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.