Am Montag 19 Oktober 2009 17:24:12 schrieb Jordan Cooper:
Whoa... how on earth does this work? How does it interpret the sections as Reader monads?
liftM2 (&&) :: (Monad m) => m Bool -> m Bool -> m Bool So for (liftM2 (&&) (< 0.5) (> -0.5)) to be well typed, we must have (< 0.5) :: m Bool for some Monad m (same for (> -0.5)). Now (< 0.5) :: Fractional a => a -> Bool, hence m must be ((->) a) for some a in the Fractional class. Reader r a is just (r -> a) wrapped in a newtype, so (r -> a) 'is' the reader monad. To use it, we must import Control.Monad for liftM2 and some module which brings the instance Monad ((->) r) into scope. That could be Control.Monad.Instances or, appropriately, Control.Monad.Reader.
That's a job for the reader monad.
Lambda Fu, form 53 - silent reader of truth
import Control.Monad import Control.Monad.Reader
filter (liftM2 (&&) (< 0.5) (> -0.5)) xs
Regards, apfelmus