Use peek to improve directObject parser avoiding a large <|> disjunction

src/PDF/Object.hs

@@ -220,16 +220,18 @@ instance Output DirectObject where
     Output.string (printf "%d %d R" (getObjectId objectId) versionNumber)
 
 directObject :: MonadParser m => m DirectObject
-directObject =
-    Boolean <$> boolean
-  <|> Reference <$> reference {- defined before Number because Number is a prefix of it -}
-  <|> NumberObject <$> number
-  <|> StringObject <$> stringObject
-  <|> NameObject <$> name
-  <|> Array <$> array
-  <|> Dictionary <$> dictionary
-  <|> const Null <$> nullObject
-  <?> "direct object"
+directObject = (peek >>= dispatch) <?> "direct object"
+  where
+    dispatch 't' = Boolean <$> boolean
+    dispatch 'f' = Boolean <$> boolean
+    dispatch '(' = StringObject <$> stringObject
+    dispatch '<' = StringObject <$> stringObject <|> Dictionary <$> dictionary
+    dispatch '/' = NameObject <$> name
+    dispatch '[' = Array <$> array
+    dispatch 'n' = nullObject *> return Null
+    dispatch _ =
+        Reference <$> reference {- defined before Number because Number is a prefix of it -}
+      <|> NumberObject <$> number
 
 --
 -- Object

src/PDF/Parser.hs

@@ -17,8 +17,8 @@ import Control.Monad.Fail (MonadFail(..))
 import Control.Monad.State (StateT(..), evalStateT)
 import Control.Monad.Trans (MonadTrans(..))
 import qualified Data.Attoparsec.ByteString.Char8 as Atto (
-      Parser, char, endOfInput, parseOnly, satisfy, string, take, takeWhile
-    , takeWhile1
+      Parser, char, endOfInput, parseOnly, peekChar', satisfy, string, take
+    , takeWhile, takeWhile1
   )
 import Data.ByteString (ByteString)
 import Data.ByteString.Char8.Util (B16Int(..))
@@ -36,6 +36,7 @@ class MonadDeps m => MonadParser m where
   endOfInput :: m ()
   hexNumber :: m B16Int
   oneOf :: String -> m Char
+  peek :: m Char
   string :: ByteString -> m ByteString
   takeAll :: (Char -> Bool) -> m ByteString
   takeAll1 :: (Char -> Bool) -> m ByteString
@@ -47,6 +48,7 @@ instance MonadParser Atto.Parser where
   decNumber = Atto.takeWhile1 (`Set.member` digits)
   hexNumber = B16Int <$> Atto.takeWhile1 (`Set.member` hexDigits)
   oneOf charSet = Atto.satisfy (`elem` charSet)
+  peek = Atto.peekChar'
   string s = Atto.string s <?> show s
   takeAll = Atto.takeWhile
   takeAll1 = Atto.takeWhile1
@@ -58,6 +60,7 @@ instance (MonadParser m, MonadTrans t, MonadDeps (t m)) => MonadParser (t m) whe
   decNumber = lift $ decNumber
   hexNumber = lift $ hexNumber
   oneOf = lift . oneOf
+  peek = lift $ peek
   string = lift . string
   takeAll = lift . takeAll
   takeAll1 = lift . takeAll1