path: root/ghc/docs/users_guide/gone_wrong.vsgml

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<sect>What to do when something goes wrong
<label id="wrong">
<p>
<nidx>problems</nidx>
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%************************************************************************

If you still have a problem after consulting this section, then you
may have found a <em>bug</em>---please report it!  See Section <ref
name="How to report a bug in the GHC system" id="bug-reports"> for a
list of things we'd like to know about your bug.  If in doubt, send a
report---we love mail from irate users :-!

(Section <ref name="Haskell 98 vs. Glasgow Haskell: language
non-compliance" id="vs-Haskell-defn">, which describes Glasgow
Haskell's shortcomings vs.~the Haskell language definition, may also
be of interest.)

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<sect1>When the compiler ``does the wrong thing''
<label id="wrong-compiler">
<p>
<nidx>compiler problems</nidx>
<nidx>problems with the compiler</nidx>
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%************************************************************************

<descrip>
%-------------------------------------------------------------------
<tag>``Help! The compiler crashed (or `panic'd)!''</tag>
These events are <em>always</em> bugs in the GHC system---please report
them.

%-------------------------------------------------------------------
<tag>``The compiler ran out of heap (or stack) when compiling itself!''</tag>
It happens.  We try to supply reasonable @-H<n>@ flags for
@ghc/compiler/@ and @ghc/lib/@, but GHC's memory consumption
can vary by platform (e.g., on a 64-bit machine).

Just say @make all EXTRA_HC_OPTS=-H<a reasonable number>@ and see
how you get along.

Note that this is less likely to happen if you are compiling with GHC
4.00 or later, since the introduction of the dynamically expanding
heap.

%-------------------------------------------------------------------
<tag>``The compiler died with a pattern-matching error.''</tag>
This is a bug just as surely as a ``panic.'' Please report it.

%-------------------------------------------------------------------
<tag>``This is a terrible error message.''</tag>
If you think that GHC could have produced a better error message,
please report it as a bug.

%-------------------------------------------------------------------
<tag>``What about these `trace' messages from GHC?''</tag>
Almost surely not a problem.  About some specific cases...
<descrip>
<tag>Simplifier still going after N iterations:</tag>
Sad, but harmless.  You can change the number with a
@-fmax-simplifier-iterations<N>@<nidx>-fmax-simplifier-iterations&lt;N&gt; option</nidx> option (no space);
and you can see what actions took place in each iteration by
turning on the @-fshow-simplifier-progress@
<nidx>-fshow-simplifier-progress option</nidx> option.

If the simplifier definitely seems to be ``looping,'' please report
it.
</descrip>

%-------------------------------------------------------------------
<tag>``What about this warning from the C compiler?''</tag>

For example: ``...warning: `Foo' declared `static' but never defined.''
Unsightly, but shouldn't be a problem.

%-------------------------------------------------------------------
<tag>Sensitivity to @.hi@ interface files:</tag>

GHC is very sensitive about interface files.  For example, if it picks
up a non-standard @Prelude.hi@ file, pretty terrible things will
happen.  If you turn on
@-fno-implicit-prelude@<nidx>-fno-implicit-prelude option</nidx>, the
compiler will almost surely die, unless you know what you are doing.

Furthermore, as sketched below, you may have big problems
running programs compiled using unstable interfaces.

%-------------------------------------------------------------------
<tag>``I think GHC is producing incorrect code'':</tag>

Unlikely :-) A useful be-more-paranoid option to give to GHC is
@-dcore-lint@<nidx>-dcore-lint option</nidx>; this causes a ``lint''
pass to check for errors (notably type errors) after each Core-to-Core
transformation pass.  We run with @-dcore-lint@ on all the time; it
costs about 5\% in compile time.

%-------------------------------------------------------------------
<tag>``Why did I get a link error?''</tag>

If the linker complains about not finding @_<something>_fast@, then
something is inconsistent: you probably didn't compile modules in the
proper dependency order.

%-------------------------------------------------------------------
<tag>``What's a `consistency error'?''</tag>
(These are reported just after linking your program.)

You tried to link incompatible object files, e.g., normal ones
(registerised, Appel garbage-collector) with profiling ones (two-space
collector).  Or those compiled by a previous version of GHC
with an incompatible newer version.

If you run @nm -o *.o | egrep 't (cc|hsc)\.'@ (or, on
unregisterised files: @what *.o@), you'll see all the consistency
tags/strings in your object files.  They must all be the same!
(ToDo: tell you what they mean...)

%-------------------------------------------------------------------
<tag>``Is this line number right?''</tag>
On this score, GHC usually does pretty well, especially
if you ``allow'' it to be off by one or two.  In the case of an
instance or class declaration, the line number
may only point you to the declaration, not to a specific method.

Please report line-number errors that you find particularly unhelpful.
</descrip>

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%*                                                                      *
<sect1>When your program ``does the wrong thing''
<label id="wrong-compilee">
<p>
<nidx>problems running your program</nidx>
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%************************************************************************

(For advice about overly slow or memory-hungry Haskell programs,
please see Section <ref name="Advice on: sooner, faster, smaller,
stingier" id="sooner-faster-quicker">).

<descrip>
%-----------------------------------------------------------------------
<tag>``Help! My program crashed!''</tag>
(e.g., a `segmentation fault' or `core dumped')
<nidx>segmentation fault</nidx>

If your program has no @_ccall_@s/@_casm_@s in it, then a crash is
always a BUG in the GHC system, except in one case: If your program is
made of several modules, each module must have been compiled after any
modules on which it depends (unless you use @.hi-boot@ files, in which
case these <em/must/ be correct with respect to the module source). 

For example, if an interface is lying about the type of an imported
value then GHC may well generate duff code for the importing module.
<em>This applies to pragmas inside interfaces too!</em>  If the pragma is
lying (e.g., about the ``arity'' of a value), then duff code may result.
Furthermore, arities may change even if types do not.

In short, if you compile a module and its interface changes, then all
the modules that import that interface <em>must</em> be re-compiled.

A useful option to alert you when interfaces change is
@-hi-diffs@<nidx>-hi-diffs option</nidx>.  It will run @diff@ on the
changed interface file, before and after, when applicable.

If you are using @make@, a useful tool to make sure that every module
<em>is</em> up-to-date with respect to its imported interfaces is
@mkdependHS@ (which comes with GHC).  Please see Section <ref
name="Makefile dependencies in Haskell: using mkdependHS"
id="mkdependHS">.

If you are down to your last-compile-before-a-bug-report, we would
recommend that you add a @-dcore-lint@ option (for extra checking) to
your compilation options.

So, before you report a bug because of a core dump, you should probably:
<tscreen><verb>
% rm *.o        # scrub your object files
% make my_prog  # re-make your program; use -hi-diffs to highlight changes;
		# as mentioned above, use -dcore-lint to be more paranoid
% ./my_prog ... # retry...
</verb></tscreen>

Of course, if you have @_ccall_@s/@_casm_@s in your program then all
bets are off, because you can trash the heap, the stack, or whatever.

If you are interested in hard-core debugging of a crashing
GHC-compiled program, please see Section <ref name="Hard-core
debugging of GHC-compiled programs" id="hard-core-debug">.

%-------------------------------------------------------------------
<tag>``My program entered an `absent' argument.''</tag>
This is definitely caused by a bug in GHC. Please report it.

%-----------------------------------------------------------------------
<tag>``What's with this `arithmetic (or `floating') exception' ''?</tag>

@Int@, @Float@, and @Double@ arithmetic is <em>unchecked</em>.
Overflows, underflows and loss of precision are either silent or
reported as an exception by the operating system (depending on the
architecture).  Divide-by-zero <em>may</em> cause an untrapped
exception (please report it if it does).

</descrip>

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%*                                                                      *
<sect1>How to report a bug in the GHC system
<label id="bug-reports">
<p>
<nidx>bug reports</nidx>
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%************************************************************************

Glasgow Haskell is a changing system so there are sure to be bugs in
it.  Please report them to <htmlurl
name="glasgow-haskell-bugs@@haskell.org"
url="mailto:glasgow-haskell-bugs@@haskell.org">!  (However, please
check the earlier part of this section to be sure it's not a known
not-really-a problem.)

The name of the bug-reporting game is: facts, facts, facts.
Don't omit them because ``Oh, they won't be interested...''

<enum>

<item> What kind of machine are you running on, and exactly what
version of the operating system are you using? (@uname -a@ or @cat
/etc/motd@ will show the desired information.)

<item> What version of GCC are you using? @gcc -v@ will tell you.

<item> Run the sequence of compiles/runs that caused the offending
behaviour, capturing all the input/output in a ``script'' (a UNIX
command) or in an Emacs shell window.  We'd prefer to see the whole
thing.

<item> Be sure any Haskell compilations are run with a @-v@ (verbose)
flag, so we can see exactly what was run, what versions of things you
have, etc.

<item> What is the program behaviour that is wrong, in your opinion?

<item> If practical, please send enough source files/interface files
for us to duplicate the problem.

<item> If you are a Hero and track down the problem in the
compilation-system sources, please send us patches relative to a known
released version of GHC, or whole files if you prefer.

</enum>

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<sect1>Hard-core debugging of GHC-compiled programs
<label id="hard-core-debug">
<p>
<nidx>debugging, hard-core</nidx>
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%************************************************************************

If your program is crashing, you should almost surely file a bug
report, as outlined in previous sections.

This section suggests ways to Make Further Progress Anyway.

The first thing to establish is: Is it a garbage-collection (GC) bug?
Try your program with a very large heap and a @-Sstderr@ RTS
flag.
<itemize>
<item>
If it crashes <em>without</em> garbage-collecting, then it is
definitely <em>not</em> a GC bug.
<item>
If you can make it crash with one heap size but not with another, then
it <em>probably is</em> a GC bug.
<item>
If it crashes with the normal
collector, but not when you force two-space collection (@-F2s@
runtime flag), then it <em>probably is</em> a GC bug.
</itemize>

If it <em>is</em> a GC bug, you may be able to avoid it by using a
particular heap size or by using a @-F2s@ runtime flag.  (But don't
forget to report the bug!!!)

ToDo: more here?