4 Feb
2008
4 Feb
'08
11:32 a.m.
On Mon, 2008-02-04 at 12:22 -0200, 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
was True regardless of 'm'. Is there any exception?
If there is, it's a bug in the library except you wouldn't normally use (==) but some meta-level equality*. From one perspective, liftM is the proof that every monad is a functor. * Usually observational equality, but one may want variants, e.g. observational equality with respect to some "observe" function