Recognize openStream was just an implementation of r for the Box m () Object ByteString, and extend it implementing the w operation while we're at it

examples/getObj.hs

@@ -6,10 +6,11 @@ import Data.ByteString.Lazy.Char8 (ByteString)
 import qualified Data.ByteString.Char8 as BS (readFile)
 import qualified Data.ByteString.Lazy.Char8 as Lazy (putStrLn)
 import PDF (Document(..), parseDocument)
+import PDF.Box (Box(..))
 import PDF.Layer (Layer(..), unify)
 import PDF.Object (Object(..))
 import PDF.Object.Navigation (
-    Error(..), (//), objectById, openStream, origin
+    Error(..), StreamContent(..), (//), objectById, origin
   )
 import PDF.Output (ObjectId(..), Output)
 import qualified PDF.Output as Output (render)
@@ -20,7 +21,7 @@ import Text.Read (readMaybe)
 
 decodedStream :: Object -> Object
 decodedStream object =
-  maybe object replaceContent $ openStream object
+  maybe object replaceContent $ r StreamContent object
   where
     replaceContent streamContent = object {streamContent}
 

src/PDF/Object/Navigation.hs

@@ -2,27 +2,30 @@
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE FlexibleInstances #-}
 module PDF.Object.Navigation (
       Error(..)
+    , StreamContent(..)
     , (./)
     , (//)
     , (>./)
     , (>//)
     , getDictionary
     , objectById
-    , openStream
     , origin
   ) where
 
-import Codec.Compression.Zlib (decompress)
+import Codec.Compression.Zlib (compress, decompress)
 import Control.Applicative (Alternative(..))
 import Control.Monad (MonadPlus(..))
 import Control.Monad.Reader (MonadReader(..))
 import Control.Monad.Fail (MonadFail(..))
 import Data.ByteString (ByteString)
-import qualified Data.ByteString.Lazy as Lazy (fromStrict, toStrict)
+import qualified Data.ByteString.Lazy as Lazy (ByteString, fromStrict, toStrict)
 import Data.Map ((!))
 import qualified Data.Map as Map (lookup)
+import PDF.Box (Box(..))
 import PDF.Layer (Layer(..))
 import PDF.Object (
       Dictionary, DirectObject(..), IndirectObjCoordinates(..)
@@ -81,10 +84,21 @@ objectById objectId = do
 origin :: PDFContent m => m Object
 origin = Direct . Dictionary . trailer . docStructure <$> ask
 
-openStream :: MonadFail m => Object -> m ByteString
-openStream (Stream {header, streamContent}) = return $
-  case Map.lookup (Name "Filter") header of
-    Just (NameObject (Name "FlateDecode")) ->
-      Lazy.toStrict . decompress . Lazy.fromStrict $ streamContent
-    _ -> streamContent
-openStream obj = expected "stream" obj
+data StreamContent = StreamContent
+
+onLazy :: (Lazy.ByteString -> Lazy.ByteString) -> ByteString -> ByteString
+onLazy f = Lazy.toStrict . f . Lazy.fromStrict
+
+instance MonadFail m => Box m StreamContent Object ByteString where
+  r StreamContent (Stream {header, streamContent}) = return $
+    case Map.lookup (Name "Filter") header of
+      Just (NameObject (Name "FlateDecode")) -> onLazy decompress streamContent
+      _ -> streamContent
+  r StreamContent obj = expected "stream" obj
+
+  w StreamContent streamContent obj@(Stream {header}) = return $
+    case Map.lookup (Name "Filter") header of
+      Just (NameObject (Name "FlateDecode")) ->
+        obj {streamContent = onLazy compress streamContent}
+      _ -> obj {streamContent}
+  w _ _ obj = expected "stream" obj

src/PDF/Pages.hs

@@ -13,6 +13,7 @@ import qualified Control.Monad.RWS as RWS (get)
 import Data.Map (Map)
 import qualified Data.Map as Map (empty, fromList, insert, lookup, toList)
 import Data.Text (Text)
+import PDF.Box (Box(..))
 import PDF.CMap (cMap)
 import qualified PDF.Content as Content (parse)
 import PDF.Content.Text (renderText)
@@ -24,7 +25,7 @@ import PDF.Object (
     , Name(..), Object(..)
   ,)
 import PDF.Object.Navigation (
-      Error(..), (//), (>./), (>//), getDictionary, objectById, openStream
+      Error(..), StreamContent(..), (//), (>./), (>//), getDictionary, objectById
     , origin
   )
 import PDF.Output (ObjectId(..))
@@ -56,7 +57,7 @@ loadFont :: ObjectId -> T Font
 loadFont objectId = objectById objectId >>= tryMappings
   where
     tryMappings object =
-        (object >./ "ToUnicode" >>= openStream >>= cMap)
+        (object >./ "ToUnicode" >>= r StreamContent >>= cMap)
       <|> (object >./ "Encoding" >>= loadEncoding)
       <|> (fail $ unknownFormat (show objectId) (show object))
     unknownFormat = printf "Unknown font format for object #%s : %s"
@@ -99,7 +100,7 @@ extractText pageObj = do
   where
     loadContent :: FontSet -> Object -> T [Text]
     loadContent fonts object =
-        openStream object
+        r StreamContent object
       >>= (either fail return . Content.parse)
       >>= renderText fonts