Calling Java from Frege. http://mmhelloworld.github.io/blog/2013/07/10/frege-hello-java/

Main1.fr

module Main1 where

data HashSet a = native java.util.HashSet where
    native new    :: () -> STMutable s (HashSet a)
    native size   :: Mutable s (HashSet a) -> ST s Int
    native add    :: Mutable s (HashSet a) -> a -> ST s Bool
    native remove :: Mutable s (HashSet a) -> a -> ST s Bool

main :: IO ()
main = do
    set <- HashSet.new ()
    println =<< set.size          -- 0

    added <- set.add "hello"
    println added                 -- true
    println =<< set.size          -- 1

    added <- set.add "hello"
    println added                 -- false
    println =<< set.size          -- 1

    removed <- set.remove "hello"
    println removed               -- true
    println =<< set.size          -- 0

    removed <- set.remove "hello"
    println removed               -- false
    println =<< set.size          -- 0

Main2.fr

module Main2 where

data HashSet a = native java.util.HashSet where
    native new  :: () -> STMutable s (HashSet a)
    native size :: Mutable s (HashSet a) -> ST s Int
    native add  :: Mutable s (HashSet a) -> a -> ST s Bool

    fromList :: [a] -> STMutable s (HashSet a)
    fromList list = do
        set <- HashSet.new ()
        let addAll :: [a] -> Mutable s (HashSet a) -> STMutable s (HashSet a)
            addAll [] s = pure s
            addAll (x : xs) s = do
                s.add x
                addAll xs s
        addAll list set

    toList :: Mutable s (HashSet a) -> ST s [a]
    toList set = do
        iterator <- set.iterator
        let loop xs i = do
                p <- i.hasNext
                if p
                then do
                    x <- i.next
                    loop (x : xs) i
                else pure xs
        loop [] iterator

main :: IO ()
main = do
    set <- HashSet.fromList [1, 2, 3]
    println =<< set.size              -- 3
    println =<< set.toList            -- [3, 2, 1]