map and fmap

John Hughes rjmh at
Mon Aug 28 15:41:08 EDT 2006

> On 8/22/06, John Meacham <john at> 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 


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