It turns out Output.concat wasn't necessary, OBuilder seems already is a Monoid so mconcat works (that fact was used in the very implementation of concat…)

src/PDF/Object.hs

@@ -44,7 +44,7 @@ import qualified Data.Map as Map (
     delete, empty, fromList, lookup, minViewWithKey, toList, union
   )
 import qualified PDF.EOL as EOL (charset, parser)
-import qualified PDF.Output as Output (concat, line, string)
+import qualified PDF.Output as Output (line, string)
 import PDF.Output (
       OBuilder, ObjectId(..), Offset(..), Output(..), Resource(..)
     , byteString, getObjectId, getOffset, getOffsets, join, newLine
@@ -167,7 +167,7 @@ instance Output Dictionary where
     where
       keyValues = join " " $ outputKeyVal <$> Map.toList aDictionary
       outputKeyVal :: (Name, DirectObject) -> OBuilder
-      outputKeyVal (key, val) = Output.concat [output key, " ", output val]
+      outputKeyVal (key, val) = mconcat [output key, " ", output val]
 
 dictionary :: MonadParser m => m Dictionary
 dictionary =
@@ -213,7 +213,7 @@ instance Output DirectObject where
   output (NumberObject n) = output n
   output (StringObject s) = output s
   output (NameObject n) = output n
-  output (Array a) = Output.concat ["[", join " " a, "]"]
+  output (Array a) = mconcat ["[", join " " a, "]"]
   output (Dictionary d) = output d
   output (Null) = "null"
   output (Reference (IndirectObjCoordinates {objectId, versionNumber})) =
@@ -244,7 +244,7 @@ data Object =
 
 instance Output Object where
   output (Direct d) = output d
-  output (Stream {header, streamContent}) = Output.concat [
+  output (Stream {header, streamContent}) = mconcat [
         output header, newLine
       , Output.line "stream"
       , byteString streamContent
@@ -259,7 +259,7 @@ data Occurrence = Comment String | Indirect IndirectObjCoordinates deriving Show
 outputOccurrence :: IndexedObjects -> Occurrence -> OBuilder
 outputOccurrence _ (Comment c) = Output.line c
 outputOccurrence objects (Indirect (IndirectObjCoordinates {objectId, versionNumber})) =
-  saveOffset (Object objectId) >> Output.concat [
+  saveOffset (Object objectId) >> mconcat [
         Output.line (printf "%d %d obj" (getObjectId objectId) versionNumber)
       , output (objects ! objectId), newLine
       , Output.line "endobj"
@@ -392,7 +392,7 @@ outputBody (occurrences, objects) =
 instance Output Content where
   output (Content {occurrences, objects, docStructure}) =
     fmap (updateXrefs xRef) <$> getOffsets (outputBody (occurrences, objects))
-    >>= \(body, (xref, startXRef)) -> Output.concat [
+    >>= \(body, (xref, startXRef)) -> mconcat [
           body
         , Output.line "xref"
         , output xref

src/PDF/Object/Navigation.hs

@@ -3,9 +3,11 @@
 {-# LANGUAGE FlexibleContexts #-}
 module PDF.Object.Navigation (
       (//)
+    , dictionaryById
     , getDictionary
     , getField
-    , getObject
+    , follow
+    , objectById
     , openStream
     , origin
   ) where
@@ -27,29 +29,42 @@ import Text.Printf (printf)
 
 type PDFContent m = (MonadReader Content m, MonadFail m)
 
-expected :: (PDFContent m, Show a) => String -> a -> m b
+castDictionary :: MonadFail m => Object -> m Dictionary
+castDictionary (Direct (Dictionary aDict)) = return aDict
+castDictionary obj = expected "dictionary : " obj
+
+castObjectId :: MonadFail m => DirectObject -> m ObjectId
+castObjectId (Reference (IndirectObjCoordinates {objectId})) = return objectId
+castObjectId directObject = expected "reference" directObject
+
+dictionaryById :: PDFContent m => ObjectId -> m Dictionary
+dictionaryById objectId = objectById objectId >>= castDictionary
+
+expected :: (MonadFail m, Show a) => String -> a -> m b
 expected name = fail . printf "Not a %s: %s" name . show
 
-getField :: MonadFail m => String -> Dictionary -> m DirectObject
-getField key aDictionary =
-  maybe (fail errorMessage) return (Map.lookup (Name key) aDictionary)
+lookupField :: String -> Dictionary -> Either String DirectObject
+lookupField key aDictionary =
+  maybe (Left errorMessage) Right (Map.lookup (Name key) aDictionary)
   where
     errorMessage =
       printf "Key %s not found in dictionary %s" key (show aDictionary)
 
-castDictionary :: PDFContent m => Object -> m Dictionary
-castDictionary (Direct (Dictionary aDict)) = return aDict
-castDictionary obj = expected "dictionary : " obj
+getField :: MonadFail m => String -> Dictionary -> m DirectObject
+getField key = either fail return . lookupField key
 
-getObject :: PDFContent m => ObjectId -> m Object
-getObject objectId = do
+follow :: PDFContent m => DirectObject -> m Object
+follow directObject = castObjectId directObject >>= objectById
+
+objectById :: PDFContent m => ObjectId -> m Object
+objectById objectId = do
   content <- ask
   return (objects content ! objectId)
 
 getDictionary :: PDFContent m => DirectObject -> m Dictionary
 getDictionary (Dictionary aDictionary) = return aDictionary
 getDictionary (Reference (IndirectObjCoordinates {objectId})) =
-  getObject objectId >>= castDictionary
+  objectById objectId >>= castDictionary
 getDictionary aDirectObject =
   expected "resource (dictionary or reference)" aDirectObject
 
@@ -61,7 +76,7 @@ getDictionary aDirectObject =
 origin :: PDFContent m => m Dictionary
 origin = trailer . docStructure <$> ask
 
-openStream :: PDFContent m => Object -> m ByteString
+openStream :: MonadFail m => Object -> m ByteString
 openStream (Stream {header, streamContent}) = return $
   case Map.lookup (Name "Filter") header of
     Just (NameObject (Name "FlateDecode")) ->

src/PDF/Output.hs

@@ -12,7 +12,6 @@ module PDF.Output (
     , Resource(..)
     , byteString
     , char
-    , concat
     , getOffsets
     , join
     , line
@@ -32,7 +31,6 @@ import qualified Data.Map as Map (singleton)
 import Data.String (IsString(..))
 import Control.Monad.RWS (RWS, runRWS, ask, get, listen, modify, tell)
 import qualified PDF.EOL as EOL (Style(..))
-import Prelude hiding (concat)
 
 newtype ObjectId = ObjectId {getObjectId :: Int} deriving (Enum, Eq, Ord, Show)
 newtype Offset = Offset {getOffset :: Int} deriving (Show)
@@ -66,9 +64,6 @@ getOffsets (OContext builder) =
 append :: OBuilder -> OBuilder -> OBuilder
 append (OContext a) (OContext b) = OContext (mappend <$> a <*> b)
 
-concat :: [OBuilder] -> OBuilder
-concat = foldl mappend mempty
-
 #if MIN_VERSION_base(4,11,0)
 instance Semigroup OBuilder where
   (<>) = append
@@ -94,7 +89,7 @@ instance Output Bool where
   output True = string "true"
 
 instance Output a => Output [a] where
-  output = concat . fmap output
+  output = mconcat . fmap output
 
 join :: Output a => String -> [a] -> OBuilder
 join _ [] = mempty