Re: [Haskell-beginners] \x -> x < 0.5 && x > -0.5

Am Freitag 23 Oktober 2009 17:25:57 schrieb Darrin Thompson:

On Fri, Oct 23, 2009 at 10:25 AM, pl wrote:

...

   filter ((<=0.5) . abs) xs

pure (&&) <*> (< 0.5) <*> (> -0.5)

liftM2 (&&) (< 0.5) (> -0.5)

Someone suggested that this was an example of the reader monad but I don't get that.

It's because ((->) r) *is* the reader monad. Control.Monad.Reader's Reader r a is just that wrapped in a newtype: newtype Reader r a = Reader { runReader :: r -> a }

...

:i (->)

data (->) a b -- Defined in GHC.Prim instance Monad ((->) r) -- Defined in Control.Monad.Instances instance Functor ((->) r) -- Defined in Control.Monad.Instances instance Applicative ((->) a) -- Defined in Control.Applicative

That's what I see working here.

-- Darrin