Implement r for access by PageNumber and clean the mess a bit

src/PDF/Pages.hs

@@ -12,13 +12,11 @@ module PDF.Pages (
     , cacheFonts
   ) where
 
-import Control.Applicative (Alternative(..))
-import Control.Applicative ((<|>))
+import Control.Applicative (Alternative, (<|>))
 import Control.Monad (foldM)
 import Control.Monad.Fail (MonadFail(..))
 import Control.Monad.Reader (runReaderT)
-import Control.Monad.State (MonadState, StateT(..), evalStateT)
-import Control.Monad.RWS (RWST(..), evalRWST, modify)
+import Control.Monad.State (MonadState, StateT(..), evalStateT, modify)
 import qualified Control.Monad.RWS as RWS (get)
 import Data.Map (Map)
 import qualified Data.Map as Map (empty, fromList, insert, lookup, toList)
@@ -35,16 +33,14 @@ import PDF.Object (
     , Name(..), Object(..)
   ,)
 import PDF.Object.Navigation (
-      Clear(..), Error(..), PDFContent, (./), (//), (>./), (>//), castObject
-    , getDictionary, objectById, origin
+      Clear(..), PDFContent, (//), (>./), (>//), castObject, getDictionary
+    , objectById, origin
   )
 import PDF.Output (ObjectId(..))
 import Prelude hiding (fail)
 import Text.Printf (printf)
 
 type CachedFonts = Map ObjectId Font
-type T = RWST Layer () CachedFonts Error
-type PageContext = StateT CachedFonts
 type FontCache m = (MonadState CachedFonts m, PDFContent m)
 data Page = Page {
       contents :: [Text]
@@ -129,6 +125,12 @@ instance (MonadFail m, Alternative m, MonadState CachedFonts m) => Box m Pages L
       numbered = Map.fromList . zip [1..]
 
 instance (MonadFail m, Alternative m, MonadState CachedFonts m) => Box m PageNumber Layer Page where
+  r (P i) layer
+    | i < 1 = fail "Pages start at 1"
+    | otherwise = runReaderT (drop (i - 1) <$> pagesList >>= firstPage) layer
+    where
+      firstPage [] = fail "Page is out of bounds"
+      firstPage (p:_) = loadPage p
 
 cacheFonts :: Monad m => StateT CachedFonts m a -> m a
 cacheFonts = flip evalStateT Map.empty