> If your datatype with a Monad instance also has a Functor instance (which it
> *can* have, you just need to declare the instance), then liftM is equivalent
> to fmap.
Only if you ignore efficiency issues, of course. Some monads have an
fmap which is significantly faster than bind.
liftM f m = do
a <- m
return (f a)
Consider []; this becomes
liftM f m
= m >>= \a -> return (f a)
= concatMap (\a -> [f a]) m
which, in the absence of other optimizations, is going to do a lot
more allocation and branching than fmap
fmap f m
= map f m
-- ryan
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