module Control.Monad.Reader where

import Data.Functor (class Functor)
import Control.Monad (class Applicative, class Monad)

data Reader e a = Reader (e -> a)

runReader :: forall e a. Reader e a -> e -> a
runReader (Reader r) = r

instance Functor (Reader e) where
  map f (Reader r) = Reader (\e -> f (r e))

instance Applicative (Reader e) where
  pure a = Reader (\_ -> a)
  apply (Reader f) (Reader r) = Reader (\e -> (f e) (r e))

instance Monad (Reader e) where
  bind (Reader r) f = Reader (\e -> runReader (f (r e)) e)

ask :: forall e. Reader e e
ask = Reader (\e -> e)

local :: forall e a. (e -> a) -> Reader e a
local f = Reader (\e -> f e)