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<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
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through the right phases (compiling, linking, etc.).
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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;
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with @-v@, 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.
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<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:
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@-fwarn-overlpapping-patterns@, @-fwarn-duplicate-exports@, and
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@-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>

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Provides the standard warnings plus @-fwarn-incomplete-patterns@,
@-fwarn-unused-imports@ and @-fwarn-unused-binds@.
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<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>

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By default, the compiler will warn you if a set of patterns are
overlapping, i.e.,
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<tscreen><verb>
f :: String -> Int
f []     = 0
f (_:xs) = 1
f "2"    = 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>

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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.

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

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.
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<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.

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<tag>@-fwarn-unused-imports@:</tag>
<nidx>-fwarn-unused-imports option</nidx>
<nidx>unused imports, warning</nidx>
<nidx>imports, unused</nidx>
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Report any objects that are explicitly imported but never used.

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

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.

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

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.
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<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.

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<tag><tt>-fwarn-type-defaults</tt>:</tag>
<nidx>-fwarn-type-defaults option</nidx>
<nidx>defaulting mechanism, warning</nidx>

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 <tt>1</tt> to be given
the type <tt>Int</tt>, whereas Haskell 98 defaults it to
<tt>Integer</tt>.  This may lead to differences in performance and
behaviour, hence the usefulness of being non-silent about this.

This warning is off by default.

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<tag>@-fwarn-missing-signatures@:</tag>
<nidx>-fwarn-missing-signatures option</nidx>
<nidx>type signatures, missing</nidx>

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.
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</descrip>
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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
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file</em> @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.
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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.

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<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.

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<tag>@-fno-implicit-prelude@</tag>
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<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.
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<tag>@-syslib <lib>@</tag>
<nidx>-syslib &lt;lib&gt; option</nidx>
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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.
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<tag>@-I<dir>@</tag>
<nidx>-I&lt;dir&gt; option</nidx>
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Once a Haskell module has been compiled to C (@.hc@ file), you may
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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.
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</descrip>

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%************************************************************************
%*                                                                      *
<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>

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See also Section <ref name="Linking and consistency-checking"
id="options-linker">, which describes how the linker finds standard
Haskell libraries.
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%************************************************************************
%*                                                                      *
<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
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@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@:
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<tscreen><verb>
depend :
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        mkdependHS -- $(HC_OPTS) -- $(SRCS)
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</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@.
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@mkdependHS@ will append the needed dependencies to your @Makefile@.
@mkdependHS@ is fully describe in Section <ref name="Makefile
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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>

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%************************************************************************
%*                                                                      *
<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
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</verb> </tscreen>
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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.

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For the example at hand, the boot interface file for A would look like
the following:
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<tscreen><verb>
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__interface A 1 where
__exports A A f;
__import PrelBase Int;
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1 newtype A = A PrelBase.Int ;
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1 f :: A -> A ;
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</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
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construction of interface files, is in the works.
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%************************************************************************
%*                                                                      *
<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
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@-fno-update-analysis@<nidx>-fno-update-analysis option</nidx> (update
analyser, because it sometimes takes a <em>long</em> time).  This one
is only enabled with -O2 anyway.
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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.

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Compiling via C will probably be slower (in compilation time) than
using GHC's native code generator.

<tag>@-funfolding-interface-threshold<n>@:</tag>
<nidx>-funfolding-interface-threshold option</nidx>
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<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
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bigger function would be assigned a bigger cost.)

<tag>@-funfolding-creation-threshold<n>@:</tag>
<nidx>-funfolding-creation-threshold option</nidx>
<nidx>inlining, controlling</nidx>
<nidx>unfolding, controlling</nidx>
(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:
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<tag>@-funfolding-use-threshold<n>@:</tag>
<nidx>-funfolding-use-threshold option</nidx>
<nidx>inlining, controlling</nidx>
<nidx>unfolding, controlling</nidx>
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(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@).

<tag>@-funfolding-con-discount<n>@:</tag>
<nidx>-funfolding-con-discount option</nidx>
<nidx>inlining, controlling</nidx>
<nidx>unfolding, controlling</nidx>
(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