[Haskell-cafe] fmap vs. liftM

[Dan Weston]

Can you do this with a GHC rule? Something like:

{-# RULES
       "join_dot_fmap_return/id"  forall x . join (fmap return x) = x
#-}

Dan

Henning Thielemann wrote:
> On Tue, 5 Feb 2008, Brandon S. Allbery KF8NH wrote:
> 
>> On Feb 5, 2008, at 8:31 , Henning Thielemann wrote:
>>
>>> How do you convince the compiler that
>>>   'join (fmap return x) == x' ?
>> How do you convince it that the current formulation of Monad obeys
>> the monad laws?  (rhetorical)
> 
> My point was that the constraint 'liftM == fmap' cannot be expressed in
> Haskell 98, and the answer by Miguel suggested that it would be possible
> by a different design of class Monad. The above was my objection to this
> suggestion.
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