oops, i already implemented an applicative

src/Language/Java/Classfile.hs

@@ -0,0 +1,6 @@
+module Language.Java.Classfile () where
+import Language.Java.Classfile.Version (Version)
+
+data Classfile = Classfile
+  { version :: Version
+  }

src/Language/Java/Classfile/Extract.hs

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+{-# LANGUAGE DeriveFunctor #-}
+{-# LANGUAGE InstanceSigs #-}
+module Language.Java.Classfile.Extract (Extract()) where
+
+import Data.ByteString.Lazy (ByteString)
+
+data Extract a = Extract (Continuation a)
+
+type Continuation a = ByteString -> Reply a
+
+data Reply a
+  = Done ByteString a -- rest, result
+  | Fail
+  deriving (Functor)
+
+instance Functor Extract where
+  fmap :: (a -> b) -> Extract a -> Extract b
+  fmap f (Extract cont) = Extract $ \ input -> f <$> cont input
+
+instance Applicative Extract where
+  pure :: a -> Extract a
+  pure x = Extract $ \ rest -> Done rest x
+  (<*>) :: Extract (a -> b) -> Extract a -> Extract b
+  (<*>) (Extract computeF) (Extract computeX) = Extract $ \ input -> case computeF input of
+    Done rest f -> case computeX rest of
+      Done rest' x -> Done rest' (f x)
+      Fail -> Fail
+    Fail -> Fail
+
+
+
+{- It seems I cannot define a lawful monad instance
+instance Monad Extract where
+  (>>=) :: Extract a -> (a -> Extract b) -> Extract b
+  (>>=) (Extract computeA) f = Extract $ \ input -> case computeA input of
+    Done rest a -> _
+    Fail -> Fail
+-}
+

src/Language/Java/Classfile/Extractable.hs

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+module M () where
+
+
+class Extractable a where
+  extract :: Extract a
+
+instance Extractable Word8 where
+  extract :: Extract Word8
+  extract = _

src/Language/Java/Classfile/Version.hs

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+module Language.Java.Classfile.Version (Version(..)) where
+
+import Data.Word (Word16)
+
+data Version = Version
+  { minor :: Word16
+  , major :: Word16
+  }