feat: Parsing the constant magic flag

java-classfile.cabal

@@ -29,6 +29,7 @@ library
       Language.Java.Classfile.Extract
       Language.Java.Classfile.Extractable
       Language.Java.Classfile.FromBigEndian
+      Language.Java.Classfile.Magic
       Language.Java.Classfile.Version
   other-modules:
       Paths_java_classfile

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

src/Language/Java/Classfile/Extractable.hs

@@ -2,16 +2,15 @@
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE UndecidableInstances #-}
-module Language.Java.Classfile.Extractable (Extractable(extract)) where
+module Language.Java.Classfile.Extractable (Extractable(extract), expectConstant) where
 
-import Language.Java.Classfile.Extract (Extract, bytes)
+import Language.Java.Classfile.Extract (Extract, bytes, expectEqual)
 import Data.Word (Word8, Word16, Word32)
 import qualified Data.ByteString.Lazy as ByteString
 import Data.ByteString.Lazy (ByteString)
 import Data.Bits (Bits(shiftL, (.|.)))
 import GHC.Generics (U1 (U1), (:*:) ((:*:)), (:+:) (L1, R1), M1 (M1), K1 (K1), Generic (Rep, to), Generically (Generically))
-import Control.Applicative ((<|>))
-
+import Control.Applicative ((<|>), empty)
 
 class Extractable a where
   extract :: Extract a
@@ -28,6 +27,9 @@ instance Extractable Word32 where
   extract :: Extract Word32
   extract = build <$> bytes 4
 
+expectConstant :: (Extractable a, Eq a) => a -> Extract a
+expectConstant = expectEqual extract
+
 build :: (Bits a, Num a) => ByteString -> a
 build = ByteString.foldl' shiftOr 0
 

src/Language/Java/Classfile/Version.hs

@@ -15,8 +15,8 @@ data Version = Version
   deriving Extractable via Generically Version
 
 -- >>> import Language.Java.Classfile.Extractable (Extractable(extract))
--- >>> import Language.Java.Classfile.Extract (runExtract)
--- >>> :set -XOVerloadedLists
--- >>> runExtract (fromList [0, 0, 0, 0]) extract :: Version
--- Variable not in scope: fromList :: [a0_a3mwi[tau:1]] -> ByteString
+-- >>> import Language.Java.Classfile.Extract (runExtract, Extract)
+-- >>> :set -XOverloadedLists
+-- >>> runExtract [1, 2, 0, 2] (extract :: Extract Version)
+-- Just ("",Version {minor = 258, major = 2})