feat: generic deriving for extraction

src/Language/Java/Classfile/Extract.hs

@@ -1,6 +1,6 @@
 {-# LANGUAGE DeriveFunctor #-}
 {-# LANGUAGE InstanceSigs #-}
-module Language.Java.Classfile.Extract (Extract(), bytes) where
+module Language.Java.Classfile.Extract (Extract(), bytes, runExtract) where
 
 import Data.ByteString.Lazy (ByteString)
 import qualified Data.ByteString.Lazy as ByteString
@@ -37,7 +37,6 @@ instance Alternative Extract where
     Fail -> right input
     t -> t
 
-
 -- | Get a specified count of bytes. Fail if there are not enough bytes available.
 
 bytes :: Word -> Extract ByteString
@@ -48,7 +47,6 @@ bytes count = Extract $ \ input -> let
     then Fail
     else Done rest bs
 
-
 {- It seems I cannot define a lawful monad instance
 instance Monad Extract where
   (>>=) :: Extract a -> (a -> Extract b) -> Extract b
@@ -57,3 +55,8 @@ instance Monad Extract where
     Fail -> Fail
 -}
 
+runExtract :: ByteString -> Extract b -> Maybe (ByteString, b)
+runExtract string (Extract computation) = case computation string of
+  Fail -> Nothing
+  Done rest x -> Just (rest, x)
+