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
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