{-# LANGUAGE Arrows #-}

bar0 f g h x =
    proc (y, z) ->
        (| test (h f . (h g) -< (y x) . y z) ((h g) . h f -< y z . (y x)) |)

bar1 f g x y = proc _ -> (f -< x) &&& (g -< y)

bar2 f g h x =
    proc (y, z) ->
        (h f . (h g) -< (y x) . y z) ||| ((h g) . h f -< y z . (y x))

bar3 f g h x =
    proc (y, z) ->
        ( (h f . h g)
            -<
                (y x) . y z
        )
            ||| ( (h g . h f)
                    -<
                        y z . (y x)
                )

bar4 = proc x ->
    case x of
        Just f -> f -< ()
        Nothing -> x -< ()
        <+> do
            g -< x

expr' = proc x ->
    do
        returnA -< x
        <+> do
            symbol Plus -< ()
            y <- term -< ()
            expr' -< x + y
        <+> do
            symbol Minus -< ()
            y <- term -< ()
            expr' -< x - y

bar f = proc (a, b) -> do
    (f a -< b)
        >-> (\y -> f b >- a)
        >-> (\y -> f b >- a)