feat[types]: ConstantPool uses GADT instead of data family

src/Language/Java/Classfile/ConstantPool.hs

@@ -8,6 +8,8 @@
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE StandaloneDeriving #-}
 module Language.Java.Classfile.ConstantPool (ConstantPool(..)) where
 import Data.Word (Word16)
 import Data.Array.IArray (Array, listArray)
@@ -18,15 +20,15 @@ import qualified Data.Text as Text
 import Language.Java.Classfile.Stage (Stage(..))
 import Data.Kind (Type)
 
+type ConstantPool :: Stage -> Type
+data ConstantPool stage where
+  ConstantPool :: (Array Word16 Entry) -> ConstantPool Parse
+  NoPool :: ConstantPool Resolve
+
+deriving instance Show (ConstantPool stage)
+
 -- | 'Stage'-indexed constant-pool. The constant-pool is erased after resolving the class file.
 
-type ConstantPool :: Stage -> Type
-data family ConstantPool stage
-
-newtype instance ConstantPool Parse = ConstantPool (Array Word16 Entry)
-  deriving stock (Show)
-
-data instance ConstantPool Resolve = NoPool
 
 instance Extractable (ConstantPool Parse) where
   extract :: Extract (ConstantPool Parse)