On Fri, Aug 22, 2008 at 2:07 PM, Quergle Quergle
Hello,
I'm a bit perplexed by what it means to do something like "ap = liftM2 id"
liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r id :: a -> a liftM2 id :: (Monad m) => m (a2 -> r) -> m a2 -> m r
My intuitive understanding is that liftM2 is expecting a two-argument function as its first argument, so it seems a little unexpected to pass it "id", a one-argument function. And I can't really see how the type signature of "liftM2 id" is derived from "liftM2" and "id". Any help much appreciated!
Just remember that 'a1 -> a2 -> r' is equivalent to '(a1) -> (a2 -> r)', and unifying this type with 'a -> a' gives a1 = a2 -> r So, liftM2's type becomes ((a2 -> r) -> a2 -> r) -> m (a2 -> r) -> m a2 -> m r Is it clearer now? HTH, -- Felipe.