andrewcoppin:
I have a small idea. I'm curios if anybody else thinks it's a good idea...
How about a {-# IMPOSSIBLE #-} pragma that documents the fact that a particular point in the program *should* be unreachable?
For example, you look up a key in a Map. You know the key is there, because you just put it there yourself two seconds ago. But, theoretically, lookup returns a Maybe x so - theoretically - it's possible it might return Nothing. No matter what you do, GHC will insert code to check whether we have Just x or Nothing, and in the Nothing case throw an exception.
Obviously there is no way in hell this code path can ever be executed. At least, assuming your code isn't broken... This is where the pragma comes in. The idea is that you write something like
case lookup k m of Just v -> process v Nothing -> {-# IMPOSSIBLE "lookup in step 3" #-}
You could try using an 'assert' , which has the property of being compiled out at higher optimisation levels. And you can use 'error' or 'undefined' for undefined behaviour. http://www.haskell.org/ghc/docs/latest/html/users_guide/assertions.html Not very Haskelly though. -- Don