GHC, using using GHC 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 make 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. structure, command-line command-line structure An invocation of GHC takes the following form: ghc [argument...] Command-line arguments are either options or file names. Command-line options begin with -. They may not be grouped: -vO is different from -v -O. Options need not precede filenames: e.g., ghc *.o -o foo. All options are processed and then applied to all files; you cannot, for example, invoke ghc -c -O1 Foo.hs -O2 Bar.hs to apply different optimisation levels to the files Foo.hs and Bar.hs. For conflicting options, e.g., -c -S, we reserve the right to do anything we want. (Usually, the last one applies.) suffixes, file file suffixes for GHC File names with “meaningful” suffixes (e.g., .lhs or .o) cause the “right thing” to happen to those files. .lhs: lhs suffix A “literate Haskell” module. .hs: A not-so-literate Haskell module. .hi: A Haskell interface file, probably compiler-generated. .hc: Intermediate C file produced by the Haskell compiler. .c: A C file not produced by the Haskell compiler. .s: An assembly-language source file, usually produced by the compiler. .o: An object file, produced by an assembler. Files with other suffixes (or without suffixes) are passed straight to the linker. help options (GHC) verbose option (GHC) A good option to start with is the -help (or -?) option. -help option -? option GHC spews a long message to standard output and then exits. The -v-v option option makes GHC verbose: it reports its version number and shows (on stderr) exactly how it invokes each phase of the compilation system. Moreover, it passes the -v flag to most phases; each reports its version number (and possibly some other information). Please, oh please, use the -v option when reporting bugs! Knowing that you ran the right bits in the right order is always the first thing we want to verify. If you're just interested in the compiler version number, the --version--version option option prints out a one-line string containing the requested info. order of passes in GHC pass ordering in GHC The basic task of the ghc driver is to run each input file through the right phases (compiling, linking, etc.). 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: Phase of the compilation system Suffix saying “start here” Flag saying “stop after” (suffix of) output file literate pre-processor .lhs - - C pre-processor (opt.) - - - Haskell compiler .hs -C, -S .hc, .s C compiler (opt.) .hc or .c -S .s assembler .s -c .o linker other - a.out -C option -S option -c option Thus, a common invocation would be: ghc -c Foo.hs Note: What the Haskell compiler proper produces depends on whether a native-code generator is used (producing assembly language) or not (producing C). The option -cpp-cpp option 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. The option -E-E option 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 -cpp, the output is the de-litted version of the original source. The option -optcpp-E-optcpp-E option runs just the pre-processing stage of the C-compiling phase, sending the result to stdout. (For debugging or obfuscation contests, usually.) output-directing options GHC's compiled output normally goes into a .hc, .o, etc., file, depending on the last-run compilation phase. The option -o foo-o option re-directs the output of that last-run phase to file foo. Note: this “feature” can be counterintuitive: ghc -C -o foo.o foo.hs will put the intermediate C code in the file foo.o, name notwithstanding! EXOTICA: But the -o option isn't of much use if you have several 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 -odir <dir>-odir <dir> option (the “Oh, dear” option). For example: % ghc -c parse/Foo.hs parse/Bar.hs gurgle/Bumble.hs -odir `arch` The output files, Foo.o, Bar.o, and Bumble.o would be put into a subdirectory named after the architecture of the executing machine (sun4, mips, etc). The directory must already exist; it won't be created. Note that the -odir option does not affect where the interface files are put. In the above example, they would still be put in parse/Foo.hi, parse/Bar.hi, and gurgle/Bumble.hi. MORE EXOTICA: The -osuf <suffix>-osuf <suffix> option will change the .o file suffix for object files to whatever you specify. (We use this in compiling the prelude.). Similarly, the -hisuf <suffix>-hisuf <suffix> option will change the .hi file suffix for non-system interface files (see ). The -hisuf/-osuf 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 .hi/.o suffixes; add -hisuf g_hi -osuf g_o to your make rule for GHC compiling… FURTHER EXOTICA: If you are doing a normal .hs-to-.o compilation but would like to hang onto the intermediate .hc C file, just throw in a -keep-hc-file-too option-keep-hc-file-too option. If you would like to look at the assembler output, toss in a -keep-s-file-too,-keep-s-file-too option too. standard error, saving Sometimes, you may cause GHC to be rather chatty on standard error; with -v, for example. You can instruct GHC to append this output to a particular log file with a -odump <blah>-odump <blah> option option. temporary files, redirecting If you have trouble because of running out of space in /tmp (or wherever your installation thinks temporary files should go), you may use the -tmpdir <dir>-tmpdir <dir> option option to specify an alternate directory. For example, -tmpdir . says to put temporary files in the current working directory. Alternatively, use your TMPDIR environment variable.TMPDIR environment variable Set it to the name of the directory where temporary files should be put. GCC and other programs will honour the TMPDIR variable as well. Even better idea: Set the TMPDIR variable when building GHC, and never worry about TMPDIR again. (see the build documentation). sanity-checking options warnings 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: -fwarn-overlpapping-patterns, -fwarn-duplicate-exports, and -fwarn-missing-methods. The following flags are simple ways to select standard “packages” of warnings: -Wnot: -Wnot option Turns off all warnings, including the standard ones. -w: -w option Synonym for -Wnot. -W: -W option Provides the standard warnings plus -fwarn-incomplete-patterns, -fwarn-unused-imports and -fwarn-unused-binds. -Wall: -Wall option Turns on all warning options. The full set of warning options is described below. To turn off any warning, simply give the corresponding -fno-warn-... option on the command line. -fwarn-name-shadowing: -fwarn-name-shadowing option shadowing, warningThis 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 let x = ... x ... in. Consequently, this option does not allow cyclic recursive definitions. -fwarn-overlapping-patterns: -fwarn-overlapping-patterns option overlapping patterns, warning patterns, overlapping By default, the compiler will warn you if a set of patterns are overlapping, i.e., f :: String -> Int f [] = 0 f (_:xs) = 1 f "2" = 2 where the last pattern match in f 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. -fwarn-incomplete-patterns: -fwarn-incomplete-patterns option incomplete patterns, warning patterns, incomplete Similarly for incomplete patterns, the function g below will fail when applied to non-empty lists, so the compiler will emit a warning about this when -fwarn-incomplete-patterns is enabled. g [] = 2 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. -fwarn-missing-methods: -fwarn-missing-methods option missing methods, warning methods, missing 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. -fwarn-missing-fields: -fwarn-missing-fields option missing fields, warning fields, missing 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. -fwarn-unused-imports: -fwarn-unused-imports option unused imports, warning imports, unused Report any objects that are explicitly imported but never used. -fwarn-unused-binds: -fwarn-unused-binds option unused binds, warning binds, unused 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. -fwarn-unused-matches: -fwarn-unused-matches option unused matches, warning matches, unused Report all unused variables which arise from pattern matches, including patterns consisting of a single variable. For instance f x y = [] would report x and y as unused. To eliminate the warning, all unused variables can be replaced with wildcards. -fwarn-duplicate-exports: -fwarn-duplicate-exports option duplicate exports, warning export lists, duplicates 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. This option is on by default. -fwarn-type-defaults: -fwarn-type-defaults option defaulting mechanism, warning 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 1 to be given the type Int, whereas Haskell 98 defaults it to Integer. This may lead to differences in performance and behaviour, hence the usefulness of being non-silent about this. This warning is off by default. -fwarn-missing-signatures: -fwarn-missing-signatures option type signatures, missing If you would like GHC to check that every top-level function/value has a type signature, use the -fwarn-missing-signatures option. This option is off by default. If you're feeling really paranoid, the -dcore-lint option-dcore-lint option is a good choice. It turns on heavyweight intra-pass sanity-checking within GHC. (It checks GHC's sanity, not yours.) separate compilation recompilation checker make and recompilation This section describes how GHC supports separate compilation. interface files .hi files When GHC compiles a source file F which contains a module A, say, it generates an object F.o, and a companion interface file A.hi. 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. NOTE: Having the name of the interface file follow the module name and not the file name, means that working with tools such as make become harder. make 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. The interface file for A contains information needed by the compiler when it compiles any module B that imports A, whether directly or indirectly. When compiling B, GHC will read A.hi to find the details that it needs to know about things defined in A. Furthermore, when compiling module C which imports B, GHC may decide that it needs to know something about A—for example, B might export a function that involves a type defined in A. In this case, GHC will go and read A.hi even though C does not explicitly import A at all. The interface file may contain all sorts of things that aren't explicitly exported from A by the programmer. For example, even though a data type is exported abstractly, A.hi will contain the full data type definition. For small function definitions, A.hi will contain the complete definition of the function. For bigger functions, A.hi will contain strictness information about the function. And so on. GHC puts much more information into .hi files when optimisation is turned on with the -O flag. Without -O it puts in just the minimum; with -O it lobs in a whole pile of stuff. optimsation, effect on .hi files A.hi should really be thought of as a compiler-readable version of A.o. If you use a .hi file that wasn't generated by the same compilation run that generates the .o file the compiler may assume all sorts of incorrect things about A, resulting in core dumps and other unpleasant happenings. interface files, finding them finding interface files In your program, you import a module Foo by saying import Foo. GHC goes looking for an interface file, Foo.hi. It has a builtin list of directories (notably including .) where it looks. -i<dirs> -i<dirs> optionThis flag prepends a colon-separated list of dirs to the “import directories” list. See also for the significance of using relative and absolute pathnames in the -i list. -i resets the “import directories” list back to nothing. -fno-implicit-prelude -fno-implicit-prelude option GHC normally imports Prelude.hi files for you. If you'd rather it didn't, then give it a -fno-implicit-prelude option. You are unlikely to get very far without a Prelude, but, hey, it's a free country. -syslib <lib> -syslib <lib> option If you are using a system-supplied non-Prelude library (e.g., the POSIX library), just use a -syslib posix option (for example). The right interface files should then be available. The accompanying HsLibs document lists the libraries available by this mechanism. -I<dir> -I<dir> option Once a Haskell module has been compiled to C (.hc file), you may wish to specify where GHC tells the C compiler to look for .h files. (Or, if you are using the -cpp option-cpp option, where it tells the C pre-processor to look…) For this purpose, use a -I option in the usual C-ish way. interface files, options The interface output may be directed to another file bar2/Wurble.iface with the option -ohi bar2/Wurble.iface-ohi <file> option (not recommended). To avoid generating an interface file at all, use a -nohi option.-nohi option The compiler does not overwrite an existing .hi interface file if the new one is byte-for-byte the same as the old one; this is friendly to make. When an interface does change, it is often enlightening to be informed. The -hi-diffs-hi-diffs option option will make GHC run diff on the old and new .hi files. You can also record the difference in the interface file itself, the -keep-hi-diffs-keep-hi-diffs option takes care of that. The .hi 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 -hi-diffs-with-usages-hi-diffs-with-usages option option. Interface files are normally jammed full of compiler-produced pragmas, 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 -fignore-interface-pragmas-fignore-interface-pragmas option option. 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 -fno-prune-tydecls and -fno-prune-instdecls. -fno-prune-tydecls option-fno-prune-instdecls option See also , which describes how the linker finds standard Haskell libraries. recompilation checker -recomp -recomp option (On by default) Turn on recompilation checking. This will stop compilation early, leaving an existing .o file in place, if it can be determined that the module does not need to be recompiled. -no-recomp -recomp option Turn off recompilation checking. In the olden days, GHC compared the newly-generated .hi 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 .hi file were not recompiled. This doesn't work any more. In our earlier example, module C does not import module A directly, yet changes to A.hi should force a recompilation of C. And some changes to A (changing the definition of a function that appears in an inlining of a function exported by B, say) may conceivably not change B.hi one jot. So now… 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 .o file's date (i.e. the source hasn't changed since the file was last compiled), and the -recomp 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! GHC only keeps detailed dependency information for “user” modules, not for “library” modules. It distinguishes the two by a hack: a module whose .hi 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 relative path names in your -i path, to force GHC to record detailed dependency information. Use absolute path names only for directories containing slowly-changing library modules. A path is considered “absolute” if it starts with “/”, or “A:/”, or “A:\” (or “B:/”, “B:\” etc). Patrick Sansom had a workshop paper about how all this is done (though the details have changed quite a bit). Ask him if you want a copy. make It is reasonably straightforward to set up a Makefile to use with GHC, assuming you name your source files the same as your modules. Thus: 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 (Sophisticated make variants may achieve some of the above more elegantly. Notably, gmake's pattern rules let you write the more comprehensible: %.o : %.lhs $(HC) -c $< $(HC_OPTS) What we've shown should work with any make.) Note the cheesy .o.hi rule: It records the dependency of the interface (.hi) file on the source. The rule says a .hi file can be made from a .o file by doing…nothing. Which is true. Note the inter-module dependencies at the end of the Makefile, which take the form Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz They tell make that if any of Foo.o, Foo.hc or Foo.s have an earlier modification date than Baz.hi, then the out-of-date file must be brought up to date. To bring it up to date, make looks for a rule to do so; one of the preceding suffix rules does the job nicely. Putting inter-dependencies of the form Foo.o : Bar.hi into your Makefile by hand is rather error-prone. Don't worry—never fear, mkdependHS is here! (and is distributed as part of GHC) Add the following to your Makefile: depend : mkdependHS -- $(HC_OPTS) -- $(SRCS) Now, before you start compiling, and any time you change the imports in your program, do make depend before you do make cool_pgm. mkdependHS will append the needed dependencies to your Makefile. mkdependHS is fully described in . A few caveats about this simple scheme: You may need to compile some modules explicitly to create their interfaces in the first place (e.g., make Bar.o to create Bar.hi). You may have to type make more than once for the dependencies to have full effect. However, a make run that does nothing does mean “everything's up-to-date.” This scheme will work with mutually-recursive modules but, again, it may take multiple iterations to “settle.” module system, recursion recursion, between modules Currently, the compiler does not have proper support for dealing with mutually recursive modules: 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 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 hi-boot files, and are placed in a file called <module>.hi-boot. To import from an hi-boot file instead of the standard .hi file, use the following syntax in the importing module: hi-boot files importing, hi-boot files import {-# SOURCE #-} A 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 everything that module A exports, only the things required by the module that imports A recursively. For the example at hand, the boot interface file for A would look like the following: __interface A 1 404 where __export A TA{MkTA} ; 1 newtype TA = MkTA PrelBase.Int ; The syntax is essentially the same as a normal .hi file (unfortunately), but you can usually tailor an existing .hi file to make a .hi-boot file. Notice that we only put the declaration for the newtype TA in the hi-boot file, not the signature for f, since f isn't used by B. 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 syntax; 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. The number “1” at the beginning of a declaration is the version number of that declaration: for the purposes of .hi-boot files these can all be set to 1. All names must be fully qualified with the original module that an object comes from: for example, the reference to Int in the interface for A comes from PrelBase, which is a module internal to GHC's prelude. It's a pain, but that's the way it is. 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: __interface A 1 404 where __export A TA; 1 data TA (You must write all the type parameters, but leave out the '=' and everything that follows it.) Note: 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. optimisation (GHC) improvement, code (GHC) The -O* options specify convenient “packages” of optimisation flags; the -f* options described later on specify individual optimisations to be turned on/off; the -m* options specify machine-specific optimisations to be turned on/off. -O options There are many 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. Once you choose a -O* “package,” stick with it—don't chop and change. Modules' interfaces will change with a shift to a new -O* option, and you may have to recompile a large chunk of all importing modules before your program can again be run safely (see ). No -O*-type option specified: -O* not specified This is taken to mean: “Please compile quickly; I'm not over-bothered about compiled-code quality.” So, for example: ghc -c Foo.hs -O or -O1: -O option -O1 option optimisenormally Means: “Generate good-quality code without taking too long about it.” Thus, for example: ghc -c -O Main.lhs -O2: -O2 option optimiseaggressively Means: “Apply every non-dangerous optimisation, even if it means significantly longer compile times.” The avoided “dangerous” optimisations are those that can make runtime or space worse if you're unlucky. They are normally turned on or off individually. At the moment, -O2 is unlikely to produce better code than -O. -O2-for-C: -O2-for-C option gcc, invoking with -O2 Says to run GCC with -O2, which may be worth a few percent in execution speed. Don't forget -fvia-C, lest you use the native-code generator and bypass GCC altogether! -Onot: -Onot option optimising, reset This option will make GHC “forget” any -Oish options it has seen so far. Sometimes useful; for example: make all EXTRA_HC_OPTS=-Onot. -Ofile <file>: -Ofile <file> option optimising, customised For those who need absolute control over exactly what options are used (e.g., compiler writers, sometimes :-), a list of options can be put in a file and then slurped in with -Ofile. In that file, comments are of the #-to-end-of-line variety; blank lines and most whitespace is ignored. Please ask if you are baffled and would like an example of -Ofile! At Glasgow, we don't use a -O* flag for day-to-day work. We use -O to get respectable speed; e.g., when we want to measure something. When we want to go for broke, we tend to use -O -fvia-C -O2-for-C (and we go for lots of coffee breaks). The easiest way to see what -O (etc.) “really mean” is to run with -v, then stand back in amazement. Alternatively, just look at the HsC_minus<blah> lists in the GHC driver script. -f* options (GHC) -fno-* options (GHC) Flags can be turned off individually. (NB: I hope you have a good reason for doing this…) To turn off the -ffoo flag, just use the -fno-foo flag.-fno-<opt> anti-option So, for example, you can say -O2 -fno-strictness, which will then drop out any running of the strictness analyser. The options you are most likely to want to turn off are: -fno-strictness-fno-strictness option (strictness analyser, because it is sometimes slow), -fno-specialise-fno-specialise option (automatic specialisation of overloaded functions, because it can make your code bigger) (US spelling also accepted), and -fno-cpr-analyse-fno-cpr-analyse option switches off the CPR (constructed product result) analyser. Should you wish to turn individual flags on, you are advised to use the -Ofile 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. Here are some “dangerous” optimisations you might want to try: -fvia-C: -fvia-C option native code generator, turning off 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 _ccall_gc_s or _casm_s, you probably have to use -fvia-C. The lower-case incantation, -fvia-c, is synonymous. Compiling via C will probably be slower (in compilation time) than using GHC's native code generator. -funfolding-interface-threshold<n>: -funfolding-interface-threshold option inlining, controlling unfolding, controlling (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.) -funfolding-creation-threshold<n>: -funfolding-creation-threshold option inlining, controlling unfolding, controlling (Default: 30) This option is similar to -funfolding-interface-threshold, 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: -funfolding-use-threshold<n>: -funfolding-use-threshold option inlining, controlling unfolding, controlling (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 -funfolding-con-discount). -funfolding-con-discount<n>: -funfolding-con-discount option inlining, controlling unfolding, controlling (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. 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. -funbox-strict-fields: -funbox-strict-fields option strict constructor fields constructor fields, strict This option causes all constructor fields which are marked strict (i.e. “!”) to be unboxed or unpacked if possible. For example: data T = T !Float !Float will create a constructor T containing two unboxed floats if the -funbox-strict-fields flag is given. This may not always be an optimisation: if the T 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). This option should only be used in conjunction with -O, in order to expose unfoldings to the compiler so the reboxing can be removed as often as possible. For example: f :: T -> Float f (T f1 f2) = f1 + f2 The compiler will avoid reboxing f1 and f2 by inlining + on floats, but only when -O is on. Any single-constructor data is eligible for unpacking; for example data T = T !(Int,Int) will store the two Ints directly in the T constructor, by flattening the pair. Multi-level unpacking is also supported: data T = T !S data S = S !Int !Int will store two unboxed Int#s directly in the T constructor. -fsemi-tagging: This option (which does not work 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.) We have not played with -fsemi-tagging enough to recommend it. (For all we know, it doesn't even work anymore… Sigh.) -m* options (GHC) platform-specific options machine-specific options Some flags only make sense for particular target platforms. -mv8: (SPARC machines)-mv8 option (SPARC only) 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 -m* GCC options for SPARC also work, actually. -mlong-calls: (HPPA machines)-mlong-calls option (HPPA only) Means to pass the like-named option to GCC. Required for Very Big modules, maybe. (Probably means you're in trouble…) -monly-[32]-regs: (iX86 machines)-monly-N-regs option (iX86 only) 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: Foo.hc:533: fixed or forbidden register was spilled. This may be due to a compiler bug or to impossible asm statements or clauses. Just give some registers back with -monly-N-regs. Try `3' first, then `2'. If `2' doesn't work, please report the bug to us. optimisation by GCC GCC optimisation The C compiler (GCC) is run with -O turned on. (It has to be, actually). If you want to run GCC with -O2—which may be worth a few percent in execution speed—you can give a -O2-for-C-O2-for-C option option. pre-processing: cpp C pre-processor options cpp, pre-processing with The C pre-processor cpp is run over your Haskell code only if the -cpp option -cpp option is given. Unless you are building a large system with significant doses of conditional compilation, you really shouldn't need it. -D<foo>: -D<name> option Define macro <foo> in the usual way. NB: does not affect -D macros passed to the C compiler when compiling via C! For those, use the -optc-Dfoo hack… (see ). -U<foo>: -U<name> option Undefine macro <foo> in the usual way. -I<dir>: -I<dir> option Specify a directory in which to look for #include files, in the usual C way. The GHC driver pre-defines several macros when processing Haskell source code (.hs or .lhs files): __HASKELL98__: __HASKELL98__ If defined, this means that GHC supports the language defined by the Haskell 98 report. __HASKELL__=98: __HASKELL__ In GHC 4.04 and later, the __HASKELL__ macro is defined as having the value 98. __HASKELL1__: __HASKELL1__ macro If defined to n, that means GHC supports the Haskell language defined in the Haskell report version 1.n. Currently 5. This macro is deprecated, and will probably disappear in future versions. __GLASGOW_HASKELL__: __GLASGOW_HASKELL__ macro For version n of the GHC system, this will be #defined to 100n. So, for version 4.00, it is 400. With any luck, __GLASGOW_HASKELL__ will be undefined in all other implementations that support C-style pre-processing. (For reference: the comparable symbols for other systems are: __HUGS__ for Hugs and __HBC__ for Chalmers.) 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. .hs, .lhs, .c and .hc files). __CONCURRENT_HASKELL__: __CONCURRENT_HASKELL__ macro 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. __PARALLEL_HASKELL__: __PARALLEL_HASKELL__ macro Only defined when -parallel is in use! This symbol is defined when pre-processing Haskell (input) and pre-processing C (GHC output). Options other than the above can be forced through to the C pre-processor with the -opt flags (see ). A small word of warning: -cpp is not friendly to “string gaps”.-cpp vs string gapsstring gaps vs -cpp. In other words, strings such as the following: strmod = "\ \ p \ \ " don't work with -cpp; /usr/bin/cpp elides the backslash-newline pairs. However, it appears that if you add a space at the end of the line, then cpp (at least GNU cpp and possibly other cpps) leaves the backslash-space pairs alone and the string gap works as expected. include-file options C compiler options GCC options 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: -ansi do ANSI C (not K&R) -pedantic be so -dgcc-lint (hack) short for “make GCC very paranoid” -ansi option (for GCC) -pedantic option (for GCC) -dgcc-lint option (GCC paranoia) If you are compiling with lots of ccalls, etc., you may need to tell the C compiler about some #include files. There is no real pretty way to do this, but you can use this hack from the command-line: % ghc -c '-#include <X/Xlib.h>' Xstuff.lhs linker options ld options GHC has to link your code with various libraries, possibly including: user-supplied, GHC-supplied, and system-supplied (-lm math library, for example). -l<FOO>: -l<lib> option Link in a library named lib<FOO>.a which resides somewhere on the library directories path. Because of the sad state of most UNIX linkers, the order of such options does matter. Thus: ghc -lbar *.o is almost certainly wrong, because it will search libbar.a before it has collected unresolved symbols from the *.o files. ghc *.o -lbar is probably better. The linker will of course be informed about some GHC-supplied libraries automatically; these are: -l equivalent description -lHSrts,-lHSclib basic runtime libraries -lHS standard Prelude library -lHS_cbits C support code for standard Prelude library -lgmp GNU multi-precision library (for Integers) -lHS library -lHS_cbits library -lHSrts library -lgmp library -syslib <name>: -syslib <name> option If you are using a Haskell “system library” (e.g., the POSIX library), just use the -syslib posix option, and the correct code should be linked in. -L<dir>: -L<dir> option Where to find user-supplied libraries… Prepend the directory <dir> to the library directories path. -static: -static option Tell the linker to avoid shared libraries. -no-link-chk and -link-chk: -no-link-chk option -link-chk option consistency checking of executables 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 .o file was compiled for a parallel machine and the others weren't.) You may turn off this check with -no-link-chk. You can turn it (back) on with -link-chk (the default). -no-hs-main: -no-hs-main option linking Haskell libraries with foreign code 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 main() at link-time, you will have to. To signal that to the driver script when linking, use -no-hs-main. 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 -v on to see what options the driver passes through to the linker. Concurrent Haskell—use 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. Concurrent and friends), use the -syslib concurrent option. Three RTS options are provided for modifying the behaviour of the threaded runtime system. See the descriptions of -C[<us>], -q, and -t<num> in . Concurrent Haskell is described in more detail in . Parallel Haskell—use [You won't be able to execute parallel Haskell programs unless PVM3 (Parallel Virtual Machine, version 3) is installed at your site.] To compile a Haskell program for parallel execution under PVM, use the -parallel option,-parallel option both when compiling and linking. You will probably want to import Parallel into your Haskell modules. To run your parallel program, once PVM is going, just invoke it “as normal”. The main extra RTS option is -N<n>, to say how many PVM “processors” your program to run on. (For more details of all relevant RTS options, please see .) 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. PVM, how to use Parallel Haskell—PVM use 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 .cshrc or equivalent: setenv PVM_ROOT /wherever/you/put/it setenv PVM_ARCH `$PVM_ROOT/lib/pvmgetarch` setenv PVM_DPATH $PVM_ROOT/lib/pvmd Creating and/or controlling your “parallel machine” is a purely-PVM business; nothing specific to Parallel Haskell. You use the pvmpvm command command to start PVM on your machine. You can then do various things to control/monitor your “parallel machine;” the most useful being: ControlD exit pvm, leaving it running halt kill off this “parallel machine” & exit add <host> add <host> as a processor delete <host> delete <host> reset kill what's going, but leave PVM up conf list the current configuration ps report processes' status pstat <pid> status of a particular process The PVM documentation can tell you much, much more about pvm! parallelism profiles profiles, parallelism visualisation tools With Parallel Haskell programs, we usually don't care about the results—only with “how parallel” it was! We want pretty pictures. Parallelism profiles (à la hbcpp) can be generated with the -q-q RTS option (concurrent, parallel) RTS option. The per-processor profiling info is dumped into files named <full-path><program>.gr. These are then munged into a PostScript picture, which you can then display. For example, to run your program a.out on 8 processors, then view the parallelism profile, do: % ./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! The scripts for processing the parallelism profiles are distributed in ghc/utils/parallel/. The “garbage-collection statistics” RTS options can be useful for seeing what parallel programs are doing. If you do either +RTS -Sstderr-Sstderr RTS option or +RTS -sstderr, 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 /tmp/pvml.nnn, courtesy of PVM. Whether doing +RTS -Sstderr or not, a handy way to watch what's happening overall is: tail -f /tmp/pvml.nnn. RTS options, concurrent RTS options, parallel Concurrent Haskell—RTS options Parallel Haskell—RTS options Besides the usual runtime system (RTS) options (), there are a few options particularly for concurrent/parallel execution. -N<N>: -N<N> RTS option (parallel) (PARALLEL ONLY) Use <N> PVM processors to run this program; the default is 2. -C[<us>]: -C<us> RTS option Sets the context switch interval to <us> microseconds. A context switch will occur at the next heap allocation after the timer expires. With -C0 or -C, 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. [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]. -q[v]: -q RTS option (PARALLEL ONLY) Produce a quasi-parallel profile of thread activity, in the file <program>.qp. In the style of hbcpp, this profile records the movement of threads between the green (runnable) and red (blocked) queues. If you specify the verbose suboption (-qv), 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 hbcpp profiling tools or if you are context switching at every heap check (with -C). -t<num>: -t<num> RTS option (PARALLEL ONLY) Limit the number of concurrent threads per processor to <num>. The default is 32. Each thread requires slightly over 1K words 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.) -d: -d RTS option (parallel) (PARALLEL ONLY) Turn on debugging. It pops up one xterm (or GDB, or something…) per PVM processor. We use the standard debugger script that comes with PVM3, but we sometimes meddle with the debugger2 script. We include ours in the GHC distribution, in ghc/utils/pvm/. -e<num>: -e<num> RTS option (parallel) (PARALLEL ONLY) Limit the number of pending sparks per processor to <num>. The default is 100. A larger number may be appropriate if your program generates large amounts of parallelism initially. -Q<num>: -Q<num> RTS option (parallel) (PARALLEL ONLY) Set the size of packets transmitted between processors to <num>. The default is 1024 words. A larger number may be appropriate if your machine has a high communication cost relative to computation speed. &runtime &debug