Java List in Frege

JavaList.fr

module hellofrege.JavaList where

data LinkedList a = native java.util.LinkedList where
    native add :: Mutable s (LinkedList a) -> a -> ST s Bool
    native get :: Mutable s (LinkedList a) -> Int -> ST s (Maybe a)
    native new :: () -> STMutable s (LinkedList a)
    
    fromFregeList :: [a] -> STMutable s (LinkedList a)
    fromFregeList xs = LinkedList.new () >>= loop xs where
        loop (x:xs) jlist = LinkedList.add jlist x >> loop xs jlist
        loop [] jlist = return jlist
        
plusTop :: Mutable s (LinkedList Int) -> ST s (Maybe Int)
plusTop xs = do
    a <- xs.get 0
    b <- xs.get 1
    return ((+) <$> a <*> b)

data Exception = native java.lang.Exception
derive Exceptional Exception

data NullPointerException = native java.lang.NullPointerException
derive Exceptional NullPointerException

pure native showThrowable toString :: Throwable -> String

main _ = do
    javaList <- LinkedList.fromFregeList [1, 2, 3]
    try (\xs -> plusTop xs >>= (println . maybe "Got a null pointer" show)) javaList 
        `catch` (\(npe :: NullPointerException) -> println $ showThrowable npe)
        `catch` (\(exception :: Exception) -> println $ showThrowable exception)