Start a Box module to describe inclusion relations between different types and get a MonadState action on the top type for any modification down there

Hufflepdf.cabal

@@ -17,6 +17,7 @@ cabal-version:       >=1.10
 
 library
   exposed-modules:     PDF
+                     , PDF.Box
                      , PDF.CMap
                      , PDF.Content
                      , PDF.EOL

src/PDF/Box.hs

@@ -0,0 +1,23 @@
+{-# 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