Add a simple Box instance that exposes IndexedInstructions within a Content

src/PDF/Content.hs

@@ -2,6 +2,9 @@
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE TypeSynonymInstances #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
 module PDF.Content (
       Content(..)
     , ContentUnit(..)
@@ -20,13 +23,14 @@ import Data.Attoparsec.ByteString.Char8 (sepBy)
 import Data.ByteString (ByteString)
 import Data.Map (Map, (!))
 import qualified Data.Map as Map (empty, insert, size)
+import PDF.Box (Box(..))
 import PDF.Content.Operator (Instruction, operator)
 import PDF.Object (blank, directObject)
 import PDF.Output (Output(..), line)
 import PDF.Parser (MonadParser, evalParser, string)
 
 newtype InstructionId = InstructionId Int deriving (Eq, Ord, Show)
-
+data Instructions = Instructions
 type IndexedInstructions = Map InstructionId Instruction
 
 data GraphicContextUnit =
@@ -45,6 +49,11 @@ data Content = Content {
 
 type InstructionParser m = (MonadParser m, MonadState IndexedInstructions m)
 
+instance Monad m => Box m Instructions Content IndexedInstructions where
+  r Instructions = return . indexedInstructions
+  w Instructions indexedInstructions someContent =
+    return $ someContent {indexedInstructions}
+
 register :: MonadState IndexedInstructions m => Instruction -> m InstructionId
 register newInstruction = do
   newInstructionID <- gets (InstructionId . Map.size)