>––print-libdir</option></primary></indexterm>
+ <listitem>
+ <para>Print the path to GHC's library directory. This is
+ the top of the directory tree containing GHC's libraries,
+ interfaces, and include files (usually something like
+ <literal>/usr/local/lib/ghc-5.04</literal> on Unix). This
+ is the value of
+ <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary>
+ </indexterm>in the package configuration file (see <xref
+ linkend="packages">).</para>
+ </listitem>
+ </varlistentry>
+
+ </variablelist>
+ </sect1>
+
+ <sect1 id="make-mode">
+ <title>Using <command>ghc</command> <option>––make</option></title>
+
+ <indexterm><primary><option>––make</option></primary>
+ </indexterm>
+ <indexterm><primary>separate compilation</primary>
+ </indexterm>
+
+ <para>When given the <option>––make</option> option, GHC will
+ build a multi-module Haskell program by following dependencies
+ from a single root module (usually <literal>Main</literal>). For
+ example, if your <literal>Main</literal> module is in a file
+ called <filename>Main.hs</filename>, you could compile and link
+ the program like this:</para>
+
+<screen>
+ghc ––make Main.hs
+</screen>
+
+ <para>The command line may contain any number of source file names
+ or module names; GHC will figure out all the modules in the
+ program by following the imports from these initial modules. It
+ will then attempt to compile each module which is out of date, and
+ finally if there is a <literal>Main</literal> module, the program
+ will also be linked into an executable.</para>
+
+ <para>The main advantages to using <literal>ghc ––make</literal>
+ over traditional <literal>Makefile</literal>s are:</para>
+
+ <itemizedlist>
+ <listitem>
+ <para>GHC doesn't have to be restarted for each compilation,
+ which means it can cache information between compilations.
+ Compiling a muli-module program with <literal>ghc
+ ––make</literal> can be up to twice as fast as running
+ <literal>ghc</literal> individually on each source
+ file.</para>
+ </listitem>
+ <listitem>
+ <para>You don't have to write a
+ <literal>Makefile</literal>.</para>
+ </listitem>
+ <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary>
+ </indexterm>
+ <listitem>
+ <para>GHC re-calculates the dependencies each time it is
+ invoked, so the dependencies never get out of sync with the
+ source.</para>
+ </listitem>
+ </itemizedlist>
+
+ <para>Any of the command-line options described in the rest of
+ this chapter can be used with <option>––make</option>, but note
+ that any options you give on the command line will apply to all
+ the source files compiled, so if you want any options to apply to
+ a single source file only, you'll need to use an
+ <literal>OPTIONS</literal> pragma (see <xref
+ linkend="source-file-options">).</para>
+
+ <para>If the program needs to be linked with additional objects
+ (say, some auxilliary C code), then the object files can be
+ given on the command line and GHC will include them when linking
+ the executable.</para>
+
+ <para>Note that GHC can only follow dependencies if it has the
+ source file available, so if your program includes a module for
+ which there is no source file, even if you have an object and an
+ interface file for the module, then GHC will complain. The
+ exception to this rule is for package modules, which may or may
+ not have source files.</para>
+
+ <para>The source files for the program don't all need to be in the
+ same directory; the <option>-i</option> option can be used to add
+ directories to the search path (see <xref
+ linkend="search-path">).</para>
+
+ </sect1>
+
+ <Sect1 id="options-order">
+ <title>GHC without <option>––make</option></title>
+
+ <para>Without <option>––make</option>, GHC will compile one or
+ more source files given on the command line.</para>
+
+ <para>The first phase to run is determined by each input-file
+ suffix, and the last phase is determined by a flag. If no
+ relevant flag is present, then go all the way through linking.
+ This table summarises:</para>
+
+ <informaltable>
+ <tgroup cols="4">
+ <colspec align="left">
+ <colspec align="left">
+ <colspec align="left">
+ <colspec align="left">
+
+ <thead>
+ <row>
+ <entry>Phase of the compilation system</entry>
+ <entry>Suffix saying “start here”</entry>
+ <entry>Flag saying “stop after”</entry>
+ <entry>(suffix of) output file</entry>
+ </row>
+ </thead>
+ <tbody>
+ <row>
+ <entry>literate pre-processor</entry>
+ <entry><literal>.lhs</literal></entry>
+ <entry>-</entry>
+ <entry><literal>.hs</literal></entry>
+ </row>
+
+ <row>
+ <entry>C pre-processor (opt.)
+ </entry>
+ <entry><literal>.hs</literal> (with
+ <option>-cpp</option>)</entry>
+ <entry><option>-E</option></entry>
+ <entry><literal>.hspp</literal></entry>
+ </row>
+
+ <row>
+ <entry>Haskell compiler</entry>
+ <entry><literal>.hs</literal></entry>
+ <entry><option>-C</option>, <option>-S</option></entry>
+ <entry><literal>.hc</literal>, <literal>.s</literal></entry>
+ </row>
+
+ <row>
+ <entry>C compiler (opt.)</entry>
+ <entry><literal>.hc</literal> or <literal>.c</literal></entry>
+ <entry><option>-S</option></entry>
+ <entry><literal>.s</literal></entry>
+ </row>
+
+ <row>
+ <entry>assembler</entry>
+ <entry><literal>.s</literal></entry>
+ <entry><option>-c</option></entry>
+ <entry><literal>.o</literal></entry>
+ </row>
+
+ <row>
+ <entry>linker</entry>
+ <entry><replaceable>other</replaceable></entry>
+ <entry>-</entry>
+ <entry><filename>a.out</filename></entry>
+ </row>
+ </tbody>
+ </tgroup>
+ </informaltable>
+
+ <indexterm><primary><option>-C</option></primary></indexterm>
+ <indexterm><primary><option>-E</option></primary></indexterm>
+ <indexterm><primary><option>-S</option></primary></indexterm>
+ <indexterm><primary><option>-c</option></primary></indexterm>
+
+ <para>Thus, a common invocation would be: <literal>ghc -c
+ Foo.hs</literal></para>
+
+ <para>Note: What the Haskell compiler proper produces depends on
+ whether a native-code generator<indexterm><primary>native-code
+ generator</primary></indexterm> is used (producing assembly
+ language) or not (producing C). See <xref
+ linkend="options-codegen"> for more details.</para>
+
+ <para>Note: C pre-processing is optional, the
+ <option>-ccp</option><indexterm><primary><option>-cpp</option></primary>
+ </indexterm>flag turns it on. See <xref
+ linkend="c-pre-processor"> for more details.</para>
+
+ <para>Note: The option <option>-E</option><IndexTerm><Primary>-E
+ option</Primary></IndexTerm> runs just the pre-processing passes
+ of the compiler, dumping the result in a file. Note that this
+ differs from the previous behaviour of dumping the file to
+ standard output.</para>
+ </sect1>
+
+ &separate;
+
+ <sect1 id="options-sanity">
+ <title>Warnings and sanity-checking</title>
+
+ <indexterm><primary>sanity-checking options</primary></indexterm>
+ <indexterm><primary>warnings</primary></indexterm>
+
+
+ <para>GHC has a number of options that select which types of
+ non-fatal error messages, otherwise known as warnings, can be
+ generated during compilation. By default, you get a standard set
+ of warnings which are generally likely to indicate bugs in your
+ program. These are:
+ <option>-fwarn-overlapping-patterns</option>,
+ <option>-fwarn-deprecations</option>,
+ <option>-fwarn-duplicate-exports</option>,
+ <option>-fwarn-missing-fields</option>, and
+ <option>-fwarn-missing-methods</option>. The following flags are
+ simple ways to select standard “packages” of warnings:
+ </para>
+
+ <VariableList>
+
+ <varlistentry>
+ <term><option>-W</option>:</term>
+ <listitem>
+ <IndexTerm><Primary>-W option</Primary></IndexTerm>
+ <para>Provides the standard warnings plus
+ <option>-fwarn-incomplete-patterns</option>,
+ <option>-fwarn-unused-matches</option>,
+ <option>-fwarn-unused-imports</option>,
+ <option>-fwarn-misc</option>, and
+ <option>-fwarn-unused-binds</option>.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-w</option>:</term>
+ <listitem>
+ <IndexTerm><Primary><option>-w</option></Primary></IndexTerm>
+ <para>Turns off all warnings, including the standard ones.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-Wall</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-Wall</option></primary></indexterm>
+ <para>Turns on all warning options.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-Werror</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-Werror</option></primary></indexterm>
+ <para>Makes any warning into a fatal error. Useful so that you don't
+ miss warnings when doing batch compilation. </para>
+ </listitem>
+ </varlistentry>
+
+ </variablelist>
+
+ <para>The full set of warning options is described below. To turn
+ off any warning, simply give the corresponding
+ <option>-fno-warn-...</option> option on the command line.</para>
+
+ <variablelist>
+
+ <varlistentry>
+ <term><option>-fwarn-deprecations</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-deprecations</option></primary>
+ </indexterm>
+ <indexterm><primary>deprecations</primary></indexterm>
+ <para>Causes a warning to be emitted when a deprecated
+ function or type is used. Entities can be marked as
+ deprecated using a pragma, see <xref
+ linkend="deprecated-pragma">.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fwarn-duplicate-exports</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
+ <indexterm><primary>duplicate exports, warning</primary></indexterm>
+ <indexterm><primary>export lists, duplicates</primary></indexterm>
+
+ <para>Have the compiler warn about duplicate entries in
+ export lists. This is useful information if you maintain
+ large export lists, and want to avoid the continued export
+ of a definition after you've deleted (one) mention of it in
+ the export list.</para>
+
+ <para>This option is on by default.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fwarn-hi-shadowing</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
+ <indexterm><primary>shadowing</primary>
+ <secondary>interface files</secondary></indexterm>
+
+ <para>Causes the compiler to emit a warning when a module or
+ interface file in the current directory is shadowing one
+ with the same module name in a library or other
+ directory.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fwarn-incomplete-patterns</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
+ <indexterm><primary>incomplete patterns, warning</primary></indexterm>
+ <indexterm><primary>patterns, incomplete</primary></indexterm>
+
+ <para>Similarly for incomplete patterns, the function
+ <function>g</function> below will fail when applied to
+ non-empty lists, so the compiler will emit a warning about
+ this when <option>-fwarn-incomplete-patterns</option> is
+ enabled.</para>
+
+<programlisting>
+g [] = 2
+</programlisting>
+
+ <para>This option isn't enabled be default because it can be
+ a bit noisy, and it doesn't always indicate a bug in the
+ program. However, it's generally considered good practice
+ to cover all the cases in your functions.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fwarn-misc</option>:</term>
+ <indexterm><primary><option>-fwarn-misc</option></primary></indexterm>
+ <listitem>
+ <para>Turns on warnings for various harmless but untidy
+ things. This currently includes: importing a type with
+ <literal>(..)</literal> when the export is abstract, and
+ listing duplicate class assertions in a qualified type.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fwarn-missing-fields</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
+ <indexterm><primary>missing fields, warning</primary></indexterm>
+ <indexterm><primary>fields, missing</primary></indexterm>
+
+ <para>This option is on by default, and warns you whenever
+ the construction of a labelled field constructor isn't
+ complete, missing initializers for one or more fields. While
+ not an error (the missing fields are initialised with
+ bottoms), it is often an indication of a programmer error.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fwarn-missing-methods</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
+ <indexterm><primary>missing methods, warning</primary></indexterm>
+ <indexterm><primary>methods, missing</primary></indexterm>
+
+ <para>This option is on by default, and warns you whenever
+ an instance declaration is missing one or more methods, and
+ the corresponding class declaration has no default
+ declaration for them.</para>
+ <para>The warning is suppressed if the method name
+ begins with an underscore. Here's an example where this is useful:
+ <programlisting>
+ class C a where
+ _simpleFn :: a -> String
+ complexFn :: a -> a -> String
+ complexFn x y = ... _simpleFn ...
+ </programlisting>
+ The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
+ never <literal>_simpleFn</literal>; and (b)
+ instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fwarn-missing-signatures</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
+ <indexterm><primary>type signatures, missing</primary></indexterm>
+
+ <para>If you would like GHC to check that every top-level
+ function/value has a type signature, use the
+ <option>-fwarn-missing-signatures</option> option. This
+ option is off by default.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fwarn-name-shadowing</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
+ <indexterm><primary>shadowing, warning</primary></indexterm>
+
+ <para>This option causes a warning to be emitted whenever an
+ inner-scope value has the same name as an outer-scope value,
+ i.e. the inner value shadows the outer one. This can catch
+ typographical errors that turn into hard-to-find bugs, e.g.,
+ in the inadvertent cyclic definition <literal>let x = ... x
+ ... in</literal>.</para>
+
+ <para>Consequently, this option does
+ <emphasis>will</emphasis> complain about cyclic recursive
+ definitions.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fwarn-overlapping-patterns</option>:</term>
+ <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
+ <indexterm><primary>overlapping patterns, warning</primary></indexterm>
+ <indexterm><primary>patterns, overlapping</primary></indexterm>
+ <listitem>
+ <para>By default, the compiler will warn you if a set of
+ patterns are overlapping, i.e.,</para>
+
+<programlisting>
+f :: String -> Int
+f [] = 0
+f (_:xs) = 1
+f "2" = 2
+</programlisting>
+
+ <para>where the last pattern match in <Function>f</Function>
+ won't ever be reached, as the second pattern overlaps
+ it. More often than not, redundant patterns is a programmer
+ mistake/error, so this option is enabled by default.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fwarn-simple-patterns</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
+ </indexterm>
+ <para>Causes the compiler to warn about lambda-bound
+ patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
+ Normally, these aren't treated as incomplete patterns by
+ <option>-fwarn-incomplete-patterns</option>.</para>
+ <para>``Lambda-bound patterns'' includes all places where there is a single pattern,
+ including list comprehensions and do-notation. In these cases, a pattern-match
+ failure is quite legitimate, and triggers filtering (list comprehensions) or
+ the monad <literal>fail</literal> operation (monads). For example:
+ <programlisting>
+ f :: [Maybe a] -> [a]
+ f xs = [y | Just y <- xs]
+ </programlisting>
+ Switching on <option>-fwarn-simple-patterns</option> will elicit warnings about
+ these probably-innocent cases, which is why the flag is off by default. </para>
+ <para> The <literal>deriving( Read )</literal> mechanism produces monadic code with
+ pattern matches, so you will also get misleading warnings about the compiler-generated
+ code. (This is arguably a Bad Thing, but it's awkward to fix.)</para>
+
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fwarn-type-defaults</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
+ <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
+ <para>Have the compiler warn/inform you where in your source
+ the Haskell defaulting mechanism for numeric types kicks
+ in. This is useful information when converting code from a
+ context that assumed one default into one with another,
+ e.g., the `default default' for Haskell 1.4 caused the
+ otherwise unconstrained value <Constant>1</Constant> to be
+ given the type <literal>Int</literal>, whereas Haskell 98
+ defaults it to <literal>Integer</literal>. This may lead to
+ differences in performance and behaviour, hence the
+ usefulness of being non-silent about this.</para>
+
+ <para>This warning is off by default.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fwarn-unused-binds</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
+ <indexterm><primary>unused binds, warning</primary></indexterm>
+ <indexterm><primary>binds, unused</primary></indexterm>
+ <para>Report any function definitions (and local bindings)
+ which are unused. For top-level functions, the warning is
+ only given if the binding is not exported.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fwarn-unused-imports</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
+ <indexterm><primary>unused imports, warning</primary></indexterm>
+ <indexterm><primary>imports, unused</primary></indexterm>
+
+ <para>Report any objects that are explicitly imported but
+ never used.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fwarn-unused-matches</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
+ <indexterm><primary>unused matches, warning</primary></indexterm>
+ <indexterm><primary>matches, unused</primary></indexterm>
+
+ <para>Report all unused variables which arise from pattern
+ matches, including patterns consisting of a single variable.
+ For instance <literal>f x y = []</literal> would report
+ <VarName>x</VarName> and <VarName>y</VarName> as unused. The
+ warning is suppressed if the variable name begins with an underscore, thus:
+ <programlisting>
+ f _x = True
+ </programlisting>
+ </para>
+ </listitem>
+ </varlistentry>
+
+ </VariableList>
+
+ <para>If you're feeling really paranoid, the
+ <option>-dcore-lint</option>
+ option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
+ is a good choice. It turns on heavyweight intra-pass
+ sanity-checking within GHC. (It checks GHC's sanity, not
+ yours.)</para>
+
+ </sect1>
+
+ &packages;
+
+ <sect1 id="options-optimise">
+ <title>Optimisation (code improvement)</title>
+
+ <indexterm><primary>optimisation</primary></indexterm>
+ <indexterm><primary>improvement, code</primary></indexterm>
+
+ <para>The <option>-O*</option> options specify convenient
+ “packages” of optimisation flags; the
+ <option>-f*</option> options described later on specify
+ <emphasis>individual</emphasis> optimisations to be turned on/off;
+ the <option>-m*</option> options specify
+ <emphasis>machine-specific</emphasis> optimisations to be turned
+ on/off.</para>
+
+ <sect2 id="optimise-pkgs">
+ <title><option>-O*</option>: convenient “packages” of optimisation flags.</title>
+
+ <para>There are <emphasis>many</emphasis> options that affect
+ the quality of code produced by GHC. Most people only have a
+ general goal, something like “Compile quickly” or
+ “Make my program run like greased lightning.” The
+ following “packages” of optimisations (or lack
+ thereof) should suffice.</para>
+
+ <para>Once you choose a <option>-O*</option>
+ “package,” stick with it—don't chop and
+ change. Modules' interfaces <emphasis>will</emphasis> change
+ with a shift to a new <option>-O*</option> option, and you may
+ have to recompile a large chunk of all importing modules before
+ your program can again be run safely (see <XRef
+ LinkEnd="recomp">).</para>
+
+ <variablelist>
+
+ <varlistentry>
+ <term>No <option>-O*</option>-type option specified:</term>
+ <indexterm><primary>-O* not specified</primary></indexterm>
+ <listitem>
+ <para>This is taken to mean: “Please compile
+ quickly; I'm not over-bothered about compiled-code
+ quality.” So, for example: <command>ghc -c
+ Foo.hs</command></para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-O0</option>:</term>
+ <indexterm><primary><option>-O0</option></primary></indexterm>
+ <listitem>
+ <para>Means “turn off all optimisation”,
+ reverting to the same settings as if no
+ <option>-O</option> options had been specified. Saying
+ <option>-O0</option> can be useful if
+ eg. <command>make</command> has inserted a
+ <option>-O</option> on the command line already.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-O</option> or <option>-O1</option>:</term>
+ <indexterm><primary>-O option</primary></indexterm>
+ <indexterm><primary>-O1 option</primary></indexterm>
+ <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
+ <listitem>
+ <para>Means: “Generate good-quality code without
+ taking too long about it.” Thus, for example:
+ <command>ghc -c -O Main.lhs</command></para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-O2</option>:</term>
+ <indexterm><primary>-O2 option</primary></indexterm>
+ <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
+ <listitem>
+ <para>Means: “Apply every non-dangerous
+ optimisation, even if it means significantly longer
+ compile times.”</para>
+
+ <para>The avoided “dangerous” optimisations
+ are those that can make runtime or space
+ <emphasis>worse</emphasis> if you're unlucky. They are
+ normally turned on or off individually.</para>
+
+ <para>At the moment, <option>-O2</option> is
+ <emphasis>unlikely</emphasis> to produce better code than
+ <option>-O</option>.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-Ofile <file></option>:</term>
+ <indexterm><primary>-Ofile <file> option</primary></indexterm>
+ <indexterm><primary>optimising, customised</primary></indexterm>
+ <listitem>
+ <para>(NOTE: not supported yet in GHC 5.x. Please ask if
+ you're interested in this.)</para>
+
+ <para>For those who need <emphasis>absolute</emphasis>
+ control over <emphasis>exactly</emphasis> what options are
+ used (e.g., compiler writers, sometimes :-), a list of
+ options can be put in a file and then slurped in with
+ <option>-Ofile</option>.</para>
+
+ <para>In that file, comments are of the
+ <literal>#</literal>-to-end-of-line variety; blank
+ lines and most whitespace is ignored.</para>
+
+ <para>Please ask if you are baffled and would like an
+ example of <option>-Ofile</option>!</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+
+ <para>We don't use a <option>-O*</option> flag for day-to-day
+ work. We use <option>-O</option> to get respectable speed;
+ e.g., when we want to measure something. When we want to go for
+ broke, we tend to use <option>-O -fvia-C</option> (and we go for
+ lots of coffee breaks).</para>
+
+ <para>The easiest way to see what <option>-O</option> (etc.)
+ “really mean” is to run with <option>-v</option>,
+ then stand back in amazement.</para>
+ </sect2>
+
+ <sect2 id="options-f">
+ <title><option>-f*</option>: platform-independent flags</title>
+
+ <indexterm><primary>-f* options (GHC)</primary></indexterm>
+ <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
+
+ <para>These flags turn on and off individual optimisations.
+ They are normally set via the <option>-O</option> options
+ described above, and as such, you shouldn't need to set any of
+ them explicitly (indeed, doing so could lead to unexpected
+ results). However, there are one or two that may be of
+ interest:</para>
+
+ <variablelist>
+ <varlistentry>
+ <term><option>-fexcess-precision</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
+ <para>When this option is given, intermediate floating
+ point values can have a <emphasis>greater</emphasis>
+ precision/range than the final type. Generally this is a
+ good thing, but some programs may rely on the exact
+ precision/range of
+ <literal>Float</literal>/<literal>Double</literal> values
+ and should not use this option for their compilation.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fignore-asserts</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
+ <para>Causes GHC to ignore uses of the function
+ <literal>Exception.assert</literal> in source code (in
+ other words, rewriting <literal>Exception.assert p
+ e</literal> to <literal>e</literal> (see <xref
+ linkend="sec-assertions">). This flag is turned on by
+ <option>-O</option>.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fno-strictness</option></term>
+ <indexterm><primary><option>-fno-strictness</option></primary>
+ </indexterm>
+ <listitem>
+ <para>Turns off the strictness analyser; sometimes it eats
+ too many cycles.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fno-cpr-analyse</option></term>
+ <indexterm><primary><option>-fno-cpr-analyse</option></primary>
+ </indexterm>
+ <listitem>
+ <para>Turns off the CPR (constructed product result)
+ analysis; it is somewhat experimental.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-funbox-strict-fields</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
+ <indexterm><primary>strict constructor fields</primary></indexterm>
+ <indexterm><primary>constructor fields, strict</primary></indexterm>
+
+ <para>This option causes all constructor fields which are
+ marked strict (i.e. “!”) to be unboxed or
+ unpacked if possible. For example:</para>
-<Para>
-If you are using a Haskell “system library” (e.g., the POSIX
-library), just use the <Option>-syslib posix</Option> option, and the correct code
-should be linked in.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-L<dir></Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-L<dir> option</Primary></IndexTerm>
-Where to find user-supplied libraries… Prepend the directory
-<Filename><dir></Filename> to the library directories path.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-static</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-static option</Primary></IndexTerm>
-Tell the linker to avoid shared libraries.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-no-link-chk</Option> and <Option>-link-chk</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-no-link-chk option</Primary></IndexTerm>
-<IndexTerm><Primary>-link-chk option</Primary></IndexTerm>
-<IndexTerm><Primary>consistency checking of executables</Primary></IndexTerm>
-By default, immediately after linking an executable, GHC verifies that
-the pieces that went into it were compiled with compatible flags; a
-“consistency check”.
-(This is to avoid mysterious failures caused by non-meshing of
-incompatibly-compiled programs; e.g., if one <Filename>.o</Filename> file was compiled
-for a parallel machine and the others weren't.) You may turn off this
-check with <Option>-no-link-chk</Option>. You can turn it (back) on with
-<Option>-link-chk</Option> (the default).
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-no-hs-main</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-no-hs-main option</Primary></IndexTerm>
-<IndexTerm><Primary>linking Haskell libraries with foreign code</Primary></IndexTerm>
-</Para>
+<ProgramListing>
+data T = T !Float !Float
+</ProgramListing>
-<Para>
-In the event you want to include ghc-compiled code as part of another
-(non-Haskell) program, the RTS will not be supplying its definition of
-<Function>main()</Function> at link-time, you will have to. To signal that to the
-driver script when linking, use <Option>-no-hs-main</Option>.
-</Para>
+ <para>will create a constructor <literal>T</literal>
+ containing two unboxed floats if the
+ <option>-funbox-strict-fields</option> flag is given.
+ This may not always be an optimisation: if the
+ <Function>T</Function> constructor is scrutinised and the
+ floats passed to a non-strict function for example, they
+ will have to be reboxed (this is done automatically by the
+ compiler).</para>
-<Para>
-Notice that since the command-line passed to the linker is rather
-involved, you probably want to use the ghc driver script to do the
-final link of your `mixed-language' application. This is not a
-requirement though, just try linking once with <Option>-v</Option> on to see what
-options the driver passes through to the linker.
-</Para>
-</ListItem>
-</VarListEntry>
-</VariableList>
-</Para>
+ <para>This option should only be used in conjunction with
+ <option>-O</option>, in order to expose unfoldings to the
+ compiler so the reboxing can be removed as often as
+ possible. For example:</para>
-</Sect2>
+<ProgramListing>
+f :: T -> Float
+f (T f1 f2) = f1 + f2
+</ProgramListing>
-</Sect1>
+ <para>The compiler will avoid reboxing
+ <Function>f1</Function> and <Function>f2</Function> by
+ inlining <Function>+</Function> on floats, but only when
+ <option>-O</option> is on.</para>
-<Sect1>
-<Title>Using Concurrent Haskell</Title>
+ <para>Any single-constructor data is eligible for
+ unpacking; for example</para>
-<Para>
-<IndexTerm><Primary>Concurrent Haskell—use</Primary></IndexTerm>
-</Para>
+<ProgramListing>
+data T = T !(Int,Int)
+</ProgramListing>
-<Para>
-GHC (as of version 4.00) supports Concurrent Haskell by default,
-without requiring a special option or libraries compiled in a certain
-way. To get access to the support libraries for Concurrent Haskell
-(i.e. <Literal>Concurrent</Literal> and friends), use the <Option>-syslib concurrent</Option> option.
-</Para>
+ <para>will store the two <literal>Int</literal>s directly
+ in the <Function>T</Function> constructor, by flattening
+ the pair. Multi-level unpacking is also supported:</para>
-<Para>
-Three RTS options are provided for modifying the behaviour of the
-threaded runtime system. See the descriptions of <Option>-C[<us>]</Option>, <Option>-q</Option>,
-and <Option>-t<num></Option> in <XRef LinkEnd="parallel-rts-opts">.
-</Para>
+<ProgramListing>
+data T = T !S
+data S = S !Int !Int
+</ProgramListing>
-<Para>
-Concurrent Haskell is described in more detail in <XRef LinkEnd="concurrent-and-parallel">.
-</Para>
+ <para>will store two unboxed <literal>Int#</literal>s
+ directly in the <Function>T</Function> constructor.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-funfolding-update-in-place<n></option></term>
+ <indexterm><primary><option>-funfolding-update-in-place</option></primary></indexterm>
+ <listitem>
+ <para>Switches on an experimental "optimisation".
+ Switching it on makes the compiler a little keener to
+ inline a function that returns a constructor, if the
+ context is that of a thunk.
+<ProgramListing>
+ x = plusInt a b
+</ProgramListing>
+ If we inlined plusInt we might get an opportunity to use
+ update-in-place for the thunk 'x'.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-funfolding-creation-threshold<n></option>:</term>
+ <listitem>
+ <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
+ <indexterm><primary>inlining, controlling</primary></indexterm>
+ <indexterm><primary>unfolding, controlling</primary></indexterm>
+
+ <para>(Default: 45) Governs the maximum size that GHC will
+ allow a function unfolding to be. (An unfolding has a
+ “size” that reflects the cost in terms of
+ “code bloat” of expanding that unfolding at
+ at a call site. A bigger function would be assigned a
+ bigger cost.) </para>
+
+ <para> Consequences: (a) nothing larger than this will be
+ inlined (unless it has an INLINE pragma); (b) nothing
+ larger than this will be spewed into an interface
+ file. </para>
+
+
+ <para> Increasing this figure is more likely to result in longer
+ compile times than faster code. The next option is more
+ useful:</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-funfolding-use-threshold<n></option>:</term>
+ <listitem>
+ <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
+ <indexterm><primary>inlining, controlling</primary></indexterm>
+ <indexterm><primary>unfolding, controlling</primary></indexterm>
+
+ <para>(Default: 8) This is the magic cut-off figure for
+ unfolding: below this size, a function definition will be
+ unfolded at the call-site, any bigger and it won't. The
+ size computed for a function depends on two things: the
+ actual size of the expression minus any discounts that
+ apply (see <option>-funfolding-con-discount</option>).</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+
+ </sect2>
+
+ </sect1>
+
+ &phases;
+
+ <sect1 id="sec-using-concurrent">
+<title>Using Concurrent Haskell</title>
+
+ <indexterm><primary>Concurrent Haskell—use</primary></indexterm>
+
+<para>
+GHC supports Concurrent Haskell by default, without requiring a
+special option or libraries compiled in a certain way. To get access
+to the support libraries for Concurrent Haskell, just import
+<literal>Control.Concurrent</literal> (details are in the accompanying
+library documentation).</para>
+
+<para>
+RTS options are provided for modifying the behaviour of the threaded
+runtime system. See <XRef LinkEnd="parallel-rts-opts">.
+</para>
+
+<para>
+Concurrent Haskell is described in more detail in the documentation
+for the <literal>Control.Concurrent</literal> module.
+</para>
</Sect1>
-<Sect1>
-<Title>Using Parallel Haskell</Title>
+<Sect1 id="sec-using-parallel">
+<title>Using Parallel Haskell</title>
-<Para>
-<IndexTerm><Primary>Parallel Haskell—use</Primary></IndexTerm>
-</Para>
+<para>
+<indexterm><primary>Parallel Haskell—use</primary></indexterm>
+</para>
-<Para>
+<para>
[You won't be able to execute parallel Haskell programs unless PVM3
-(Parallel Virtual Machine, version 3) is installed at your site.]
+(Parallel Virtual Machine, version 3) is installed at your site.]
</Para>
-<Para>
+<para>
To compile a Haskell program for parallel execution under PVM, use the
-<Option>-parallel</Option> option,<IndexTerm><Primary>-parallel option</Primary></IndexTerm> both when compiling
-<Emphasis>and linking</Emphasis>. You will probably want to <Literal>import Parallel</Literal>
-into your Haskell modules.
-</Para>
-
-<Para>
-To run your parallel program, once PVM is going, just invoke it “as
-normal”. The main extra RTS option is <Option>-N<n></Option>, to say how many
-PVM “processors” your program to run on. (For more details of
-all relevant RTS options, please see <XRef LinkEnd="parallel-rts-opts">.)
-</Para>
-
-<Para>
+<Option>-parallel</Option> option,<IndexTerm><Primary>-parallel
+option</Primary></IndexTerm> both when compiling <Emphasis>and
+linking</Emphasis>. You will probably want to <Literal>import
+Parallel</Literal> into your Haskell modules.
+</Para>
+
+<para>
+To run your parallel program, once PVM is going, just invoke it
+“as normal”. The main extra RTS option is
+<Option>-qp<n></Option>, to say how many PVM
+“processors” your program to run on. (For more details of
+all relevant RTS options, please see <XRef
+LinkEnd="parallel-rts-opts">.)
+</para>
+
+<para>
In truth, running Parallel Haskell programs and getting information
out of them (e.g., parallelism profiles) is a battle with the vagaries of
PVM, detailed in the following sections.
-</Para>
+</para>
-<Sect2>
+<Sect2 id="pvm-dummies">
<Title>Dummy's guide to using PVM</Title>
-<Para>
-<IndexTerm><Primary>PVM, how to use</Primary></IndexTerm>
-<IndexTerm><Primary>Parallel Haskell—PVM use</Primary></IndexTerm>
+<para>
+<indexterm><primary>PVM, how to use</primary></indexterm>
+<indexterm><primary>Parallel Haskell—PVM use</primary></indexterm>
Before you can run a parallel program under PVM, you must set the
required environment variables (PVM's idea, not ours); something like,
-probably in your <Filename>.cshrc</Filename> or equivalent:
+probably in your <filename>.cshrc</filename> or equivalent:
<ProgramListing>
setenv PVM_ROOT /wherever/you/put/it
setenv PVM_DPATH $PVM_ROOT/lib/pvmd
</ProgramListing>
+</para>
+
+<para>
+Creating and/or controlling your “parallel machine” is a purely-PVM
+business; nothing specific to Parallel Haskell. The following paragraphs
+describe how to configure your parallel machine interactively.
</Para>
<Para>
-Creating and/or controlling your “parallel machine” is a purely-PVM
-business; nothing specific to Parallel Haskell.
+If you use parallel Haskell regularly on the same machine configuration it
+is a good idea to maintain a file with all machine names and to make the
+environment variable PVM_HOST_FILE point to this file. Then you can avoid
+the interactive operations described below by just saying
</Para>
+<ProgramListing>
+pvm $PVM_HOST_FILE
+</ProgramListing>
+
<Para>
You use the <Command>pvm</Command><IndexTerm><Primary>pvm command</Primary></IndexTerm> command to start PVM on your
machine. You can then do various things to control/monitor your
“parallel machine;” the most useful being:
-</Para>
+</para>
-<Para>
+<para>
<InformalTable>
<TGroup Cols=2>
<ColSpec Align="Left">
<TBody>
-<Row>
-<Entry><KeyCombo><KeyCap>Control</KeyCap><KeyCap>D</KeyCap></KeyCombo></Entry>
-<Entry>exit <Command>pvm</Command>, leaving it running</Entry>
-</Row>
-
-<Row>
-<Entry><Command>halt</Command></Entry>
-<Entry>kill off this “parallel machine” & exit</Entry>
-</Row>
-
-<Row>
-<Entry><Command>add <host></Command></Entry>
-<Entry>add <Command><host></Command> as a processor</Entry>
-</Row>
-
-<Row>
-<Entry><Command>delete <host></Command></Entry>
-<Entry>delete <Command><host></Command></Entry>
-</Row>
-
-<Row>
-<Entry><Command>reset</Command></Entry>
-<Entry>kill what's going, but leave PVM up</Entry>
-</Row>
-
-<Row>
-<Entry><Command>conf</Command></Entry>
-<Entry>list the current configuration</Entry>
-</Row>
-
-<Row>
-<Entry><Command>ps</Command></Entry>
-<Entry>report processes' status</Entry>
-</Row>
-
-<Row>
-<Entry><Command>pstat <pid></Command></Entry>
-<Entry>status of a particular process</Entry>
-</Row>
+<row>
+<entry><KeyCombo><KeyCap>Control</KeyCap><KeyCap>D</KeyCap></KeyCombo></entry>
+<entry>exit <command>pvm</command>, leaving it running</entry>
+</row>
+
+<row>
+<entry><command>halt</command></entry>
+<entry>kill off this “parallel machine” & exit</entry>
+</row>
+
+<row>
+<entry><command>add <host></command></entry>
+<entry>add <command><host></command> as a processor</entry>
+</row>
+
+<row>
+<entry><command>delete <host></command></entry>
+<entry>delete <command><host></command></entry>
+</row>
+
+<row>
+<entry><command>reset</command></entry>
+<entry>kill what's going, but leave PVM up</entry>
+</row>
+
+<row>
+<entry><command>conf</command></entry>
+<entry>list the current configuration</entry>
+</row>
+
+<row>
+<entry><command>ps</command></entry>
+<entry>report processes' status</entry>
+</row>
+
+<row>
+<entry><command>pstat <pid></command></entry>
+<entry>status of a particular process</entry>
+</row>
</TBody>
</TGroup>
</InformalTable>
-</Para>
+</para>
-<Para>
-The PVM documentation can tell you much, much more about <Command>pvm</Command>!
-</Para>
+<para>
+The PVM documentation can tell you much, much more about <command>pvm</command>!
+</para>
-</Sect2>
+</sect2>
-<Sect2>
+<Sect2 id="par-profiles">
<Title>Parallelism profiles</Title>
-<Para>
-<IndexTerm><Primary>parallelism profiles</Primary></IndexTerm>
-<IndexTerm><Primary>profiles, parallelism</Primary></IndexTerm>
-<IndexTerm><Primary>visualisation tools</Primary></IndexTerm>
-</Para>
+<para>
+<indexterm><primary>parallelism profiles</primary></indexterm>
+<indexterm><primary>profiles, parallelism</primary></indexterm>
+<indexterm><primary>visualisation tools</primary></indexterm>
+</para>
-<Para>
+<para>
With Parallel Haskell programs, we usually don't care about the
results—only with “how parallel” it was! We want pretty pictures.
-</Para>
+</para>
<Para>
Parallelism profiles (à la <Command>hbcpp</Command>) can be generated with the
-<Option>-q</Option><IndexTerm><Primary>-q RTS option (concurrent, parallel)</Primary></IndexTerm> RTS option. The
+<Option>-qP</Option><IndexTerm><Primary>-qP RTS option (concurrent, parallel)</Primary></IndexTerm> RTS option. The
per-processor profiling info is dumped into files named
<Filename><full-path><program>.gr</Filename>. These are then munged into a PostScript picture,
which you can then display. For example, to run your program
<Para>
<Screen>
-% ./a.out +RTS -N8 -q
-% grs2gr *.???.gr > temp.gr # combine the 8 .gr files into one
-% gr2ps -O temp.gr # cvt to .ps; output in temp.ps
-% ghostview -seascape temp.ps # look at it!
+<prompt>$</prompt> ./a.out +RTS -qP -qp8
+<prompt>$</prompt> grs2gr *.???.gr > temp.gr # combine the 8 .gr files into one
+<prompt>$</prompt> gr2ps -O temp.gr # cvt to .ps; output in temp.ps
+<prompt>$</prompt> ghostview -seascape temp.ps # look at it!
</Screen>
</Para>
-<Para>
+<para>
The scripts for processing the parallelism profiles are distributed
-in <Filename>ghc/utils/parallel/</Filename>.
-</Para>
+in <filename>ghc/utils/parallel/</filename>.
+</para>
-</Sect2>
+</sect2>
<Sect2>
<Title>Other useful info about running parallel programs</Title>
<Option>+RTS -Sstderr</Option><IndexTerm><Primary>-Sstderr RTS option</Primary></IndexTerm> or <Option>+RTS -sstderr</Option>, then
you'll get mutator, garbage-collection, etc., times on standard
error. The standard error of all PE's other than the `main thread'
-appears in <Filename>/tmp/pvml.nnn</Filename>, courtesy of PVM.
-</Para>
+appears in <filename>/tmp/pvml.nnn</filename>, courtesy of PVM.
+</para>
-<Para>
-Whether doing <Option>+RTS -Sstderr</Option> or not, a handy way to watch
-what's happening overall is: <Command>tail -f /tmp/pvml.nnn</Command>.
-</Para>
+<para>
+Whether doing <option>+RTS -Sstderr</option> or not, a handy way to watch
+what's happening overall is: <command>tail -f /tmp/pvml.nnn</command>.
+</para>
-</Sect2>
+</sect2>
<Sect2 id="parallel-rts-opts">
-<Title>RTS options for Concurrent/Parallel Haskell
-</Title>
+<title>RTS options for Concurrent/Parallel Haskell
+</title>
-<Para>
-<IndexTerm><Primary>RTS options, concurrent</Primary></IndexTerm>
-<IndexTerm><Primary>RTS options, parallel</Primary></IndexTerm>
-<IndexTerm><Primary>Concurrent Haskell—RTS options</Primary></IndexTerm>
-<IndexTerm><Primary>Parallel Haskell—RTS options</Primary></IndexTerm>
-</Para>
+<para>
+<indexterm><primary>RTS options, concurrent</primary></indexterm>
+<indexterm><primary>RTS options, parallel</primary></indexterm>
+<indexterm><primary>Concurrent Haskell—RTS options</primary></indexterm>
+<indexterm><primary>Parallel Haskell—RTS options</primary></indexterm>
+</para>
-<Para>
+<para>
Besides the usual runtime system (RTS) options
(<XRef LinkEnd="runtime-control">), there are a few options particularly
for concurrent/parallel execution.
-</Para>
+</para>
-<Para>
+<para>
<VariableList>
<VarListEntry>
-<Term><Option>-N<N></Option>:</Term>
+<Term><Option>-qp<N></Option>:</Term>
<ListItem>
<Para>
-<IndexTerm><Primary>-N<N> RTS option (parallel)</Primary></IndexTerm>
+<IndexTerm><Primary>-qp<N> RTS option</Primary></IndexTerm>
(PARALLEL ONLY) Use <Literal><N></Literal> PVM processors to run this program;
the default is 2.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-C[<us>]</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-C<us> RTS option</Primary></IndexTerm>
-Sets the context switch interval to <Literal><us></Literal> microseconds. A context
-switch will occur at the next heap allocation after the timer expires.
-With <Option>-C0</Option> or <Option>-C</Option>, context switches will occur as often as
-possible (at every heap allocation). By default, context switches
-occur every 10 milliseconds. Note that many interval timers are only
-capable of 10 millisecond granularity, so the default setting may be
-the finest granularity possible, short of a context switch at every
-heap allocation.
-</Para>
-
-<Para>
-[NOTE: this option currently has no effect (version 4.00). Context
-switches happen when the current heap block is full, i.e. every 4k of
-allocation].
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term><Option>-q[v]</Option>:</Term>
-<ListItem>
-<Para>
-<IndexTerm><Primary>-q RTS option</Primary></IndexTerm>
+</para>
+</listitem>
+</varlistentry>
+<varlistentry>
+<term><option>-C[<us>]</option>:</term>
+<listitem>
+<para>
+<indexterm><primary>-C<us> RTS option</primary></indexterm> Sets
+the context switch interval to <literal><s></literal> seconds.
+A context switch will occur at the next heap block allocation after
+the timer expires (a heap block allocation occurs every 4k of
+allocation). With <option>-C0</option> or <option>-C</option>,
+context switches will occur as often as possible (at every heap block
+allocation). By default, context switches occur every 20ms
+milliseconds. Note that GHC's internal timer ticks every 20ms, and
+the context switch timer is always a multiple of this timer, so 20ms
+is the maximum granularity available for timed context switches.
+</para>
+</listitem>
+</varlistentry>
+<varlistentry>
+<term><option>-q[v]</option>:</term>
+<listitem>
+<para>
+<indexterm><primary>-q RTS option</primary></indexterm>
(PARALLEL ONLY) Produce a quasi-parallel profile of thread activity,
-in the file <FIlename><program>.qp</FIlename>. In the style of <Command>hbcpp</Command>, this profile
+in the file <FIlename><program>.qp</FIlename>. In the style of <command>hbcpp</command>, this profile
records the movement of threads between the green (runnable) and red
-(blocked) queues. If you specify the verbose suboption (<Option>-qv</Option>), the
+(blocked) queues. If you specify the verbose suboption (<option>-qv</option>), the
green queue is split into green (for the currently running thread
only) and amber (for other runnable threads). We do not recommend
that you use the verbose suboption if you are planning to use the
<Command>hbcpp</Command> profiling tools or if you are context switching at every heap
check (with <Option>-C</Option>).
+-->
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
-<Term><Option>-t<num></Option>:</Term>
+<Term><Option>-qt<num></Option>:</Term>
<ListItem>
<Para>
-<IndexTerm><Primary>-t<num> RTS option</Primary></IndexTerm>
-(PARALLEL ONLY) Limit the number of concurrent threads per processor
-to <Literal><num></Literal>. The default is 32. Each thread requires slightly over 1K
-<Emphasis>words</Emphasis> in the heap for thread state and stack objects. (For
-32-bit machines, this translates to 4K bytes, and for 64-bit machines,
-8K bytes.)
+<IndexTerm><Primary>-qt<num> RTS option</Primary></IndexTerm>
+(PARALLEL ONLY) Limit the thread pool size, i.e. the number of concurrent
+threads per processor to <Literal><num></Literal>. The default is
+32. Each thread requires slightly over 1K <Emphasis>words</Emphasis> in
+the heap for thread state and stack objects. (For 32-bit machines, this
+translates to 4K bytes, and for 64-bit machines, 8K bytes.)
</Para>
</ListItem>
</VarListEntry>
+<!-- no more -HWL
<VarListEntry>
<Term><Option>-d</Option>:</Term>
<ListItem>
</Para>
</ListItem>
</VarListEntry>
+-->
<VarListEntry>
-<Term><Option>-e<num></Option>:</Term>
+<Term><Option>-qe<num></Option>:</Term>
<ListItem>
<Para>
-<IndexTerm><Primary>-e<num> RTS option (parallel)</Primary></IndexTerm>
-(PARALLEL ONLY) Limit the number of pending sparks per processor to
-<Literal><num></Literal>. The default is 100. A larger number may be appropriate if
-your program generates large amounts of parallelism initially.
+<IndexTerm><Primary>-qe<num> RTS option
+(parallel)</Primary></IndexTerm> (PARALLEL ONLY) Limit the spark pool size
+i.e. the number of pending sparks per processor to
+<Literal><num></Literal>. The default is 100. A larger number may be
+appropriate if your program generates large amounts of parallelism
+initially.
</Para>
</ListItem>
</VarListEntry>
<VarListEntry>
-<Term><Option>-Q<num></Option>:</Term>
+<Term><Option>-qQ<num></Option>:</Term>
<ListItem>
<Para>
-<IndexTerm><Primary>-Q<num> RTS option (parallel)</Primary></IndexTerm>
+<IndexTerm><Primary>-qQ<num> RTS option (parallel)</Primary></IndexTerm>
(PARALLEL ONLY) Set the size of packets transmitted between processors
to <Literal><num></Literal>. The default is 1024 words. A larger number may be
appropriate if your machine has a high communication cost relative to
</Para>
</ListItem>
</VarListEntry>
-</VariableList>
+<VarListEntry>
+<Term><Option>-qh<num></Option>:</Term>
+<ListItem>
+<Para>
+<IndexTerm><Primary>-qh<num> RTS option (parallel)</Primary></IndexTerm>
+(PARALLEL ONLY) Select a packing scheme. Set the number of non-root thunks to pack in one packet to
+<num>-1 (0 means infinity). By default GUM uses full-subgraph
+packing, i.e. the entire subgraph with the requested closure as root is
+transmitted (provided it fits into one packet). Choosing a smaller value
+reduces the amount of pre-fetching of work done in GUM. This can be
+advantageous for improving data locality but it can also worsen the balance
+of the load in the system.
+</Para>
+</ListItem>
+</VarListEntry>
+<VarListEntry>
+<Term><Option>-qg<num></Option>:</Term>
+<ListItem>
+<Para>
+<IndexTerm><Primary>-qg<num> RTS option
+(parallel)</Primary></IndexTerm> (PARALLEL ONLY) Select a globalisation
+scheme. This option affects the
+generation of global addresses when transferring data. Global addresses are
+globally unique identifiers required to maintain sharing in the distributed
+graph structure. Currently this is a binary option. With <num>=0 full globalisation is used
+(default). This means a global address is generated for every closure that
+is transmitted. With <num>=1 a thunk-only globalisation scheme is
+used, which generated global address only for thunks. The latter case may
+lose sharing of data but has a reduced overhead in packing graph structures
+and maintaining internal tables of global addresses.
</Para>
+</ListItem>
+</VarListEntry>
+</VariableList>
+</para>
-</Sect2>
+</sect2>
</Sect1>
-&runtime
-&debug
+ <sect1 id="options-platform">
+ <title>Platform-specific Flags</title>
+
+ <indexterm><primary>-m* options</primary></indexterm>
+ <indexterm><primary>platform-specific options</primary></indexterm>
+ <indexterm><primary>machine-specific options</primary></indexterm>
+
+ <para>Some flags only make sense for particular target
+ platforms.</para>
+
+ <variablelist>
+
+ <varlistentry>
+ <term><option>-mv8</option>:</term>
+ <listitem>
+ <para>(SPARC machines)<indexterm><primary>-mv8 option (SPARC
+ only)</primary></indexterm> Means to pass the like-named
+ option to GCC; it says to use the Version 8 SPARC
+ instructions, notably integer multiply and divide. The
+ similiar <option>-m*</option> GCC options for SPARC also
+ work, actually.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-monly-[32]-regs</option>:</term>
+ <listitem>
+ <para>(iX86 machines)<indexterm><primary>-monly-N-regs
+ option (iX86 only)</primary></indexterm> GHC tries to
+ “steal” four registers from GCC, for performance
+ reasons; it almost always works. However, when GCC is
+ compiling some modules with four stolen registers, it will
+ crash, probably saying:
+
+<Screen>
+Foo.hc:533: fixed or forbidden register was spilled.
+This may be due to a compiler bug or to impossible asm
+statements or clauses.
+</Screen>
+
+ Just give some registers back with
+ <option>-monly-N-regs</option>. Try `3' first, then `2'.
+ If `2' doesn't work, please report the bug to us.</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+
+ </sect1>
+
+&runtime;
+
+<sect1 id="ext-core">
+ <title>Generating and compiling External Core Files</title>
+
+ <indexterm><primary>intermediate code generation</primary></indexterm>
+
+ <para>GHC can dump its optimized intermediate code (said to be in “Core” format)
+ to a file as a side-effect of compilation. Core files, which are given the suffix
+ <filename>.hcr</filename>, can be read and processed by non-GHC back-end
+ tools. The Core format is formally described in <ulink url="http://www.haskell.org/ghc/docs/papers/core.ps.gz"
+ <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
+ and sample tools (in Haskell)
+ for manipulating Core files are available in the GHC source distribution
+ directory <literal>/fptools/ghc/utils/ext-core</literal>.
+ Note that the format of <literal>.hcr</literal>
+ files is <emphasis>different</emphasis> (though similar) to the Core output format generated
+ for debugging purposes (<xref linkend="options-debugging">).</para>
+
+ <para>The Core format natively supports notes which you can add to
+ your source code using the <literal>CORE</literal> pragma (see <xref
+ linkend="pragmas">).</para>
+
+ <variablelist>
+
+ <varlistentry>
+ <term><option>-fext-core</option></term>
+ <indexterm>
+ <primary><option>-fext-core</option></primary>
+ </indexterm>
+ <listitem>
+ <para>Generate <literal>.hcr</literal> files.</para>
+ </listitem>
+ </varlistentry>
+
+ </variablelist>
+
+<para>GHC can also read in External Core files as source; just give the <literal>.hcr</literal> file on
+the command line, instead of the <literal>.hs</literal> or <literal>.lhs</literal> Haskell source.
+A current infelicity is that you need to give teh <literal>-fglasgow-exts</literal> flag too, because
+ordinary Haskell 98, when translated to External Core, uses things like rank-2 types.</para>
+</sect1>
+
+&debug;
+&flags;
</Chapter>
+
+<!-- Emacs stuff:
+ ;;; Local Variables: ***
+ ;;; mode: sgml ***
+ ;;; sgml-parent-document: ("users_guide.sgml" "book" "chapter") ***
+ ;;; End: ***
+ -->
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