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

[Daniel Fischer]

Am Freitag 23 Oktober 2009 17:25:57 schrieb Darrin Thompson:
> On Fri, Oct 23, 2009 at 10:25 AM, pl <pl.listas at gmail.com> 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