On 4 Feb 2008, at 6:22 AM, Felipe Lessa wrote:
Hi there,
Reading http://www.haskell.org/haskellwiki/Things_to_avoid I found an interesting saying:
"By the way, in the case of IO monad the Functor class method fmap and the Monad based function liftM are the same."
I always tought that
prop :: (Functor m, Monad m, Eq (m b)) => (a -> b) -> m a -> Bool prop f x = fmap f x == liftM f x
Indeed, this is an equation from the equivalence of Haskell and category-theoretic monads. Furthermore, the same thing is explicitly asserted by the Haskell 98 Report: [1] Instances of both Monad and Functor should additionally satisfy the law: fmap f xs = xs >>= return . f
was True regardless of 'm'. Is there any exception?
There is only one case I'm aware of: if the author of the type forgot to define a functor instance for his monad (I've been guilty of that before, at least). jcc (I fixed the wiki, btw.) [1] http://haskell.org/onlinereport/basic.html#sect6.3.6