Hi
Tools like Neil Mitchell's Catch can do more sophisticated checking, as long as your program can be compiled by YHC. Sometimes fromJust can be quite useful, though, especially in tandem with isJust. For example,
prop_foobar :: SomeType -> Property prop_foobar x = isJust (someTypeToMaybe x) ==> fromJust x == expectedResult
I was thinking of jumping in on this thread, to advertise Catch, but unfortunately "can be compiled with Yhc" makes it a fairly niche tool :-( However, the fundamental bit of Catch works on an a normal Core language, and I do want to hook it up to GHC's Core language at some point. It could certainly deal with prop_foobar, and things that are far more complex. To get an idea of what Catch can do, I recommend reading the draft paper and skipping to section 5.4 where I verify the entire HsColour program with one small refactoring and no annotations. I even found 3 real crashing bugs in HsColour, that were fixed because Catch pointed them out. Incomplete patterns + a checker (Catch) are just a faster and more efficient way of writing complete patterns without dead code :-) Catch: http://www-users.cs.york.ac.uk/~ndm/catch/ Thanks Neil