[project @ 1998-02-02 13:30:57 by simonm]

[ghc-hetmet.git] / ghc / docs / users_guide / using.vsgml

<sect> Using GHC
<label id="using-GHC">
<p>
<nidx>GHC, using</nidx>
<nidx>using GHC</nidx>

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.

%************************************************************************
%*                                                                      *
<sect1> Overall command-line structure
<label id="command-line-structure">
<p>
<nidx>structure, command-line</nidx>
<nidx>command-line structure</nidx>
%*                                                                      *
%************************************************************************

An invocation of GHC takes the following form:

<tscreen> <verb>
ghc [argument...]
</verb> </tscreen>

Command-line arguments are either options or file names.

Command-line options begin with @-@.  They may <em>not</em> 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.)

%************************************************************************
%*                                                                      *
<sect1>Meaningful file suffixes
<label id="file-suffixes">
<p>
<nidx>suffixes, file</nidx>
<nidx>file suffixes for GHC</nidx>
%*                                                                      *
%************************************************************************

File names with ``meaningful'' suffixes (e.g., @.lhs@ or @.o@)
cause the ``right thing'' to happen to those files.

<descrip>
<tag>@.lhs@:</tag>
<nidx>lhs suffix</nidx>
A ``literate Haskell'' module.

<tag>@.hs@:</tag> 
A not-so-literate Haskell module.

<tag>@.hi@:</tag>
A Haskell interface file, probably compiler-generated.

<tag>@.hc@:</tag>
Intermediate C file produced by the Haskell compiler.

<tag>@.c@:</tag>
A C~file not produced by the Haskell compiler.

% <tag>@.i@:</tag>
% C code after it has be preprocessed by the C compiler (using the
% @-E@ flag).

<tag>@.s@:</tag>
An assembly-language source file, usually
produced by the compiler.

<tag>@.o@:</tag>
An object file, produced by an assembler.
</descrip>

Files with other suffixes (or without suffixes) are passed straight
to the linker.

%************************************************************************
%*                                                                      *
<sect1>Help and verbosity options
<label id="options-help">
<p>
<nidx>help options (GHC)</nidx>
<nidx>verbose option (GHC)</nidx>
%*                                                                      *
%************************************************************************

A good option to start with is the @-help@ (or @-?@) option.
<nidx>-help option</nidx>
<nidx>-? option</nidx>
GHC spews a long message to standard output and then exits.

The @-v@<nidx>-v option</nidx> option makes GHC <em>verbose</em>: 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@<nidx>--version option</nidx> option prints out a
one-line string containing the requested info.

%************************************************************************
%*                                                                      *
<sect1>Running the right phases in the right order
<label id="options-order">
<p>
<nidx>order of passes in GHC</nidx>
<nidx>pass ordering in GHC</nidx>
%*                                                                      *
%************************************************************************

The basic task of the @ghc@ driver is to run each input file
through the right phases (parsing, 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:

<tabular ca="llll">
Phase of the           | Suffix saying | Flag saying   | (suffix of) @@
compilation system     | ``start here''| ``stop after''| 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 @@
</tabular>
<nidx>-C option</nidx>
<nidx>-S option</nidx>
<nidx>-c option</nidx>

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@<nidx>-cpp option</nidx> 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@<nidx>-E option</nidx> 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@<nidx>-optcpp-E option</nidx> runs just the
pre-processing stage of the C-compiling phase, sending the result to
stdout.  (For debugging or obfuscation contests, usually.)

%************************************************************************
%*                                                                      *
<sect1>Re-directing the compilation output(s)
<label id="options-output">
<p>
<nidx>output-directing options</nidx>
%*                                                                      *
%************************************************************************

GHC's compiled output normally goes into a @.hc@, @.o@, etc., file,
depending on the last-run compilation phase.  The option @-o
foo@<nidx>-o option</nidx> 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
<em>several</em> 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>@<nidx>-odir &lt;dir&gt; option</nidx> (the
``Oh, dear'' option).  For example:

<tscreen><verb>
% ghc -c parse/Foo.hs parse/Bar.hs gurgle/Bumble.hs -odir `arch`
</verb></tscreen>

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 <em>not</em> 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>@<nidx>-osuf &lt;suffix&gt;
option</nidx> will change the @.o@ file suffix for object files to
whatever you specify.  (We use this in compiling the prelude.).
Similarly, the @-hisuf <suffix>@<nidx>-hisuf &lt;suffix&gt;
option</nidx> will change the @.hi@ file suffix for non-system
interface files (see Section <ref name="Other options related to
interface files" id="hi-options">).

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<nidx>-keep-hc-file-too option</nidx>.
If you would like to look at the assembler output, toss in a
@-keep-s-file-too@,<nidx>-keep-s-file-too option</nidx> too.

<sect2> Saving GHC's standard error output
<label id="saving-ghc-stderr">
<p>
<nidx>standard error, saving</nidx>

Sometimes, you may cause GHC to be rather chatty on standard error;
with @-fshow-import-specs@, for example.  You can instruct GHC to
<em>append</em> this output to a particular log file with a @-odump
<blah>@<nidx>-odump &lt;blah&gt; option</nidx> option.

<sect2> Redirecting temporary files
<label id="temp-files">
<p>
<nidx>temporary files, redirecting</nidx>

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>@<nidx>-tmpdir &lt;dir&gt; option</nidx> 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.<nidx>TMPDIR
environment variable</nidx> 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).

%************************************************************************
%*                                                                      *
<sect1>Warnings and sanity-checking
<label id="options-sanity">
<p>
<nidx>sanity-checking options</nidx>
<nidx>warnings</nidx>
%*                                                                      *
%************************************************************************

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:

<descrip>

<tag>@-Wnot@:</tag>
<nidx>-Wnot option</nidx>

Turns off all warnings, including the standard ones.

<tag>@-w@:</tag>
<nidx>-w option</nidx>

Synonym for @-Wnot@.

<tag>@-W@:</tag>
<nidx>-W option</nidx>

Provides the standard warnings plus @-fwarn-incomplete-patterns@
and @-fwarn-unused-names@.

<tag>@-Wall@:</tag>
<nidx>-Wall option</nidx>

Turns on all warning options.

</descrip>

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.

<descrip>

<tag>@-fwarn-name-shadowing@:</tag> 
<nidx>-fwarn-name-shadowing option</nidx>
<nidx>shadowing, warning</nidx>

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
@let x = ... x ... in@.

Consequently, this option does <em>not</em> allow cyclic recursive
definitions.

<tag>@-fwarn-overlapping-patterns@:</tag>
<nidx>-fwarn-overlapping-patterns option</nidx>
<nidx>overlapping patterns, warning</nidx>
<nidx>patterns, overlapping</nidx>

By default, the compiler will warn you if a set of patterns are either
incomplete (i.e., you're only matching on a subset of an algebraic
data type's constructors), or overlapping, i.e.,

<tscreen><verb>
f :: String -> Int
f []     = 0
f (_:xs) = 1
f "2"    = 2

g [] = 2
</verb></tscreen>

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.

<tag>@-fwarn-incomplete-patterns@:</tag>
<nidx>-fwarn-incomplete-patterns option</nidx>
<nidx>incomplete patterns, warning</nidx>
<nidx>patterns, incomplete</nidx>

Similarly for incomplete patterns, the function @g@ will fail when
applied to non-empty lists, so the compiler will emit a warning about
this when this option is enabled.

<tag>@-fwarn-missing-methods@:</tag>
<nidx>-fwarn-missing-methods option</nidx>
<nidx>missing methods, warning</nidx>
<nidx>methods, missing</nidx>

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.

<tag>@-fwarn-unused-names@:</tag>
<nidx>-fwarn-unused-names option</nidx>
<nidx>unused names, warning</nidx>
<nidx>names, unused</nidx>

Have the renamer report which locally defined names are not used/exported.

<tag>@-fwarn-duplicate-exports@:</tag>
<nidx>-fwarn-duplicate-exports option</nidx>
<nidx>duplicate exports, warning</nidx>
<nidx>export lists, duplicates</nidx>

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.

</descrip>

If you would like GHC to check that every top-level value has a type
signature, use the @-fsignatures-required@
option.<nidx>-fsignatures-required option</nidx>

If you're feeling really paranoid, the @-dcore-lint@
option<nidx>-dcore-lint option</nidx> is a good choice.  It turns on
heavyweight intra-pass sanity-checking within GHC.  (It checks GHC's
sanity, not yours.)

%************************************************************************
%*                                                                      *
<sect1>Separate compilation
<label id="separate-compilation">
<p>
<nidx>separate compilation</nidx>
<nidx>recompilation checker</nidx>
<nidx>make and recompilation</nidx>
%*                                                                      *
%************************************************************************

This section describes how GHC supports separate compilation.

<sect2>Interface files
<label id="hi-files">
<p>
<nidx>interface files</nidx>
<nidx>.hi files</nidx>

When GHC compiles a source file @F@ which contains a module @A@, say,
it generates an object @F.o@, <em>and</em> a companion <em>interface
file</em> @A.hi@.  

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(1)@
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.
<nidx>optimsation, effect on .hi files</nidx>

@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.

%************************************************************************
%*                                                                      *
<sect2>Finding interface files
<label id="options-finding-imports">
<p>
<nidx>interface files, finding them</nidx>
<nidx>finding interface files</nidx>
%*                                                                      *
%************************************************************************

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.

<descrip>

<tag>@-i<dirs>@</tag><nidx>-i&lt;dirs&gt; option</nidx> This flag
prepends a colon-separated list of @dirs@ to the ``import
directories'' list.

<tag>@-i@</tag> resets the ``import directories'' list back to nothing.

<tag>@-fno-implicit-prelude@
<nidx>-fno-implicit-prelude option</nidx>
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.

<tag>@-syslib <lib>@</tag>
<nidx>-syslib &lt;lib&gt; option</nidx>

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.  Section <ref
name="The GHC Prelude and Libraries" id="ghc-prelude"> lists the
libraries available by this mechanism.

<tag>@-I<dir>@</tag>
<nidx>-I&lt;dir&gt; option</nidx>

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<nidx>-cpp option</nidx>, where
it tells the C pre-processor to look...)  For this purpose, use a @-I@
option in the usual C-ish way.

%************************************************************************
%*                                                                      *
<sect2>Other options related to interface files
<label id="hi-options">
<p>
<nidx>interface files, options</nidx>
%*                                                                      *
%************************************************************************

The interface output may be directed to another file
@bar2/Wurble.iface@ with the option @-ohi bar2/Wurble.iface@<nidx>-ohi
&lt;file&gt; option</nidx> (not recommended).

To avoid generating an interface file at all, use a @-nohi@
option.<nidx>-nohi option</nidx>

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@<nidx>-hi-diffs option</nidx> 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@<nidx>-keep-hi-diffs</nidx> 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@<nidx>-hi-diffs-with-usages option</nidx>
option.

Interface files are normally jammed full of compiler-produced
<em>pragmas</em>, 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@<nidx>-fignore-interface-pragmas
option</nidx> 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@.
<nidx>-fno-prune-tydecls option</nidx><nidx>-fno-prune-instdecls
option</nidx>

See also Section <ref name="Linking and consistency-checking"
id="options-linker">, which describes how the linker finds standard
Haskell libraries.

%************************************************************************
%*                                                                      *
<sect2>The recompilation checker
<label id="recomp">
<p>
<nidx>recompilation checker</nidx>
%*                                                                      *
%************************************************************************

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 you give GHC the @-recomp@<nidx>-recomp
option</nidx> flag, then 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!

It's still an experimental feature (that's why @-recomp@ is off by
default), so tell us if you think it doesn't work.

Patrick Sansom has a workshop paper about how all this is done.  Ask
him (email: <htmlurl name="sansom@@dcs.gla.ac.uk"
url="mailto:sansom@@dcs.gla.ac.uk">) if you want a copy.

%************************************************************************
%*                                                                      *
<sect2>Using @make@
<label id="using-make">
<p>
<ncdx>make</ncdx>
%*                                                                      *
%************************************************************************

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:

<tscreen><verb>
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
</verb></tscreen>

(Sophisticated @make@ variants may achieve some of the above more
elegantly.  Notably, @gmake@'s pattern rules let you write the more
comprehensible:

<tscreen><verb>
%.o : %.lhs
        $(HC) -c $< $(HC_OPTS)
</verb></tscreen>

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

<tscreen><verb>
Foo.o Foo.hc Foo.s    : Baz.hi          # Foo imports Baz
</verb></tscreen>

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@:

<tscreen><verb>
depend :
        mkdependHS -- $(HC_OPTS) -- $(SRCS)
</verb></tscreen>

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 describe in Section <ref name="Makefile
dependencies in Haskell: using mkdependHS" id="mkdependHS">.

A few caveats about this simple scheme:

<itemize>

<item> 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@).

<item> You may have to type @make@ more than once for the dependencies
to have full effect.  However, a @make@ run that does nothing
<em>does</em> mean ``everything's up-to-date.''

<item> This scheme will work with mutually-recursive modules but,
again, it may take multiple iterations to ``settle.''

</itemize>

%************************************************************************
%*                                                                      *
<sect2>How to compile mutually recursive modules
<label id="mutual-recursion">
<p>
<nidx>module system, recursion</nidx>
<nidx>recursion, between modules</nidx>
%*                                                                      *
%************************************************************************

Currently, the compiler does not have proper support for dealing with
mutually recursive modules:

<tscreen><verb>
module A where

import B

newtype A = A Int

f :: B -> A
f (B x) = A x
--------
module B where

import A

data B = B !Int

g :: A -> B
g (A x) = B x
</verb></tscreen>

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:
<nidx>hi-boot files</nidx>
<nidx>importing, hi-boot files</nidx>

<tscreen> <verb>
import {-# SOURCE #-} A
</verb> <tscreen>

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 <em/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 like the
following:

<tscreen><verb>
_interface_ A 1
_exports_
A(A);
_declarations_
1 newtype A = A PrelBase.Int ;
</verb></tscreen>

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 @A@ in the
@hi-boot@ file, not the signature for @f@, since @f@ isn't used by
@B@.

The number ``1'' at the beginning of a declaration is the <em>version
number</em> 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
<em/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.

<bf>Note:</bf> This is all a temporary solution, a version of the
compiler that handles mutually recursive properly without the manual
construction of interface file, is in the works.

%************************************************************************
%*                                                                      *
<sect1>Optimisation (code improvement)
<label id="options-optimise">
<p>
<nidx>optimisation (GHC)</nidx>
<nidx>improvement, code (GHC)</nidx>
%*                                                                      *
%************************************************************************

The @-O*@ options specify convenient ``packages'' of optimisation
flags; the @-f*@ options described later on specify
<em>individual</em> optimisations to be turned on/off; the @-m*@
options specify <em>machine-specific</em> optimisations to be turned
on/off.

%----------------------------------------------------------------------
<sect2>@-O*@: convenient ``packages'' of optimisation flags.
<label id="optimise-pkgs">
<p>
<nidx>-O options</nidx>

There are <em>many</em> 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 <em>will</em> 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 Section <ref name="The recompilation checker" id="recomp">).

<descrip>
<tag>No @-O*@-type option specified:</tag>
<nidx>-O* not specified</nidx>
This is taken to mean: ``Please compile quickly; I'm not over-bothered
about compiled-code quality.''  So, for example: @ghc -c Foo.hs@

<tag>@-O@ or @-O1@:</tag>
<nidx>-O option</nidx>
<nidx>-O1 option</nidx>
<nidx>optimise normally</nidx>
Means: ``Generate good-quality code without taking too long about it.''
Thus, for example: @ghc -c -O Main.lhs@

<tag>@-O2@:</tag>
<nidx>-O2 option</nidx>
<nidx>optimise aggressively</nidx>
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 <em>worse</em> if you're unlucky.  They are
normally turned on or off individually.

At the moment, @-O2@ is <em>unlikely</em> to produce
better code than @-O@.

<tag>@-O2-for-C@:</tag>
<nidx>-O2-for-C option</nidx>
<nidx>gcc, invoking with -O2</nidx>

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!

<tag>@-Onot@:</tag>
<nidx>-Onot option</nidx>
<nidx>optimising, reset</nidx>

This option will make GHC ``forget'' any -Oish options it has seen so
far.  Sometimes useful; for example: @make all EXTRA_HC_OPTS=-Onot@.

<tag>@-Ofile <file>@:</tag>
<nidx>-Ofile &lt;file&gt; option</nidx>
<nidx>optimising, customised</nidx>

For those who need <em>absolute</em> control over <em>exactly</em>
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@!
</descrip>

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.

%----------------------------------------------------------------------
<sect2>@-f*@: platform-independent flags
<p>
<nidx>-f* options (GHC)</nidx>
<nidx>-fno-* options (GHC)</nidx>

Flags can be turned <em>off</em> individually.  (NB: I hope you have a
good reason for doing this....) To turn off the @-ffoo@ flag, just use
the @-fno-foo@ flag.<nidx>-fno-&lt;opt&gt; anti-option</nidx> 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@<nidx>-fno-strictness option</nidx> (strictness
analyser [because it is sometimes slow]),
@-fno-specialise@<nidx>-fno-specialise option</nidx> (automatic
specialisation of overloaded functions [because it makes your code
bigger]) [US spelling also accepted], and
@-fno-update-analyser@<nidx>-fno-update-analyser option</nidx>
(update analyser, because it sometimes takes a <em>long</em> time).

Should you wish to turn individual flags <em>on</em>, 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 <em>might</em> want to try:
<descrip>
%------------------------------------------------------------------
<tag>@-fvia-C@:</tag>
<nidx>-fvia-C option</nidx>
<nidx>native code generator, turning off</nidx>

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 <em>have to</em> use @-fvia-C@.

The lower-case incantation, @-fvia-c@, is synonymous.

<tag>@-funfolding-creation-threshold<n>@:</tag>
<nidx>-funfolding-creation-threshold option</nidx>
<nidx>inlining, controlling</nidx>
<nidx>unfolding, controlling</nidx>
(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 Core expression would be assigned a bigger cost.)

<tag>@-funfolding-use-threshold<n>@:</tag>
<nidx>-funfolding-use-threshold option</nidx>
<nidx>inlining, controlling</nidx>
<nidx>unfolding, controlling</nidx>
(Default: 3) By raising or lowering this number, you can make the
compiler more or less keen to expand unfoldings.

OK, folks, these magic numbers `30' and `3' are mildly arbitrary; they
are of the ``seem to be OK'' variety.  The `3' is the more critical
one; it's what determines how eager GHC is about expanding unfoldings.

% <tag>@-funfolding-override-threshold<n>@:</tag>
% (Default: 8) [Pretty obscure]
W hen deciding what unfoldings from a module should be made available
% to the rest of the world (via this module's interface), the compiler
% normally likes ``small'' expressions.

% For example, if it sees @foo = bar@, it will decide that the very
% small expression @bar@ is a great unfolding for @foo@.  But if
% @bar@ turns out to be @(True,False,True)@, we would probably
% prefer <em>that</em> for the unfolding for @foo@.

% Should we ``override'' the initial small unfolding from @foo=bar@
% with the bigger-but-better one?  Yes, if the bigger one's ``size'' is
% still under the ``override threshold.''  You can use this flag to
% adjust this threshold (why, I'm not sure).

% <tag>@-fliberated-case-threshold<n>@:</tag>
% (Default: 12) [Vastly obscure: NOT IMPLEMENTED YET]
% ``Case liberation'' lifts evaluation out of recursive functions; it
% does this by duplicating code.  Done without constraint, you can get
% serious code bloat; so we only do it if the ``size'' of the duplicated
% code is smaller than some ``threshold.''  This flag can fiddle that
% threshold.

<tag>@-fsemi-tagging@:</tag>
This option (which <em>does not work</em> 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.)
</descrip>

%----------------------------------------------------------------------
<sect2>@-m*@: platform-specific flags
<p>
<nidx>-m* options (GHC)</nidx>
<nidx>platform-specific options</nidx>
<nidx>machine-specific options</nidx>

Some flags only make sense for particular target platforms.

<descrip>
<tag>@-mv8@:</tag>
(SPARC machines)<nidx>-mv8 option (SPARC only)</nidx>
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.

<tag>@-mlong-calls@:</tag>
(HPPA machines)<nidx>-mlong-calls option (HPPA only)</nidx>
Means to pass the like-named option to GCC.  Required for Very Big
modules, maybe.  (Probably means you're in trouble...)

<tag>@-monly-[32]-regs@:</tag>
(iX86 machines)<nidx>-monly-N-regs option (iX86 only)</nidx>
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:
<tscreen><verb>
Foo.hc:533: fixed or forbidden register was spilled.
This may be due to a compiler bug or to impossible asm
statements or clauses.
</verb></tscreen>
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.
</descrip>

%----------------------------------------------------------------------
<sect2>Code improvement by the C compiler.
<label id="optimise-C-compiler">
<p>
<nidx>optimisation by GCC</nidx>
<nidx>GCC optimisation</nidx>

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@<nidx>-O2-for-C option</nidx> option.

%************************************************************************
%*                                                                      *
<sect1>Options related to a particular phase
<label id="options-phases">
<p>
%*                                                                      *
%************************************************************************

<sect2> The C pre-processor
<label id="c-pre-processor">
<p>
<nidx>pre-processing: cpp</nidx>
<nidx>C pre-processor options</nidx>
<nidx>cpp, pre-processing with</nidx>

The C pre-processor @cpp@ is run over your Haskell code only if the
@-cpp@ option <nidx>-cpp option</nidx> is given.  Unless you are
building a large system with significant doses of conditional
compilation, you really shouldn't need it.
<descrip>
<tag>@-D<foo>@:</tag>
<nidx>-D&lt;name&gt; option</nidx>
Define macro @<foo>@ in the usual way.  NB: does <em>not</em> affect
@-D@ macros passed to the C~compiler when compiling via C!  For
those, use the @-optc-Dfoo@ hack...

<tag>@-U<foo>@:</tag>
<nidx>-U&lt;name&gt; option</nidx>
Undefine macro @<foo>@ in the usual way.

<tag>@-I<dir>@:</tag>
<nidx>-I&lt;dir&gt; option</nidx>
Specify a directory in which to look for @#include@ files, in
the usual C way.
</descrip>

The @ghc@ driver pre-defines several macros:
<descrip>
<tag>@__HASKELL1__@:</tag>
<nidx>__HASKELL1__ macro</nidx>
If defined to $n$, that means GHC supports the
Haskell language defined in the Haskell report version $1.n$.
Currently 4.

NB: This macro is set both when pre-processing Haskell source and
when pre-processing generated C (@.hc@) files.

<tag>@__GLASGOW_HASKELL__@:</tag>
<nidx>__GLASGOW_HASKELL__ macro</nidx>
For version $n$ of the GHC system, this will be @#define@d to
$100 \times n$.  So, for version~3.00, it is 300.

This macro is <em>only</em> set when pre-processing Haskell source.
(<em>Not</em> when pre-processing generated C.)

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.)

<tag>@__CONCURRENT_HASKELL__@:</tag>
<nidx>__CONCURRENT_HASKELL__ macro</nidx>
Only defined when @-concurrent@ is in use!
This symbol is defined when pre-processing Haskell (input) and
pre-processing C (GHC output).

<tag>@__PARALLEL_HASKELL__@:</tag>
<nidx>__PARALLEL_HASKELL__ macro</nidx>
Only defined when @-parallel@ is in use!  This symbol is defined when
pre-processing Haskell (input) and pre-processing C (GHC output).
</descrip>

Options other than the above can be forced through to the C
pre-processor with the @-opt@ flags (see
Section <ref name="Forcing options to a particular phase." id="forcing-options-through">).

A small word of warning: @-cpp@ is not friendly to ``string
gaps''.<nidx>-cpp vs string gaps</nidx><nidx>string gaps vs -cpp</nidx>.  In
other words, strings such as the following:

<tscreen><verb>
        strmod = "\
        \ p \
        \ "
</verb></tscreen>

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 @cpp@s)
leaves the backslash-space pairs alone and the string gap works as
expected.

%************************************************************************
%*                                                                      *
<sect2>Options affecting the C compiler (if applicable)
<label id="options-C-compiler">
<p>
<nidx>include-file options</nidx>
<nidx>C compiler options</nidx>
<nidx>GCC options</nidx>
%*                                                                      *
%************************************************************************

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:

<tabular ca="ll">
@-ansi@      | do ANSI C (not K&amp;R) @@
@-pedantic@  | be so@@
@-dgcc-lint@ | (hack) short for ``make GCC very paranoid''@@
</tabular>
<nidx>-ansi option (for GCC)</nidx>
<nidx>-pedantic option (for GCC)</nidx>
<nidx>-dgcc-lint option (GCC paranoia)</nidx>

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:

<tscreen><verb>
% ghc -c '-#include <X/Xlib.h>' Xstuff.lhs
</verb></tscreen>


%************************************************************************
%*                                                                      *
<sect2>Linking and consistency-checking
<label id="options-linker">
<p>
<nidx>linker options</nidx>
<nidx>ld options</nidx>
%*                                                                      *
%************************************************************************

GHC has to link your code with various libraries, possibly including:
user-supplied, GHC-supplied, and system-supplied (@-lm@ math
library, for example).

<descrip>
<tag>@-l<FOO>@:</tag>
<nidx>-l&lt;lib&gt; option</nidx>
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@ <em>before</em> 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:

<tabular ca="ll">
<bf>-l equivalent</bf> | <bf>description</bf> @@
@@
@-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)@@
</tabular>

<nidx>-lHS library</nidx>
<nidx>-lHS_cbits library</nidx>
<nidx>-lHSrts library</nidx>
<nidx>-lgmp library</nidx>

<tag>@-syslib <name>@:</tag>
<nidx>-syslib &lt;name&gt; option</nidx>

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.

<tag>@-L<dir>@:</tag>
<nidx>-L&lt;dir&gt; option</nidx>
Where to find user-supplied libraries...  Prepend the directory
@<dir>@ to the library directories path.

<tag>@-static@:</tag>
<nidx>-static option</nidx>
Tell the linker to avoid shared libraries.

<tag>@-no-link-chk@ and @-link-chk@:</tag>
<nidx>-no-link-chk option</nidx>
<nidx>-link-chk option</nidx>
<nidx>consistency checking of executables</nidx>
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).
</descrip>

%************************************************************************
%*                                                                      *
<sect1>Using Concurrent Haskell
<p>
<nidx>Concurrent Haskell---use</nidx>
%*                                                                      *
%************************************************************************

To compile a program as Concurrent Haskell, use the @-concurrent@
option,<nidx>-concurrent option</nidx> both when compiling <em>and
linking</em>.  You will probably need the @-fglasgow-exts@ option, too.

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 Section <ref name="RTS options for Concurrent/Parallel Haskell" id="parallel-rts-opts">.

%************************************************************************
%*                                                                      *
<sect2>Potential problems with Concurrent Haskell
<label id="concurrent-problems">
<p>
<nidx>Concurrent Haskell problems</nidx>
<nidx>problems, Concurrent Haskell</nidx>
%*                                                                      *
%************************************************************************

The main thread in a Concurrent Haskell program is given its own
private stack space, but all other threads are given stack space from
the heap.  Stack space for the main thread can be
adjusted as usual with the @-K@ RTS
option,<nidx>-K RTS option (concurrent, parallel)</nidx> but if this
private stack space is exhausted, the main thread will switch to stack
segments in the heap, just like any other thread.  Thus, problems
which would normally result in stack overflow in ``sequential Haskell''
can be expected to result in heap overflow when using threads.

The concurrent runtime system uses black holes as synchronisation
points for subexpressions which are shared among multiple threads.  In
``sequential Haskell'', a black hole indicates a cyclic data
dependency, which is a fatal error.  However, in concurrent execution, a
black hole may simply indicate that the desired expression is being
evaluated by another thread.  Therefore, when a thread encounters a
black hole, it simply blocks and waits for the black hole to be
updated.  Cyclic data dependencies will result in deadlock, and the
program will fail to terminate.

Because the concurrent runtime system uses black holes as
synchronisation points, it is not possible to disable black-holing
with the @-N@ RTS option.<nidx>-N RTS option</nidx> Therefore, the use
of signal handlers (including timeouts) with the concurrent runtime
system can lead to problems if a thread attempts to enter a black hole
that was created by an abandoned computation.  The use of signal
handlers in conjunction with threads is strongly discouraged.


%************************************************************************
%*                                                                      *
<sect1>Using Parallel Haskell
<p>
<nidx>Parallel Haskell---use</nidx>
%*                                                                      *
%************************************************************************

[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,<nidx>-parallel option</nidx> both when compiling
<em>and linking</em>.  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 Section <ref name="RTS options for Concurrent/Parallel Haskell" id="parallel-rts-opts">.)

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.

%************************************************************************
%*                                                                      *
<sect2>Dummy's guide to using PVM
<p>
<nidx>PVM, how to use</nidx>
<nidx>Parallel Haskell---PVM use</nidx>
%*                                                                      *
%************************************************************************

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:
<tscreen><verb>
setenv PVM_ROOT /wherever/you/put/it
setenv PVM_ARCH `$PVM_ROOT/lib/pvmgetarch`
setenv PVM_DPATH $PVM_ROOT/lib/pvmd
</verb></tscreen>

Creating and/or controlling your ``parallel machine'' is a purely-PVM
business; nothing specific to Parallel Haskell.

You use the @pvm@<nidx>pvm command</nidx> command to start PVM on your
machine.  You can then do various things to control/monitor your
``parallel machine;'' the most useful being:

\begin{tabular}{ll}
@Control-D@ & 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 \\
\end{tabular}

The PVM documentation can tell you much, much more about @pvm@!

%************************************************************************
%*                                                                      *
<sect2>Parallelism profiles
<p>
<nidx>parallelism profiles</nidx>
<nidx>profiles, parallelism</nidx>
<nidx>visualisation tools</nidx>
%*                                                                      *
%************************************************************************

With Parallel Haskell programs, we usually don't care about the
results---only with ``how parallel'' it was!  We want pretty pictures.

Parallelism profiles (\`a la @hbcpp@) can be generated with the
@-q@<nidx>-q RTS option (concurrent, parallel)</nidx> 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:

<tscreen><verb>
% ./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!
</verb></tscreen>

The scripts for processing the parallelism profiles are distributed
in @ghc/utils/parallel/@.

%************************************************************************
%*                                                                      *
<sect2>Other useful info about running parallel programs
<p>
%*                                                                      *
%************************************************************************

The ``garbage-collection statistics'' RTS options can be useful for
seeing what parallel programs are doing.  If you do either
@+RTS -Sstderr@<nidx>-Sstderr RTS option</nidx> 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@.

%************************************************************************
%*                                                                      *
<sect2>RTS options for Concurrent/Parallel Haskell
<label id="parallel-rts-opts">
<p>
<nidx>RTS options, concurrent</nidx>
<nidx>RTS options, parallel</nidx>
<nidx>Concurrent Haskell---RTS options</nidx>
<nidx>Parallel Haskell---RTS options</nidx>
%*                                                                      *
%************************************************************************

Besides the usual runtime system (RTS) options
(Section <ref name="Running a compiled program" id="runtime-control">), there are a few options particularly
for concurrent/parallel execution.

<descrip>
<tag>@-N<N>@:</tag>
<nidx>-N&lt;N&gt; RTS option (parallel)</nidx>
(PARALLEL ONLY) Use @<N>@ PVM processors to run this program;
the default is 2.

<tag>@-C[<us>]@:</tag>
<nidx>-C&lt;us&gt; RTS option</nidx>
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.

<tag>@-q[v]@:</tag>
<nidx>-q RTS option</nidx>
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@).

<tag>@-t<num>@:</tag>
<nidx>-t&lt;num&gt; RTS option</nidx>
Limit the number of concurrent threads per processor to @<num>@.
The default is 32.  Each thread requires slightly over 1K <em>words</em>
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.)

<tag>@-d@:</tag>
<nidx>-d RTS option (parallel)</nidx>
(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/@.

<tag>@-e<num>@:</tag>
<nidx>-e&lt;num&gt; RTS option (parallel)</nidx>
(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.

<tag>@-Q<num>@:</tag>
<nidx>-Q&lt;num&gt; RTS option (parallel)</nidx>
(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.
</descrip>

%************************************************************************
%*                                                                      *
<sect2>Potential problems with Parallel Haskell
<label id="parallel-problems">
<p>
<nidx>Parallel Haskell---problems</nidx> 
<nidx>problems, Parallel Haskell</nidx> 
%*                                                                      *
%************************************************************************

The ``Potential problems'' for Concurrent Haskell also apply for
Parallel Haskell.  Please see Section <ref name="Potential problems with Concurrent Haskell" id="concurrent-problems">.