On Sun, Jan 18, 2009 at 3:23 AM, Andrew Coppin
<andrewcoppin@btinternet.com> wrote:
Jonathan Cast wrote:
On Sat, 2009-01-17 at 12:04 +0000, Andrew Coppin wrote:
instance (Monad m) => Functor m where
fmap f ma = do a <- ma; return (f a)
While that's quite interesting from a mathematical point of view, how is this "useful" for programming purposes?
Good Lord. fmap (as above) is *at least* useful enough to be in the
standard library! (Control.Monad.liftM).
Given that liftM exists, why is having an identical implementation for fmap useful?
Because liftM works on Monads and fmap works on Functors?
I believe you can make data that are Functors but are not Monads.
The example that leaps out at me is that (>>=) is identical to concatMap within the list monad. But using lists as a monad is a generally useful thing to do, and being able to substitute arbitrary monads has obvious utility. I'm not seeing how being able to treat something that isn't a container as if it was a container is useful.