Re: map and fmap
On 8/20/06, John Hughes <[EMAIL PROTECTED]> wrote: From: "Jon Fairbairn" <[EMAIL PROTECTED]> > 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. There's a proposal http://hackage.haskell.org/trac/haskell-prime/wiki/Defaulting that mentions extending defaulting to other typeclasses. That seems to fix this particular problem, but above you mentioned that this was "a whole new can of worms." Could you elaborate or point me to a discussion of the worms? Thanks, Jeffrey ___ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
Re: map and fmap
On 2006-08-20, John Hughes <[EMAIL PROTECTED]> wrote: > > From: "Jon Fairbairn" <[EMAIL PROTECTED]> > >> 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 -><- ___ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
Re: map and fmap
From: "Jon Fairbairn" <[EMAIL PROTECTED]> 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. 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. John ___ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
Re: map and fmap
From: "Jon Fairbairn" <[EMAIL PROTECTED]> 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. 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. John ___ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
Re: map and fmap
From: "Jon Fairbairn" <[EMAIL PROTECTED]> 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. 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. John ___ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
