18. What to do when something goes wrong

If you still have a problem after consulting this section, then you may have found a bug—please report it! See Reporting bugs in GHC for details on how to report a bug and a list of things we’d like to know about your bug. If in doubt, send a report — we love mail from irate users :-!

(Haskell standards vs. Glasgow Haskell: language non-compliance, which describes Glasgow Haskell’s shortcomings vs. the Haskell language definition, may also be of interest.)

18.1. When the compiler “does the wrong thing”

“Help! The compiler crashed (or panic’d)!”

These events are always bugs in the GHC system—please report them.

“This is a terrible error message.”

If you think that GHC could have produced a better error message, please report it as a bug.

“What about this warning from the C compiler?”

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

Sensitivity to .hi interface files

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 -XNoImplicitPrelude-XNoImplicitPrelude option, 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.

“I think GHC is producing incorrect code”

Unlikely :-) A useful be-more-paranoid option to give to GHC is -dcore-lint-dcore-lint option; 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.

Why did I get a link error?

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

“Is this line number right?”

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.

18.2. When your program “does the wrong thing”

(For advice about overly slow or memory-hungry Haskell programs, please see Advice on: sooner, faster, smaller, thriftier).

“Help! My program crashed!”

(e.g., a “segmentation fault” or “core dumped”) segmentation fault

If your program has no foreign calls in it, and no calls to known-unsafe functions (such as unsafePerformIO) 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 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. This applies to pragmas inside interfaces too! 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 must be re-compiled.

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

If you are using make, GHC can automatically generate the dependencies required in order to make sure that every module is up-to-date with respect to its imported interfaces. Please see Dependency generation.

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:

% rm *.o        # scrub your object files
% make my_prog  # re-make your program; use -ddump-hi-diffs to highlight changes;
                # as mentioned above, use -dcore-lint to be more paranoid
% ./my_prog ... # retry...

Of course, if you have foreign calls in your program then all bets are off, because you can trash the heap, the stack, or whatever.

“My program entered an ‘absent’ argument.”

This is definitely caused by a bug in GHC. Please report it (see Reporting bugs in GHC).

“What’s with this arithmetic (or floating-point) exception?”

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