A complex program of mine fails with this message:

Fail: Maybe.fromJust: Nothing

I tried to extract more information about the error by compiling with -prof -auto-all and running the program with +RTS -xc, as advised on http://www.haskell.org/hawiki/TipsAndTricks . This yielded exactly one additional line:

. Fail: Maybe.fromJust: Nothing

(Same result for GHC 6.0.1 and 6.2.)

That is rather unfortunate, but it's the "correct" behaviour. fromJust is defined like this: fromJust Nothing = error "Maybe.fromJust: Nothing" fromJust (Just a) = a and GHC has compiled it like this: _lift = error "Maybe.fromJust: Nothing" fromJust Nothing = _lift fromJust (Just a) = a so that it can inline fromJust without creating too much code explosion. Unfortunately, this means that _lift is a CAF, and all CAF evaluation appears at the top level in the cost-centre-stack semantics (see the section on profiling in the User's Guide for details). There's no obvious way to work around this, I'm afraid. Cheers, Simon