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

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

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


modifyAt :: (MonadState a m, Box m i a b) => i -> (b -> m b) -> m ()
modifyAt i f = do
  a <- get
  r i a >>= f >>= w i a >>= put

instance (Box m i a b, Box m j b c) => Box m (i, j) a c