module Control.Monad.Writer where

import Data.Unit (Unit, unit)
import Data.Semigroup ((<>))
import Data.Monoid (class Monoid, mempty)
import Data.Functor (class Functor)
import Control.Monad (class Applicative, class Monad)

data Writer w a = Writer (a, w)

runWriter :: forall w a. Writer w a -> (a, w)
runWriter (Writer x) = x

instance Functor (Writer w) where
  map f (Writer (a, w)) = Writer (f a, w)

instance Monoid w => Applicative (Writer w) where
  pure a = Writer (a, mempty)
  apply (Writer (f, u)) (Writer (a, v)) =
    Writer (f a, u <> v)

instance Monoid w => Monad (Writer w) where
  bind (Writer (a, u)) f =
    let (Writer (b, v)) = f a in 
    Writer (b, u <> v)

tell :: forall w. w -> Writer w ()
tell w = Writer ((), w)

listen :: forall w a. Writer w a -> Writer w (a, w)
listen (Writer (a, w)) = Writer ((a, w), w)

pass :: forall w a. Writer w (a, w -> w) -> Writer w a
pass (Writer ((a, f), w)) = Writer (a, f w)