feat: Alternative instance + Generic work

src/Language/Java/Classfile/Extract.hs

@@ -4,6 +4,7 @@ module Language.Java.Classfile.Extract (Extract(), bytes) where
 
 import Data.ByteString.Lazy (ByteString)
 import qualified Data.ByteString.Lazy as ByteString
+import Control.Applicative (Alternative (empty, (<|>)))
 
 data Extract a = Extract (Continuation a)
 
@@ -28,6 +29,14 @@ instance Applicative Extract where
       Fail -> Fail
     Fail -> Fail
 
+instance Alternative Extract where
+  empty :: Extract a
+  empty = Extract $ const Fail
+  (<|>) :: Extract a -> Extract a -> Extract a
+  (<|>) (Extract left) (Extract right) = Extract $ \ input -> case left input of
+    Fail -> right input
+    t -> t
+
 
 -- | Get a specified count of bytes. Fail if there are not enough bytes available.
 

src/Language/Java/Classfile/Extractable.hs

@@ -1,11 +1,13 @@
 {-# LANGUAGE InstanceSigs #-}
+{-# LANGUAGE TypeOperators #-}
 module Language.Java.Classfile.Extractable () where
 import Language.Java.Classfile.Extract (Extract, bytes)
 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))
+import GHC.Generics (U1 (U1), (:*:))
+import Control.Applicative ((<|>))
 
 
 class Extractable a where
@@ -29,3 +31,16 @@ build = ByteString.foldl' shiftOr 0
 shiftOr :: (Bits a1, Num a1) => a1 -> Word8 -> a1
 shiftOr word appendix = shiftL word 8 .|. fromIntegral appendix
 
+class GenericExtractable a where
+  genericExtract :: Extract (a i)
+
+-- unit data types
+instance GenericExtractable U1 where
+  genericExtract :: Extract (U1 i)
+  genericExtract = pure U1
+
+-- product types
+instance GenericExtractable (l :*: r) where
+  genericExtract :: Extract ((:*:) l r i)
+  genericExtract = genericExtract <|> genericExtract
+