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?
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.
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