MyCollection.hs

{-# LANGUAGE MagicHash           #-}
{-# LANGUAGE DataKinds           #-}
{-# LANGUAGE TypeFamilies        #-}
{-# LANGUAGE FlexibleContexts    #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}

module MyCollections where

import Java
import Java.Collections
import Control.Monad (forM_)

data {-# CLASS "java.util.Collection" #-} Collection = Collection (Object# Collection)
  deriving Class

data {-# CLASS "java.util.List" #-} List = List (Object# List)
  deriving Class

data {-# CLASS "java.util.ArrayList" #-} ArrayList = ArrayList (Object# ArrayList)
  deriving (Class, Show)

foreign import java unsafe "@new" newArrayList :: Java c ArrayList

foreign import java unsafe "@interface add" add ::
  (Extends a Object, Extends b Collection) => a -> Java b Bool

foreign import java unsafe "@interface addAll" addAll ::
  (Extends b Collection) => b -> Java b Bool

type instance Inherits List = '[Collection]

type instance Inherits ArrayList = '[List]

instance Monoid ArrayList where
  mempty          = pureJava newArrayList
  a1 `mappend` a2 = pureJava $ do
    al <- newArrayList
    forM_ [a1, a2] $ \x ->
      al <.> addAll x
    return al