Generalize register to all IdMap a b, since it's gonna be needed by Indexed Text too

src/Data/Id.hs

@@ -1,4 +1,5 @@
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE FlexibleContexts #-}
 module Data.Id (
       Id(..)
     , IdMap
@@ -13,15 +14,17 @@ module Data.Id (
     , mapWithKey
     , member
     , minViewWithKey
+    , register
     , singleton
     , size
     , union
   ) where
 
+import Control.Monad.State (MonadState, modify, gets)
 import Data.IntMap (IntMap, (!))
 import qualified Data.IntMap as IntMap (
-      delete, empty, fromList, keysSet, insert, lookup, mapWithKey, member
-    , minViewWithKey, size, union
+      delete, empty, fromList, keysSet, insert, lookup, mapWithKey, maxViewWithKey
+    , member, minViewWithKey, size, union
   )
 import Data.IntSet (IntSet)
 import Prelude hiding (lookup)
@@ -75,3 +78,9 @@ fromList = IdMap . IntMap.fromList . fmap (\(key, b) -> (getId key, b))
 
 keysSet :: IdMap a b -> IntSet
 keysSet = IntMap.keysSet . intMap
+
+register :: MonadState (IdMap a b) m => b -> m (Id a)
+register b = do
+  newId <- gets (Id . maybe 0 ((+1) . fst . fst) . IntMap.maxViewWithKey . intMap)
+  modify (insert newId b)
+  return newId

src/PDF/Content.hs

@@ -16,10 +16,10 @@ module PDF.Content (
 import Control.Applicative ((<|>))
 import Control.Monad.Fail (MonadFail)
 import Control.Monad.Reader (asks, runReader)
-import Control.Monad.State (evalStateT, gets, modify)
+import Control.Monad.State (evalStateT, modify)
 import Data.Attoparsec.ByteString.Char8 (sepBy)
 import Data.ByteString (ByteString)
-import Data.Id (Id(..), Indexed, at, empty, insert, size)
+import Data.Id (Id(..), Indexed, at, empty, register)
 import PDF.Box (Box(..))
 import PDF.Content.Operator (Instruction, operator)
 import PDF.Object (blank, directObject)
@@ -49,12 +49,6 @@ instance Monad m => Box m Instructions Content (Indexed Instruction) where
   w Instructions indexedInstructions someContent =
     return $ someContent {indexedInstructions}
 
-register :: Instruction -> InstructionParser (Id Instruction)
-register newInstruction = do
-  newInstructionID <- gets (Id . size)
-  modify (insert newInstructionID newInstruction)
-  return newInstructionID
-
 parse :: MonadFail m => ByteString -> m Content
 parse =
   either fail (return . uncurry Content) . runParser contentUnits empty