On 8/22/06, John Meacham <[EMAIL PROTECTED]> wrote:
I am not talking about type signatures, I am talking about having to
annotate in the middle of a term.
f x y | x `member` map g freeVars y = ....
having to become
f x y | x `member` map g (freeVars y :: [Id]) = ....
There is no need to write such types... In this particular case the
type of 'elem' indicates that the argument is a list. I don't think
that a polymorphic 'map' function requires any more signatures than,
say, '>>='. This certainly is not my experience when I use 'fmap'...
So, I am not saying renaming fmap to map is bad outright, I am just
saying that the question is trickier than just the error message problem
it was previously stated in terms of.
Do you have an example illustrating what is tricky about 'fmap'? As
far as I understand 'map' used to be polymorphic, and later the
distinction between 'map' and 'fmap' was specifically introduced to
avoid the error messages that may confuse beginners.
-Iavor
No, map was never overloaded--it was list comprehensions that were
overloaded as monad comprehensions in Haskell 1.4. That certainly did lead
to problems of exactly the sort John M is describing.
As for an example of fmap causing trouble, recall the code I posted last
week sometime:
class Foldable f where
fold :: (a -> a -> a) -> a -> f a -> a
instance Foldable [] where
fold = foldr
example = fold (+) 0 (fmap (+1) (return 2))
Here nothing fixes the type to be lists. When I posted this, someone called
it contrived because I wrote return 2 rather than [2], which would have
fixed the type of fmap to work over lists. But I don't think this is
contrived, except perhaps that I reused return from the Monad class, rather
than defining a new collection class with overloaded methods for both
creating a singleton collection and folding an operator over a collection.
This is a natural thing to do, in my opinion, and it leads directly to this
example.
John
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