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.

"The compiler ran out of heap (or stack) when compiling itself!"

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.

"The compiler died with a pattern-matching error."

This is a bug just as surely as a "panic." Please report it.

"Some confusion about a value specialised to a type..." Huh???

(A deeply obscure and unfriendly error message.) This message crops up when the typechecker sees a reference in an interface pragma to a specialisation of an overloaded value (function); for example, `elem' specialised for type `[Char]' (`String'). The problem is: it doesn't know that such a specialisation exists! The cause of this problem is (please report any other cases...): The compiler has imported pragmatic info for the value in question from more than one interface, and the multiple interfaces did not agree exactly about the value's pragmatic info. Since the compiler doesn't know whom to believe, it believes none of them. The cure is to re-compile the modules that re-export the offending value (after possibly re-compiling its defining module). Now the pragmatic info should be exactly the same in every case, and things should be fine.

"Can't see the data constructors for a ccall/casm" Huh?

GHC "unboxes" C-call arguments and "reboxes" C-call results for you. To do this, it {@em has} to be able to see the types fully; abstract types won't do! Thus, if you say `data Foo = Foo Int#' (a cool "boxed primitive" type), but then make it abstract (only `data Foo' appears in the interface), then GHC can't figure out what to do with `Foo' arguments/results to C-calls. Solutions: either make the type unabstract, or compile with `-O'. With the latter, the constructor info will be passed along in the interface pragmas.

"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 these `trace' messages from GHC?"

Almost surely not a problem. About some specific cases...

Simplifier still going after N iterations:

Sad, but harmless. You can change the number with a `-fmax-simplifier-iterations<N>' option (no space); and you can see what actions took place in each iteration by turning on the `-fshow-simplifier-progress' option. If the simplifier definitely seems to be "looping," please report it.

"What about this warning from the C compiler?"

For example: "...warning: `Foo' declared `static' but never defined." Unsightly, but not 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 `-fno-implicit-prelude', 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'; 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. (Or maybe 25%; who knows?)

"Why did I get a link error?"

If the linker complains about not finding `_<something>_fast', then your interface files haven't settled -- keep on compiling! (In particular, this error means that arity information, which you can see in any `.hi' file, has changed.)

"What's a `consistency error'?"

(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...)

"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.