map and fmap
rjmh at cs.chalmers.se
Sun Aug 20 09:52:10 EDT 2006
From: "Jon Fairbairn" <jon.fairbairn at cl.cam.ac.uk>
> 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.
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.
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