Both of these messages (from Neil and Serge) suggest use debugging ideas. Would anyone like to add them to http://haskell.org/haskellwiki/Debugging Simon | Use a safe module: | http://neilmitchell.blogspot.com/2006/11/library-idea-safe-library.html | - always works, a little bit of effort (I have such a module in my | code). You then get: | | f x = h $ fromJustNote "Foo.f" $ g x | | Use Hat: http://haskell.org/hat - sometimes works | | Use Catch to prove the absence of pattern match errors: | http://www-users.cs.york.ac.uk/~ndm/projects/catch.php - I'd say this | is still too immature for anyone but me to use, but it can be useful. | | Use Yhi-stack - still unreleased, but gives out a stack trace on | crashing. I am trying to push for this to be released soon! | | Thanks | | More about finding the source of fromJust Nothing. | | For g n = fromJust $ f n, | | ghc-6.6 often looses the reference to f in its run-time error | report -- when f returns Nothing. | And this is difficult to locate the source. | But one could write | g n = let Just m = f n in m, | | for which GHC reports | Main: M1.hs:9:11-22: | Irrefutable pattern failed for pattern Data.Maybe.Just m | | -- it points to the source line!