feat: Extracting word types

src/Language/Java/Classfile/Extract.hs

@@ -1,8 +1,9 @@
 {-# LANGUAGE DeriveFunctor #-}
 {-# LANGUAGE InstanceSigs #-}
-module Language.Java.Classfile.Extract (Extract()) where
+module Language.Java.Classfile.Extract (Extract(), bytes) where
 
 import Data.ByteString.Lazy (ByteString)
+import qualified Data.ByteString.Lazy as ByteString
 
 data Extract a = Extract (Continuation a)
 
@@ -28,6 +29,16 @@ instance Applicative Extract where
     Fail -> Fail
 
 
+-- | Get a specified count of bytes. Fail if there are not enough bytes available.
+
+bytes :: Word -> Extract ByteString
+bytes count = Extract $ \ input -> let
+  count' = fromIntegral count
+  (bs, rest) = ByteString.splitAt count' input
+  in if ByteString.length bs /= count'
+    then Fail
+    else Done rest bs
+
 
 {- It seems I cannot define a lawful monad instance
 instance Monad Extract where