Move Error type from Pages to Navigation as a candidate for MonadFail required by PDFContent defined there

src/PDF/Object/Navigation.hs

@@ -1,8 +1,10 @@
 {-# LANGUAGE NamedFieldPuns #-}
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
 module PDF.Object.Navigation (
-      (//)
+      Error(..)
+    , (//)
     , dictionaryById
     , getDictionary
     , getField
@@ -13,6 +15,8 @@ module PDF.Object.Navigation (
   ) where
 
 import Codec.Compression.Zlib (decompress)
+import Control.Applicative (Alternative(..))
+import Control.Monad (MonadPlus(..))
 import Control.Monad.Reader (MonadReader(..))
 import Control.Monad.Fail (MonadFail(..))
 import Data.ByteString (ByteString)
@@ -27,7 +31,12 @@ import PDF.Output (ObjectId)
 import Prelude hiding (fail)
 import Text.Printf (printf)
 
-type PDFContent m = (MonadReader Content m, MonadFail m)
+type PDFContent m = (Alternative m, MonadReader Content m, MonadFail m)
+newtype Error a = Error {
+    runError :: Either String a
+  } deriving (Alternative, Functor, Applicative, Monad, MonadPlus)
+instance MonadFail Error where
+  fail = Error . Left
 
 castDictionary :: MonadFail m => Object -> m Dictionary
 castDictionary (Direct (Dictionary aDict)) = return aDict

src/PDF/Pages.hs

@@ -1,13 +1,12 @@
 {-# LANGUAGE NamedFieldPuns #-}
-{-# LANGUAGE GeneralizedNewtypeDeriving #-}
 module PDF.Pages (
       Page(..)
     , get
     , getAll
   ) where
 
-import Control.Applicative (Alternative(..), (<|>))
+import Control.Applicative ((<|>))
-import Control.Monad (MonadPlus(..), foldM)
+import Control.Monad (foldM)
 import Control.Monad.Fail (MonadFail(..))
 import Control.Monad.RWS (RWST(..), evalRWST, modify)
 import qualified Control.Monad.RWS as RWS (get)
@@ -24,25 +23,19 @@ import PDF.Object (
     , Name(..)
   ,)
 import PDF.Object.Navigation (
-    (//), dictionaryById, getDictionary, getField, follow, openStream, origin
+    Error(..), (//), dictionaryById, getDictionary, getField, follow, openStream, origin
   )
 import PDF.Output (ObjectId(..))
 import Prelude hiding (fail)
 import Text.Printf (printf)
 
 type CachedFonts = Map ObjectId Font
-newtype Error a = Error {
-    runError :: Either String a
-  } deriving (Alternative, Functor, Applicative, Monad, MonadPlus)
 type T = RWST Content () CachedFonts Error
 data Page = Page {
       contents :: [Text]
     , source :: ObjectId
   }
 
-instance MonadFail Error where
-  fail = Error . Left
-
 getFontDictionary :: Dictionary -> T Dictionary
 getFontDictionary pageDict =
     ((pageDict // ["Resources", "Font"]) >>= getDictionary)