On 2006-08-20, John Hughes
From: "Jon Fairbairn"
To reinforce what Aaron said, if a programme works now, it'll still work if map suddenly means fmap.
Well, this isn't quite true, is it? Here's an example:
class Foldable f where fold :: (a -> a -> a) -> a -> f a -> a
instance Foldable [] where fold = foldr
example = fold (+) 0 (map (+1) (return 2))
example has the value 3 (of course), but if you replace map by fmap then the code no longer compiles.
Solely due to the compiler no longer seeing that list is the only intermediate type allowed. But you have to admit this code is a bit forced. People won't be combining things quite this way, and will be passing in values rather than bare returns.
In any case, I'm dubious about this as a criterion. I would guess that the majority if compiler runs for beginners (and perhaps for the rest of us too!) end in a type error, not a successful compilation, so arguably the quality of error messages when a type-check fails is more important than which programs compile.
Right, like I said, we need to work on better error messages. -- Aaron Denney -><-