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
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