{-# LANGUAGE NamedFieldPuns #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE UndecidableInstances #-}
module PDF.Box (
      Box(..)
    , (.@)
    , edit
    , modifyAt
  ) where

import Control.Monad.State (MonadState(..))

class Monad m => Box m i a b | m a i -> b where
  r :: i -> a -> m b
  w :: i -> a -> b -> m a

edit :: Box m i a b => i -> (b -> m b) -> a -> m a
edit i f a =
  r i a >>= f >>= w i a

modifyAt :: (MonadState a m, Box m i a b) => i -> (b -> m b) -> m ()
modifyAt i f = get >>= edit i f >>= put

instance (Box m i a b, Box m j b c) => Box m (i, j) a c where
  r (i, j) a = r i a >>= r j
  w (i, j) a c = r i a >>= flip (w j) c >>= w i a

infixr 6 .@
(.@) :: a -> b -> (a, b)
(.@) = (,)