[project @ 1997-06-05 23:38:01 by sof]

[ghc-hetmet.git] / ghc / docs / users_guide / how_to_run.lit

\section[invoking-GHC]{Invoking GHC: Command-line options}
\index{command-line options}
\index{options, GHC command-line}

Command-line arguments are either options or file names.

Command-line options begin with \tr{-}.  They may {\em not} be
grouped: \tr{-vO} is different from \tr{-v -O}.  Options need not
precede filenames: e.g., \tr{ghc *.o -o foo}.  All options are
processed and then applied to all files; you cannot, for example, invoke
\tr{ghc -c -O1 Foo.hs -O2 Bar.hs} to apply different optimisation
levels to the files \tr{Foo.hs} and \tr{Bar.hs}.  For conflicting
options, e.g., \tr{-c -S}, we reserve the right to do anything we
want.  (Usually, the last one applies.)

Options related to profiling, Glasgow extensions to Haskell (e.g.,
unboxed values), Concurrent and Parallel Haskell are described in
\sectionref{profiling}, \sectionref{glasgow-exts}, and
\sectionref{concurrent-and-parallel}, respectively.

%************************************************************************
%*                                                                      *
\subsection[file-suffixes]{Meaningful file suffixes}
\index{suffixes, file}
\index{file suffixes for GHC}
%*                                                                      *
%************************************************************************

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

\begin{description}
\item[\tr{.lhs}:]
\index{lhs suffix@.lhs suffix}
A ``literate Haskell'' module.

\item[\tr{.hs}:] 
A not-so-literate Haskell module.

\item[\tr{.hi}:]
A Haskell interface file, probably compiler-generated.

\item[\tr{.hc}:]
Intermediate C file produced by the Haskell compiler.

\item[\tr{.c}:]
A C~file not produced by the Haskell compiler.

% \item[\tr{.i}:]
% C code after it has be preprocessed by the C compiler (using the
% \tr{-E} flag).

\item[\tr{.s}:]
An assembly-language source file, usually
produced by the compiler.

\item[\tr{.o}:]
An object file, produced by an assembler.
\end{description}

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

%************************************************************************
%*                                                                      *
\subsection[options-help]{Help and verbosity options}
\index{help options (GHC)}
\index{verbose option (GHC)}
%*                                                                      *
%************************************************************************

A good option to start with is the \tr{-help} (or \tr{-?}) option.
\index{-help option}
\index{-? option}
GHC spews a long message to standard output and then exits.

The \tr{-v}\index{-v option} option makes GHC {\em verbose}: it
reports its version number and shows (on stderr) exactly how it invokes each 
phase of the compilation system.  Moreover, it passes
the \tr{-v} flag to most phases; each reports
its version number (and possibly some other information).

Please, oh please, use the \tr{-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
\tr{--version}\index{--version option} option prints out a
one-line string containing the requested info.

%************************************************************************
%*                                                                      *
\subsection[options-order]{Running the right phases in the right order}
\index{order of passes in GHC}
\index{pass ordering in GHC}
%*                                                                      *
%************************************************************************

The basic task of the \tr{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:

\begin{tabular}{llll}
phase of the       & suffix saying & flag saying   & (suffix of) \\
compilation system & ``start here''& ``stop after''& output file \\ \hline

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 \\
\end{tabular}
\index{-C option}
\index{-S option}
\index{-c option}

Thus, a common invocation would be: \tr{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 suffix information may be overridden with a \tr{-lang <suf>}
%\index{-lang <suf> option} option.  This says: process all inputs
%files as if they had suffix \pl{<suf>}. [NOT IMPLEMENTED YET]

The option \tr{-cpp}\index{-cpp option} must be given for the C
pre-processor phase to be run, that is, the pre-processor will be run
over your Haskell source file before continuing.

The option \tr{-E}\index{-E option} runs just the C-preprocessor part
of the C-compiling phase, sending the result to stdout [I think].  (For
debugging, usually.)

%************************************************************************
%*                                                                      *
\subsection[options-optimise]{Optimisation (code improvement)}
\index{optimisation (GHC)}
\index{improvement, code (GHC)}
%*                                                                      *
%************************************************************************

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

%----------------------------------------------------------------------
\subsubsection[optimise-pkgs]{\tr{-O*}: convenient ``packages'' of optimisation flags.}
\index{-O options (GHC)}

There are {\em many} options that affect the quality of code produced by
GHC.  Most people only have a general goal, something like ``Compile
quickly'' or ``Make my program run like greased lightning.''  The
following ``packages'' of optimisations (or lack thereof) should suffice.

Once you choose a \tr{-O*} ``package,'' stick with it---don't chop and
change.  Modules' interfaces {\em will} change with a shift to a new
\tr{-O*} option, and you may have to recompile a large chunk of all
importing modules before your program can again be run
safely\sectionref{recomp}.

\begin{description}
\item[No \tr{-O*}-type option specified:]
\index{-O* not specified}
This is taken to mean: ``Please compile quickly; I'm not over-bothered
about compiled-code quality.''  So, for example: \tr{ghc -c Foo.hs}

\item[\tr{-O} or \tr{-O1}:]
\index{-O option}
\index{-O1 option}
\index{optimise normally}
Means: ``Generate good-quality code without taking too long about it.''
Thus, for example: \tr{ghc -c -O Main.lhs}

\item[\tr{-O2}:]
\index{-O2 option}
\index{optimise aggressively}
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} if you're unlucky.  They are
normally turned on or off individually.

At the moment, \tr{-O2} is {\em unlikely} to produce
better code than \tr{-O}.

% \item[\tr{-O0}:]
% \index{-O0 option}
% \index{optimise minimally}
% [``Oh zero''] Means: ``Turn {\em off} as many optimisations (e.g.,
% simplifications) as possible.''  This is the only optimisation level
% at which the GCC-register-trickery is turned off.  {\em You can't use
% it unless you have a suitably-built Prelude to match.} Intended for
% hard-core debugging.

\item[\tr{-fvia-C}:]
\index{-fvia-C option}
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
\tr{_ccall_}s or \tr{_casm_}s, you probably {\em have to} use
\tr{-fvia-C}.

\item[\tr{-O2-for-C}:]
\index{-O2-for-C option}
Says to run GCC with \tr{-O2}, which may be worth a few percent in
execution speed.  Don't forget \tr{-fvia-C}, lest you use the
native-code generator and bypass GCC altogether!

\item[\tr{-Onot}:]
\index{-Onot option}
\index{optimising, reset}
This option will make GHC ``forget'' any -Oish options it has seen
so far.  Sometimes useful; for example: \tr{make all EXTRA_HC_OPTS=-Onot}.

\item[\tr{-Ofile <file>}:]
\index{-Ofile <file> option}
\index{optimising, customised}
For those who need {\em absolute} control over {\em exactly} what
options are used (e.g., compiler writers, sometimes :-), a list of
options can be put in a file and then slurped in with \tr{-Ofile}.

In that file, comments are of the \tr{#}-to-end-of-line variety; blank
lines and most whitespace is ignored.

Please ask if you are baffled and would like an example of \tr{-Ofile}!
\end{description}

At Glasgow, we don't use a \tr{-O*} flag for day-to-day work.  We use
\tr{-O} to get respectable speed; e.g., when we want to measure
something.  When we want to go for broke, we tend to use
\tr{-O -fvia-C -O2-for-C} (and we go for lots of coffee breaks).

%Here is a table to summarise whether pragmatic interface information
%is used or not, whether the native-code generator is used (if
%available), and whether we use GCC register tricks (for speed!) on the
%generated C code:
%
%\begin{tabular}{lccl}
%\tr{-O*}    & Interface & Native code & `Registerised' C \\
%            & pragmas?  & (if avail.) & (if avail.) \\ \hline
%%
%\pl{<none>} & no        & yes         & yes, only if \tr{-fvia-C} \\
%\tr{-O,-O1} & yes       & yes         & yes, only if \tr{-fvia-C} \\
%\tr{-O2}    & yes       & no         & yes \\
%\tr{-Ofile} & yes      & yes         & yes, only if \tr{-fvia-C} \\
%\end{tabular}

The easiest way to see what \tr{-O} (etc) ``really mean'' is to run
with \tr{-v}, then stand back in amazement.
Alternatively, just look at the
\tr{@HsC_minus<blah>} lists in the \tr{ghc} driver script.

%----------------------------------------------------------------------
\subsubsection{\tr{-f*}: platform-independent flags}
\index{-f* options (GHC)}
\index{-fno-* options (GHC)}

Flags can be turned {\em off} individually.  (NB: I hope
you have a good reason for doing this....) To turn off the \tr{-ffoo}
flag, just use the \tr{-fno-foo} flag.\index{-fno-<opt> anti-option}
So, for example, you can say
\tr{-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:
\tr{-fno-strictness}\index{-fno-strictness option} (strictness
analyser [because it is sometimes slow]),
\tr{-fno-specialise}\index{-fno-specialise option} (automatic
specialisation of overloaded functions [because it makes your code
bigger]) [US spelling also accepted],
and
\tr{-fno-foldr-build}\index{-fno-foldr-build option}.

Should you wish to turn individual flags {\em on}, you are advised to
use the \tr{-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} want to try:
\begin{description}
%------------------------------------------------------------------
\item[\tr{-funfolding-creation-threshold<n>}:]
(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.)

\item[\tr{-funfolding-use-threshold<n>}:]
(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.

\item[\tr{-funfolding-override-threshold<n>}:]
(Default: 8) [Pretty obscure]
When 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 \tr{foo = bar}, it will decide that the very
small expression \tr{bar} is a great unfolding for \tr{foo}.  But if
\tr{bar} turns out to be \tr{(True,False,True)}, we would probably
prefer {\em that} for the unfolding for \tr{foo}.

Should we ``override'' the initial small unfolding from \tr{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).

% \item[\tr{-fliberated-case-threshold<n>}:]
% (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.

\item[\tr{-fsemi-tagging}:]
This option (which {\em does not work} with the native-code generator)
tells the compiler to add extra code to test for already-evaluated
values.  You win if you have lots of such values during a run of your
program, you lose otherwise.  (And you pay in extra code space.)

We have not played with \tr{-fsemi-tagging} enough to recommend it.
(For all we know, it doesn't even work anymore...  Sigh.)
\end{description}

%----------------------------------------------------------------------
% \subsubsection[optimise-simplifier]{Controlling ``simplification'' in the Haskell compiler.}
%
%Almost everyone turns program transformation
% (a.k.a. ``simplification'') on/off via one of the ``packages'' above,
%but you can exert absolute control if you want to.  Do a \tr{ghc -v -O ...},
%and you'll see there are plenty of knobs to turn!
%
%The Core-to-Core and STG-to-STG passes can be run multiple times, and
%in varying orders (though you may live to regret it).  The on-or-off
%global flags, however, are simply, well, on or off.
%
%The best way to give an exact list of options is the \tr{-Ofile}
%option, described elsewhere.
%
% [Check out \tr{ghc/compiler/simplCore/SimplCore.lhs} and
%\tr{simplStg/SimplStg.lhs} if you {\em really} want to see every
%possible Core-to-Core and STG-to-STG pass, respectively.  The
%on-or-off global flags that effect what happens {\em within} one of
%these passes are defined by the \tr{GlobalSwitch} datatype in
%\tr{compiler/main/CmdLineOpts.lhs}.]

%----------------------------------------------------------------------
\subsubsection{\tr{-m*}: platform-specific flags}
\index{-m* options (GHC)}
\index{platform-specific options}
\index{machine-specific options}

Some flags only make sense for particular target platforms.

\begin{description}
\item[\tr{-mv8}:]
(SPARC machines)\index{-mv8 option (SPARC only)}
Means to pass the like-named option to GCC; it says to use the
Version 8 SPARC instructions, notably integer multiply and divide.
The similiar \tr{-m*} GCC options for SPARC also work, actually.

\item[\tr{-mlong-calls}:]
(HPPA machines)\index{-mlong-calls option (HPPA only)}
Means to pass the like-named option to GCC.  Required for Very Big
modules, maybe.  (Probably means you're in trouble...)

\item[\tr{-monly-[32]-regs}:]
(iX86 machines)\index{-monly-N-regs option (iX86 only)}
GHC tries to ``steal'' four registers from GCC, for performance
reasons; it almost always works.  However, when GCC is compiling some
modules with four stolen registers, it will crash, probably saying:
\begin{verbatim}
Foo.hc:533: fixed or forbidden register was spilled.
This may be due to a compiler bug or to impossible asm
statements or clauses.
\end{verbatim}
Just give some registers back with \tr{-monly-N-regs}.  Try `3' first,
then `2'.  If `2' doesn't work, please report the bug to us.
\end{description}

%----------------------------------------------------------------------
\subsubsection[optimise-C-compiler]{Code improvement by the C compiler.}
\index{optimisation by GCC}
\index{GCC optimisation}

The C~compiler (GCC) is run with \tr{-O} turned on.  (It has
to be, actually).

If you want to run GCC with \tr{-O2}---which may be worth a few
percent in execution speed---you can give a
\tr{-O2-for-C}\index{-O2-for-C option} option.

%If you are brave or foolish, you might want to omit some checking code
% (e.g., for stack-overflow checks), as sketched in
%\sectionref{omit-checking}.

%************************************************************************
%*                                                                      *
\subsection[options-sanity]{Warnings and sanity-checking}
\index{sanity-checking options}
%*                                                                      *
%************************************************************************

If you would like GHC to check that every top-level value has a type
signature, use the \tr{-fsignatures-required}
option.\index{-fsignatures-required option}

If you would like to disallow ``name shadowing,'' i.e., an inner-scope
value has the same name as an outer-scope value, then use the
\tr{-fwarn-name-shadowing}
option.\index{-fwarn-name-shadowing option}
This option catches typographical errors that turn into hard-to-find
bugs, e.g., in the inadvertent cyclic definition \tr{let x = ... x ... in}.

Consequently, this option does {\em not} allow cyclic recursive
definitions.

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.,

\begin{verbatim}
f :: String -> Int
f []     = 0
f (_:xs) = 1
f "2"    = 2

g [] = 2
\end{verbatim}

where the last pattern match in \tr{f} won't ever be reached, as the
second pattern overlaps it. More often than not, redundant patterns
is a programmer mistake/error, but if you don't want the compiler to
``baby-sit'', use the \tr{-fno-warn-overlapping-patterns} option to
turn these warnings off.\index{-fno-warn-overlapping-patterns option}

Similarly for incomplete patterns, the function \tr{g} will fail when
applied to non-empty lists, so the compiler will by default emit a
warning about this. The option \tr{-fno-warn-incomplete-patterns}
turns rhis off.\index{-fno-warn-incomplete-pattern option}


If you're feeling really paranoid, the \tr{-dcore-lint}
option\index{-dcore-lint option} is a good choice.  It turns on
heavyweight intra-pass sanity-checking within GHC.  (It checks GHC's
sanity, not yours.)

%************************************************************************
%*                                                                      *
\subsection[options-output]{Re-directing the compilation output(s)}
\index{output-directing options}
%*                                                                      *
%************************************************************************

When compiling a Haskell module, GHC may produce several files of
output (usually two).

One file is usually an {\em interface file}.  If compiling
\tr{bar/Foo.hs}, the interface file would normally be \tr{bar/Foo.hi}.
The interface output may be directed to another file
\tr{bar2/Wurble.iface} with the option
\tr{-ohi bar2/Wurble.iface}\index{-ohi <file> option} (not recommended).

To avoid generating an interface file at all, use a \tr{-nohi}
option.\index{-nohi option}

The compiler does not overwrite an existing \tr{.hi} interface file if
the new one is byte-for-byte the same as the old one; this is friendly to
\tr{make}.  When an interface does change, it is often enlightening to
be informed.  The \tr{-hi-diffs}\index{-hi-diffs option} option will
make \tr{ghc} run \tr{diff} on the old and new \tr{.hi} files.

The \tr{.hi} files from GHC 2.xx contain ``usage'' information which
changes often and uninterestingly.  If you really want to see these
changes reported, you need to use the
\tr{-hi-diffs-with-usages}\index{-hi-diffs-with-usages option} option.

GHC's non-interface output normally goes into a \tr{.hc}, \tr{.o},
etc., file, depending on the last-run compilation phase.  The option
\tr{-o foo}\index{-o option} re-directs the output of that last-run
phase to file \tr{foo}.

Note: this ``feature'' can be counterintuitive:
\tr{ghc -C -o foo.o foo.hs} will put the intermediate C code in the
file \tr{foo.o}, name notwithstanding!

EXOTICA: But the \tr{-o} option isn't of much use if you have {\em
several} input files... Non-interface output files are normally put
in the same directory as their corresponding input file came from.
You may specify that they be put in another directory using the
\tr{-odir <dir>}\index{-odir <dir> option} (the ``Oh, dear'' option).
For example:

\begin{verbatim}
% ghc -c parse/Foo.hs parse/Bar.hs gurgle/Bumble.hs -odir `arch`
\end{verbatim}

The output files, \tr{Foo.o}, \tr{Bar.o}, and \tr{Bumble.o} would be
put into a subdirectory named after the architecture of the executing
machine (\tr{sun4}, \tr{mips}, etc).  The directory must already
exist; it won't be created.

Note that the \tr{-odir} option does {\em not} affect where the
interface files are put.  In the above example, they would still be
put in \tr{parse/Foo.hi}, \tr{parse/Bar.hi}, and
\tr{gurgle/Bumble.hi}.

MORE EXOTICA: The \tr{-osuf <suffix>}\index{-osuf <suffix> option}
will change the \tr{.o} file suffix for object files to whatever
you specify.  (We use this in compiling the prelude.)

Similarly, the \tr{-hisuf <suffix>}\index{-hisuf <suffix> option} will
change the \tr{.hi} file suffix for non-system interface files.  This
can be useful when you are trying to compile a program several ways,
all in the same directory.  The suffix given is used for {\em all}
interfaces files written, {\em and} for all non-system interface files
that your read.

The \tr{-hisuf}/\tr{-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 \tr{.hi}/\tr{.o} suffixes; add
\tr{-hisuf g_hi -osuf g_o} to your \tr{make} rule for GHC compiling...

NB: {\em A change from 0.26 and before:} Before, you might have said
\tr{-hisuf _g.hi -osuf _g.o}; now, the \tr{.} is assumed and you
specify what comes {\em after} it.  (This is a more portable solution
for the long term.)

% THIS SHOULD HAPPEN AUTOMAGICALLY:
% If you want to change the suffix looked for on system-supplied
% interface files (notably the \tr{Prelude.hi} file), use the
% \tr{-hisuf-prelude <suffix>}\index{-hisuf-prelude <suffix> option}
% option.  (This may be useful if you've built GHC in various funny
% ways, and you are running tests in even more funny ways.  It happens.)

FURTHER EXOTICA: If you are doing a normal \tr{.hs}-to-\tr{.o} compilation
but would like to hang onto the intermediate \tr{.hc} C file, just
throw in a \tr{-keep-hc-file-too} option\index{-keep-hc-file-too option}.
If you would like to look at the assembler output, toss in a
\tr{-keep-s-file-too},\index{-keep-hc-file-too option} too.

SAVING GHC STDERR OUTPUT: Sometimes, you may cause GHC to be rather
chatty on standard error; with \tr{-fshow-import-specs}, for example.
You can instruct GHC to {\em append} this output to a particular log
file with a \tr{-odump <blah>}\index{-odump <blah> option} option.

TEMPORARY FILES: If you have trouble because of running out of space
in \tr{/tmp/} (or wherever your installation thinks temporary files
should go), you may use the \tr{-tmpdir <dir>}\index{-tmpdir <dir> option}
option to specify an alternate directory.  For example, \tr{-tmpdir .}
says to put temporary files in the current working directory.

BETTER IDEA FOR TEMPORARY FILES: Use your \tr{TMPDIR} environment
variable.\index{TMPDIR environment variable}  Set it to the name of
the directory where temporary files should be put.  GCC and other
programs will honour the \tr{TMPDIR} variable as well.

EVEN BETTER IDEA: Set the \tr{TMPDIR} variable when building
GHC, and never worry about \tr{TMPDIR} again. (see the build
documentation).

%************************************************************************
%*                                                                      *
\subsection[options-finding-imports-etc]{For finding interface files, etc.}
\index{interface files, finding them}
\index{finding interface files}
%*                                                                      *
%************************************************************************

In your program, you import a module \tr{Foo} by saying
\tr{import Foo}.  GHC goes looking for an interface file, \tr{Foo.hi}.
It has a builtin list of directories (notably including \tr{.}) where
it looks.

The \tr{-i<dirs>} option\index{-i<dirs> option} prepends a
colon-separated list of \tr{dirs} to the ``import directories'' list.

A plain \tr{-i} resets the ``import directories'' list back to nothing.

GHC normally imports \tr{Prelude.hi} files for you.  If you'd rather
it didn't, then give it a \tr{-fno-implicit-prelude}
option\index{-fno-implicit-prelude option}.  You are unlikely to get
very far without a Prelude, but, hey, it's a free country.

If you are using a system-supplied non-Prelude library (e.g., the HBC
library), just use a \tr{-syslib hbc}\index{-syslib <lib> option}
option (for example).  The right interface files should then be
available.

Once a Haskell module has been compiled to C (\tr{.hc} file), you may
wish to specify where GHC tells the C compiler to look for \tr{.h}
files.  (Or, if you are using the \tr{-cpp} option\index{-cpp option},
where it tells the C pre-processor to look...)  For this purpose, use
a \tr{-I<dir>}\index{-I<dir> option} in the usual C-ish way.

Pragmas: Interface files are normally jammed full of
compiler-produced {\em pragmas}, which record arities, strictness
info, etc.  If you think these pragmas are messing you up (or you are
doing some kind of weird experiment), you can tell GHC to ignore them
with the \tr{-fignore-interface-pragmas}\index{-fignore-interface-pragmas option}
option.

When compiling without optimisations on, the compiler is extra-careful
about not slurping in data constructors and instance declarations that
it will not need. If you believe it is getting it wrong and not
importing stuff which you think it should, this optimisation can be
turned off with \tr{-fno-prune-tydecls} and \tr{-fno-prune-instdecls}.
\index{-fno-prune-tydecls option}\index{-fno-prune-instdecls}

See also \sectionref{options-linker}, which describes how the linker
finds standard Haskell libraries.

%************************************************************************
%*                                                                      *
%\subsection[options-names]{Fiddling with namespaces}
%*                                                                      *
%************************************************************************

%-split-objs and -fglobalise-toplev-names.  You don't need them and you
%don't want to know; used for the prelude (ToDo).

%************************************************************************
%*                                                                      *
\subsection[options-CPP]{Related to the C pre-processor}
\index{C pre-processor options}
\index{pre-processor (cpp) options}
%*                                                                      *
%************************************************************************

The C pre-processor \tr{cpp} is run over your Haskell code only if the
\tr{-cpp} option \index{-cpp option} is given.  Unless you are
building a large system with significant doses of conditional
compilation, you really shouldn't need it.
\begin{description}
\item[\tr{-D<foo>}:]
\index{-D<name> option}
Define macro \tr{<foo>} in the usual way.  NB: does {\em not} affect
\tr{-D} macros passed to the C~compiler when compiling via C!  For
those, use the \tr{-optc-Dfoo} hack...

\item[\tr{-U<foo>}:]
\index{-U<name> option}
Undefine macro \tr{<foo>} in the usual way.

\item[\tr{-I<dir>}:]
\index{-I<dir> option}
Specify a directory in which to look for \tr{#include} files, in
the usual C way.
\end{description}

The \tr{ghc} driver pre-defines several macros:
\begin{description}
\item[\tr{__HASKELL1__}:]
\index{__HASKELL1__ macro}
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 (\tr{.hc}) files.

% If you give the \tr{-fhaskell-1.3} flag\index{-fhaskell-1.3 option},
% then \tr{__HASKELL1__} is set to 3.  Obviously.

\item[\tr{__GLASGOW_HASKELL__}:]
\index{__GLASGOW_HASKELL__ macro}
For version $n$ of the GHC system, this will be \tr{#define}d to
$100 \times n$.  So, for version~2.02, it is 202.

This macro is {\em only} set when pre-processing Haskell source.
({\em Not} when pre-processing generated C.)

With any luck, \tr{__GLASGOW_HASKELL__} will be undefined in all other
implementations that support C-style pre-processing.

(For reference: the comparable symbols for other systems are:
\tr{__HUGS__} for Hugs and \tr{__HBC__} for Chalmers.)

\item[\tr{__CONCURRENT_HASKELL__}:]
\index{__CONCURRENT_HASKELL__ macro}
Only defined when \tr{-concurrent} is in use!
This symbol is defined when pre-processing Haskell (input) and
pre-processing C (GHC output).

\item[\tr{__PARALLEL_HASKELL__}:]
\index{__PARALLEL_HASKELL__ macro}
Only defined when \tr{-parallel} is in use!  This symbol is defined when
pre-processing Haskell (input) and pre-processing C (GHC output).
\end{description}

Options other than the above can be forced through to the C
pre-processor with the \tr{-opt} flags (see
\sectionref{forcing-options-through}).

A small word of warning: \tr{-cpp} is not friendly to
``string gaps''.\index{-cpp vs string gaps}\index{string gaps vs -cpp}


%************************************************************************
%*                                                                      *
\subsection[options-C-compiler]{Options affecting the C compiler (if applicable)}
\index{include-file-option}
\index{C compiler options}
\index{GCC options}
%*                                                                      *
%************************************************************************

At the moment, quite a few common C-compiler options are passed on
quietly to the C compilation of Haskell-compiler-generated C files.
THIS MAY CHANGE.  Meanwhile, options so sent are:

\begin{tabular}{ll}
\tr{-Wall}      & get all warnings from GCC \\
\tr{-ansi}      & do ANSI C (not K\&R) \\
\tr{-pedantic}  & be so\\
\tr{-dgcc-lint} & (hack) short for ``make GCC very paranoid''\\
\end{tabular}
\index{-Wall option (for GCC)}
\index{-ansi option (for GCC)}
\index{-pedantic option (for GCC)}
\index{-dgcc-lint option (GCC paranoia)}

If you are compiling with lots of \tr{ccalls}, etc., you may need to
tell the C~compiler about some \tr{#include} files.  There is no real
pretty way to do this, but you can use this hack from the
command-line:
\begin{verbatim}
% ghc -c '-#include <X/Xlib.h>' Xstuff.lhs
\end{verbatim}


%************************************************************************
%*                                                                      *
%\subsection[options-native-code]{Options affecting the native-code generator(s)}
%*                                                                      *
%************************************************************************

%The only option is to select the target architecture.  Right now,
%you have only at most one choice: \tr{-fasm-sparc}.\index{-fasm-<target> option}
%
%EXPECT this native-code stuff to change in the future.

%************************************************************************
%*                                                                      *
\subsection[options-linker]{Linking and consistency-checking}
\index{linker options}
\index{ld options}
%*                                                                      *
%************************************************************************

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

\begin{description}
\item[\tr{-l<FOO>}:]
\index{-l<lib> option}
Link in a library named \tr{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: \tr{ghc -lbar *.o} is almost certainly
wrong, because it will search \tr{libbar.a} {\em before} it has
collected unresolved symbols from the \tr{*.o} files.
\tr{ghc *.o -lbar} is probably better.

The linker will of course be informed about some GHC-supplied
libraries automatically; these are:

\begin{tabular}{ll}
-l equivalent & description \\ \hline

-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)\\
\end{tabular}
\index{-lHS library}
\index{-lHS_cbits library}
\index{-lHSrts library}
\index{-lgmp library}

\item[\tr{-syslib <name>}:]
\index{-syslib <name> option}

If you are using a Haskell ``system library'' (e.g., the HBC
library), just use the \tr{-syslib hbc} option, and the correct code
should be linked in.

%Please see \sectionref{syslibs} for information about
%``system libraries.''

\item[\tr{-L<dir>}:]
\index{-L<dir> option}
Where to find user-supplied libraries...  Prepend the directory
\tr{<dir>} to the library directories path.

\item[\tr{-static}:]
\index{-static option}
Tell the linker to avoid shared libraries.

\item[\tr{-no-link-chk} and \tr{-link-chk}:]
\index{-no-link-chk option}
\index{-link-chk option}
\index{consistency checking of executables}
By default, immediately after linking an executable, GHC verifies that
the pieces that went into it were compiled with compatible flags; a
``consistency check''.
(This is to avoid mysterious failures caused by non-meshing of
incompatibly-compiled programs; e.g., if one \tr{.o} file was compiled
for a parallel machine and the others weren't.)  You may turn off this
check with \tr{-no-link-chk}.  You can turn it (back) on with
\tr{-link-chk} (the default).
\end{description}

%************************************************************************
%*                                                                      *
\subsection[options-compiler-RTS]{For the compiler's RTS: heap, stack sizes, etc.}
\index{heap-size options (for GHC)}
\index{stack-size options (for GHC)}
%*                                                                      *
%************************************************************************

The compiler is itself a Haskell program, so it has a tweakable
runtime-system (RTS), just like any other Haskell program.

\begin{description}
\item[\tr{-H<size>} or \tr{-Rmax-heapsize <size>}:]
\index{-H<size> option}
\index{-Rmax-heapsize <size> option}
Don't use more than \tr{<size>} {\em bytes} for heap space.  If more
than one of these arguments is given, the largest will be taken.

A size of zero can be used to reset the heap size downwards.  For
example, to run GHC with a heap of 250KB (the default is 6MB), do
\tr{-H0 -H250k}.

\item[\tr{-K<size>} or \tr{-Rmax-stksize <size>}:]
\index{-K<size> option}
\index{-Rmax-stksize <size> option}
Set the stack space to \tr{<size>} bytes.  If you have to set it very
high [a megabyte or two, say], the compiler is probably looping, which
is a BUG (please report).

A size of zero can be used to rest the stack size downwards, as above.

\item[\tr{-Rscale-sizes<factor>}:]
\index{-Rscale-sizes<factor> option}
Multiply the given (or default) heap and stack sizes by \tr{<factor>}.
For example, on a DEC Alpha (a 64-bit machine), you might want to
double those space sizes; just use \tr{-Rscale-sizes2}.

A non-integral factor is OK, too: \tr{-Rscale-sizes1.2}.

\item[\tr{-Rghc-timing}:]
\index{-Rghc-timing option}
Reports a one-line useful collection of time- and space- statistics
for a module's compilation.

\item[\tr{-Rgc-stats}:]
\index{-Rgc-stats option}
Report garbage-collection statistics.  It will create a
\tr{<foo>.stat} file, in some obvious place (I hope).

Alternatively, if you'd rather the GC stats went straight to standard
error, you can ``cheat'' by using, instead: \tr{-optCrts-Sstderr}.
%
%\item[\tr{-Rhbc}:]
%\index{-Rhbc option}
%Tell the compiler it has an HBC-style RTS; i.e., it was compiled with
%HBC.  Not used in Real Life.
%
%\item[\tr{-Rghc}:]
%\index{-Rghc option}
%Tell the compiler it has a GHC-style RTS; i.e., it was compiled with
%GHC.  Not used in Real Life.
\end{description}

For all \tr{<size>}s: If the last character of \tr{size} is a K,
multiply by 1000; if an M, by 1,000,000; if a G, by 1,000,000,000.
Sizes are always in {\em bytes}, not words.  Good luck on the G's (I
think the counter is still only 32-bits [WDP])!

%************************************************************************
%*                                                                      *
%\subsection[options-cross-compiling]{For cross-compiling to another architecture}
%*                                                                      *
%************************************************************************
%
% (We do this for GRIP at Glasgow; it's hacked in---not proper
%cross-compiling support.  But you could do the same, if required...)
%
%The \tr{-target <arch>} option\index{-target <arch> option} says to
%generate code for the \tr{<arch>} architecture.

%************************************************************************
%*                                                                      *
\subsection[options-parallel]{For Concurrent and Parallel Haskell}
%*                                                                      *
%************************************************************************

For the full story on using GHC for concurrent \& parallel Haskell
programming, please see \Sectionref{concurrent-and-parallel}.

%The \tr{-fparallel} option\index{-fparallel option} tells the compiler
%to generate code for parallel execution.  The \tr{-mgrip}
%option\index{-mgrip option} says that the code should be explicitly
%suitable for the GRIP multiprocessor (the one in our Glasgow basement).

%************************************************************************
%*                                                                      *
%\subsection[options-experimental]{For experimental purposes}
%\index{experimental options}
%*                                                                      *
%************************************************************************

%From time to time, we provide GHC options for ``experimenting.''  Easy
%come, easy go.  In version~0.26, the ``experimental'' options are:
%\begin{description}
%\item[\tr{-firrefutable-tuples} option:]
%\index{-firrefutable-tuples option (experimental)}
%Pretend that every tuple pattern is irrefutable; i.e., has a
%``twiddle'' (\tr{~}) in front of it.
%
%Some parts of the GHC system {\em depend} on strictness properties which
%\tr{-firrefutable-tuples} may undo, notably the low-level state-transformer
%stuff, which includes I/O (!).  You're on your own...
%
%\item[\tr{-fall-strict} option:]
%\index{-fall-strict option (experimental)}
% (DOESN'T REALLY WORK, I THINK) Changes the strictness analyser so
%that, when it asks the question ``Is this function argument certain to
%be evaluated?'', the answer is always ``yes''.
%
%Compilation is changed in no other way.
%\end{description}

% -firrefutable-everything
% -fall-demanded

%************************************************************************
%*                                                                      *
\subsection[options-debugging]{For debugging the compiler}
\index{debugging options (for GHC)}
%*                                                                      *
%************************************************************************

HACKER TERRITORY. HACKER TERRITORY.
(You were warned.)

%----------------------------------------------------------------------
\subsubsection[replacing-phases]{Replacing the program for one or more phases.}
\index{GHC phases, changing}
\index{phases, changing GHC}

You may specify that a different program
be used for one of the phases of the compilation system, in place of
whatever the driver \tr{ghc} has wired into it.  For example, you
might want to try a different assembler.  The
\tr{-pgm<phase-code><program-name>}\index{-pgm<phase><stuff> option} option to
\tr{ghc} will cause it to use \pl{<program-name>} for phase
\pl{<phase-code>}, where the codes to indicate the phases are:

\begin{tabular}{ll}
code & phase \\ \hline
L    & literate pre-processor \\
P    & C pre-processor (if -cpp only) \\
C    & Haskell compiler \\
c    & C compiler\\
a    & assembler \\
l    & linker \\
\end{tabular}

%----------------------------------------------------------------------
\subsubsection[forcing-options-through]{Forcing options to a particular phase.}
\index{forcing GHC-phase options}

The preceding sections describe driver options that are mostly
applicable to one particular phase.  You may also {\em force} a
specific option \tr{<option>} to be passed to a particular phase
\tr{<phase-code>} by feeding the driver the option
\tr{-opt<phase-code><option>}.\index{-opt<phase><stuff> option} The
codes to indicate the phases are the same as in the previous section.

So, for example, to force an \tr{-Ewurble} option to the assembler, you
would tell the driver \tr{-opta-Ewurble} (the dash before the E is
required).

Besides getting options to the Haskell compiler with \tr{-optC<blah>},
you can get options through to its runtime system with
\tr{-optCrts<blah>}\index{-optCrts<blah> option}.

So, for example: when I want to use my normal driver but with my
profiled compiler binary, I use this script:
\begin{verbatim}
#! /bin/sh
exec /local/grasp_tmp3/simonpj/ghc-BUILDS/working-alpha/ghc/driver/ghc \
     -pgmC/local/grasp_tmp3/simonpj/ghc-BUILDS/working-hsc-prof/hsc \
     -optCrts-i0.5 \
     -optCrts-PT \
     "$@"
\end{verbatim}

%----------------------------------------------------------------------
\subsubsection[dumping-output]{Dumping out compiler intermediate structures}
\index{dumping GHC intermediates}
\index{intermediate passes, output}

\begin{description}
\item[\tr{-noC}:]
\index{-noC option}
Don't bother generating C output {\em or} an interface file.  Usually
used in conjunction with one or more of the \tr{-ddump-*} options; for
example: \tr{ghc -noC -ddump-simpl Foo.hs}

\item[\tr{-hi}:]
\index{-hi option}
{\em Do} generate an interface file.  This would normally be used in
conjunction with \tr{-noC}, which turns off interface generation;
thus: \tr{-noC -hi}.

\item[\tr{-dshow-passes}:]
\index{-dshow-passes option}
Prints a message to stderr as each pass starts.  Gives a warm but
undoubtedly misleading feeling that GHC is telling you what's
happening.

\item[\tr{-ddump-<pass>}:]
\index{-ddump-<pass> options}
Make a debugging dump after pass \tr{<pass>} (may be common enough to
need a short form...).  Some of the most useful ones are:

\begin{tabular}{ll}
\tr{-ddump-rdr} & reader output (earliest stuff in the compiler) \\
\tr{-ddump-rn} & renamer output \\
\tr{-ddump-tc} & typechecker output \\
\tr{-ddump-deriv} & derived instances \\
\tr{-ddump-ds} & desugarer output \\
\tr{-ddump-simpl} & simplifer output (Core-to-Core passes) \\
\tr{-ddump-stranal} & strictness analyser output \\
\tr{-ddump-occur-anal} & `occurrence analysis' output \\
\tr{-ddump-spec} & dump specialisation info \\
\tr{-ddump-stg} & output of STG-to-STG passes \\
\tr{-ddump-absC} & {\em un}flattened Abstract~C \\
\tr{-ddump-flatC} & {\em flattened} Abstract~C \\
\tr{-ddump-realC} & same as what goes to the C compiler \\
\tr{-ddump-asm} & assembly language from the native-code generator \\
\end{tabular}
\index{-ddump-rdr option}%
\index{-ddump-rn option}%
\index{-ddump-tc option}%
\index{-ddump-deriv option}%
\index{-ddump-ds option}%
\index{-ddump-simpl option}%
\index{-ddump-stranal option}%
\index{-ddump-occur-anal option}%
\index{-ddump-spec option}%
\index{-ddump-stg option}%
\index{-ddump-absC option}%
\index{-ddump-flatC option}%
\index{-ddump-realC option}%
\index{-ddump-asm option}

%For any other \tr{-ddump-*} options: consult the source, notably
%\tr{ghc/compiler/main/CmdLineOpts.lhs}.

\item[\tr{-dverbose-simpl} and \tr{-dverbose-stg}:]
\index{-dverbose-simpl option}
\index{-dverbose-stg option}
Show the output of the intermediate Core-to-Core and STG-to-STG
passes, respectively.  ({\em Lots} of output!) So: when we're 
really desperate:
\begin{verbatim}
% ghc -noC -O -ddump-simpl -dverbose-simpl -dcore-lint Foo.hs
\end{verbatim}

\item[\tr{-dppr-{user,debug,all}}:]
\index{-dppr-user option}
\index{-dppr-debug option}
\index{-dppr-all option}
Debugging output is in one of several ``styles.''  Take the printing
of types, for example.  In the ``user'' style, the compiler's internal
ideas about types are presented in Haskell source-level syntax,
insofar as possible.  In the ``debug'' style (which is the default for
debugging output), the types are printed in the most-often-desired
form, with explicit foralls, etc.  In the ``show all'' style, very
verbose information about the types (e.g., the Uniques on the
individual type variables) is displayed.

\item[\tr{-ddump-raw-asm}:]
\index{-ddump-raw-asm option}
Dump out the assembly-language stuff, before the ``mangler'' gets it.

\item[\tr{-ddump-rn-trace}:]
\index{-ddump-rn-trace}
Make the renamer be *real* chatty about what it is upto.

\item[\tr{-dshow-rn-stats}:]
\index{-dshow-rn-stats}
Print out summary of what kind of information the renamer had to bring
in.
\item[\tr{-dshow-unused-imports}:]
\index{-dshow-unused-imports}
Have the renamer report what imports does not contribute.

\item[\tr{-fwarn-unused-names}:]
\index{-fwarn-unused-names}
Have the renamer report which locally defined names are not used/exported.

%
%\item[\tr{-dgc-debug}:]
%\index{-dgc-debug option}
%Enables some debugging code related to the garbage-collector.
\end{description}

%ToDo: -ddump-asm-insn-counts
%-ddump-asm-globals-info

%----------------------------------------------------------------------
\subsubsection{How to read Core syntax (from some \tr{-ddump-*} flags)}
\index{reading Core syntax}
\index{Core syntax, how to read}

Let's do this by commenting an example.  It's from doing
\tr{-ddump-ds} on this code:
\begin{verbatim}
skip2 m = m : skip2 (m+2)
\end{verbatim}

Before we jump in, a word about names of things.  Within GHC,
variables, type constructors, etc., are identified by their
``Uniques.''  These are of the form `letter' plus `number' (both
loosely interpreted).  The `letter' gives some idea of where the
Unique came from; e.g., \tr{_} means ``built-in type variable'';
\tr{t} means ``from the typechecker''; \tr{s} means ``from the
simplifier''; and so on.  The `number' is printed fairly compactly in
a `base-62' format, which everyone hates except me (WDP).

Remember, everything has a ``Unique'' and it is usually printed out
when debugging, in some form or another.  So here we go...

\begin{verbatim}
Desugared:
Main.skip2{-r1L6-} :: _forall_ a$_4 =>{{Num a$_4}} -> a$_4 -> [a$_4]

--# `r1L6' is the Unique for Main.skip2;
--# `_4' is the Unique for the type-variable (template) `a'
--# `{{Num a$_4}}' is a dictionary argument

_NI_

--# `_NI_' means "no (pragmatic) information" yet; it will later
--# evolve into the GHC_PRAGMA info that goes into interface files.

Main.skip2{-r1L6-} =
    /\ _4 -> \ d.Num.t4Gt ->
        let {
          {- CoRec -}
          +.t4Hg :: _4 -> _4 -> _4
          _NI_
          +.t4Hg = (+{-r3JH-} _4) d.Num.t4Gt

          fromInt.t4GS :: Int{-2i-} -> _4
          _NI_
          fromInt.t4GS = (fromInt{-r3JX-} _4) d.Num.t4Gt

--# The `+' class method (Unique: r3JH) selects the addition code
--# from a `Num' dictionary (now an explicit lamba'd argument).
--# Because Core is 2nd-order lambda-calculus, type applications
--# and lambdas (/\) are explicit.  So `+' is first applied to a
--# type (`_4'), then to a dictionary, yielding the actual addition
--# function that we will use subsequently...

--# We play the exact same game with the (non-standard) class method
--# `fromInt'.  Unsurprisingly, the type `Int' is wired into the
--# compiler.

          lit.t4Hb :: _4
          _NI_
          lit.t4Hb =
              let {
                ds.d4Qz :: Int{-2i-}
                _NI_
                ds.d4Qz = I#! 2#
              } in  fromInt.t4GS ds.d4Qz

--# `I# 2#' is just the literal Int `2'; it reflects the fact that
--# GHC defines `data Int = I# Int#', where Int# is the primitive
--# unboxed type.  (see relevant info about unboxed types elsewhere...)

--# The `!' after `I#' indicates that this is a *saturated*
--# application of the `I#' data constructor (i.e., not partially
--# applied).

          skip2.t3Ja :: _4 -> [_4]
          _NI_
          skip2.t3Ja =
              \ m.r1H4 ->
                  let { ds.d4QQ :: [_4]
                        _NI_
                        ds.d4QQ =
                    let {
                      ds.d4QY :: _4
                      _NI_
                      ds.d4QY = +.t4Hg m.r1H4 lit.t4Hb
                    } in  skip2.t3Ja ds.d4QY
                  } in
                  :! _4 m.r1H4 ds.d4QQ

          {- end CoRec -}
        } in  skip2.t3Ja
\end{verbatim}

(``It's just a simple functional language'' is an unregisterised
trademark of Peyton Jones Enterprises, plc.)

%----------------------------------------------------------------------
\subsubsection[source-file-options]{Command line options in source files}
\index{source-file options}

Sometimes it is useful to make the connection between a source file
and the command-line options it requires, quite tight. For instance,
if a (Glasgow) Haskell source file uses \tr{casm}s, the C back-end
often needs to be told about which header files to include. Rather than
maintaining the list of files the source depends on in a
\tr{Makefile} (using the \tr{-#include} command-line option), it is
possible to do this directly in the source file using the \tr{OPTIONS}
pragma \index{OPTIONS pragma}: 

\begin{verbatim}
{-# OPTIONS -#include "foo.h" #-}
module X where

...
\end{verbatim}

\tr{OPTIONS} pragmas are only looked for at the top of your source
files, upto the first (non-literate,non-empty) line not containing
\tr{OPTIONS}. Multiple \tr{OPTIONS} pragmas are recognised. Note
that your command shell does not get to the source file options, they
are just included literally in the array of command-line arguments
the compiler driver maintains internally, so you'll be desperately
disappointed if you try to glob etc. inside \tr{OPTIONS}.

It is not recommended to move all the contents of your Makefiles into
your source files, but in some circumstances, the \tr{OPTIONS} pragma
is the Right Thing. (If you use \tr{-keep-hc-file-too} and have OPTION
flags in your module, the OPTIONS will get put into the generated .hc
file).

%----------------------------------------------------------------------
\subsubsection{How to compile mutually recursive modules}
\index{module system, recursion}

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

\begin{verbatim}
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
\end{verbatim}

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. For the example at hand, the
boot interface file for A would like the following:

\begin{verbatim}
_interface_ A 1
_exports_
A A(A) f;
_declarations_
1 newtype A = A PrelBase.Int ;
1 f _:_ B.B -> A.A ;;
\end{verbatim}

To make sure you get the syntax right, tailoring an existing interface
file is a Good Idea.

{\bf Note:} 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.

%----------------------------------------------------------------------
%\subsubsection[arity-checking]{Options to insert arity-checking code}
%\index{arity checking}
%
%The \tr{-darity-checks}\index{-darity-checks option} option inserts
%code to check for arity violations.  Unfortunately, it's not that
%simple: you have to link with a prelude that was also built with arity
%checks.  If you have one, then great; otherwise...
%
%The \tr{-darity-checks-C-only}\index{-darity-checks-C-only option}
%option inserts the self-same arity checking code into \tr{.hc} files,
%but doesn't compile it into the \tr{.o} files.  We use this flag with
%the \tr{-keep-hc-file-too}\index{-keep-hc-file-too option}, where we
%are keeping \tr{.hc} files around for debugging purposes.

%----------------------------------------------------------------------
%\subsubsection[omit-checking]{Options to omit checking code}
%\index{omitting runtime checks}
%
%By default, the GHC system emits all possible not-too-expensive
%runtime checking code.  If you are brave or experimenting, you might
%want to turn off some of this (not recommended):
%
%\begin{tabular}{ll}
%-dno-black-holing & won't buy you much (even if it works) \\
%-dno-updates & you're crazy if you do this \\
%-dno-stk-stubbing & omit stack stubbing (NOT DONE YET) \\
%\end{tabular}
%\index{-dno-black-holing option}%
%\index{-dno-updates option}%
%\index{-dno-stk-stubbing option}
%
%Warning: all very lightly tested, if at all...

%% %************************************************************************
%% %*                                                                   *
%% \subsection[options-GC]{Choosing a garbage collector}
%% %*                                                                   *
%% %************************************************************************
%% 
%% (Note: you need a Good Reason before launching into this territory.)
%% 
%% There are up to four garbage collectors to choose from (it depends how
%% your local system was built); the Appel-style generational collector
%% is the default.
%% 
%% If you choose a non-default collector, you must specify it both when
%% compiling the modules and when linking them together into an
%% executable.  Also, the native-code generator only works with the
%% default collector (a small point to bear in mind).
%% 
%% \begin{description}
%% \item[\tr{-gc-ap} option:]
%% \index{-gc-ap option}
%% Appel-like generational collector (the default).
%% 
%% \item[\tr{-gc-2s} option:]
%% \index{-gc-2s option}
%% Two-space copying collector.
%% 
%% \item[\tr{-gc-1s} option:]
%% \index{-gc-1s option}
%% One-space compacting collector.
%% 
%% \item[\tr{-gc-du} option:]
%% \index{-gc-du option}
%% Dual-mode collector (swaps between copying and compacting).
%% \end{description}