feat: Parsing the constant magic flag

src/Language/Java/Classfile/Extract.hs

@@ -1,6 +1,6 @@
 {-# LANGUAGE DeriveFunctor #-}
 {-# LANGUAGE InstanceSigs #-}
-module Language.Java.Classfile.Extract (Extract(), bytes, runExtract) where
+module Language.Java.Classfile.Extract (Extract(), bytes, runExtract, expectRaw, expectEqual) where
 
 import Data.ByteString.Lazy (ByteString)
 import qualified Data.ByteString.Lazy as ByteString
@@ -47,6 +47,20 @@ bytes count = Extract $ \ input -> let
     then Fail
     else Done rest bs
 
+expectRaw :: ByteString -> Extract ByteString
+expectRaw e = Extract $ \ input -> let
+     (actual, rest) = ByteString.splitAt (ByteString.length e) input
+     in case actual == e of
+       True -> Done rest actual
+       False -> Fail 
+
+expectEqual :: Eq a => Extract a -> a -> Extract a
+expectEqual (Extract computeActual) expected = Extract $ \ input -> case computeActual input of
+  Done rest actual -> if actual == expected
+    then Done rest actual
+    else Fail
+  Fail -> Fail
+
 {- It seems I cannot define a lawful monad instance
 instance Monad Extract where
   (>>=) :: Extract a -> (a -> Extract b) -> Extract b