Hughes' Arrowed FilterA Exercise

Question

I've been reading through John Hughes' Programming with Arrows, and I got to first exercise in which he asks the reader to implement a generic filter function for arrows, filterA. The spec says that it should behave like filter over (->) arrows and filterM over Kliesli arrows. I attempted to implement it as follows but what I ended up with seems somewhat simpler than the other examples I have seen of the solution online. I fear that I've missed something but my answer seems to work as the spec suggests. It matches filter for -> and filterM for Kliesli arrows. It also works in a reasonable way for stream functions.

listcase [] = Left ()
listcase (x:xs) = Right (x,xs)


filterA :: forall a arr. ArrowChoice arr => arr a Bool -> arr [a] [a]
filterA f = arr listcase >>>
            arr (const []) ||| (switchA *** filterA f >>> arr (uncurry (++)))
                where switchA :: ArrowChoice arr => arr a [a]
                      switchA = (f &&& id) >>> arr (\(b,x) -> if b then [x] else [])

Is this an acceptable implementation?

Answer 1

It seems acceptable to me. As an alternative you could consider using uncurry ($) and

switchA :: ArrowChoice arr =>> arr a ([a] -> [a])
switchA = (f &&& arr id) >>> arr test
  where
    test (False, _) = id
    test (True, x)  = (x :)