Silly me -- that code works with the current GHC (module attached). I still think the generalization is worth doing, though. ------------------------------------------------------------------------- {-# LANGUAGE Arrows #-} module ArrowTest where import Control.Applicative import Control.Arrow import Control.Category import Prelude hiding (id, (.), repeat) -- copied from Control.Arrow.Transformer.Static (in the arrows package) newtype StaticArrow f a b c = StaticArrow (f (a b c)) instance (Category a, Applicative f) => Category (StaticArrow f a) where id = StaticArrow (pure id) StaticArrow f . StaticArrow g = StaticArrow ((.) <$> f <*> g) instance (Arrow a, Applicative f) => Arrow (StaticArrow f a) where arr f = StaticArrow (pure (arr f)) first (StaticArrow f) = StaticArrow (first <$> f) newtype MyArr b c = MyArr (b -> c) instance Category MyArr instance Arrow MyArr repeat :: Int -> (Int -> MyArr e a) -> MyArr e a repeat = undefined func1 :: [Double] -> Double func1 = undefined job1 :: MyArr [Double] Double job1 = undefined job3 :: Int -> MyArr Double String job3 = undefined repeat' :: Int -> StaticArrow ((->) Int) MyArr e a -> MyArr e a repeat' n (StaticArrow f) = repeat n f test2 :: MyArr [Double] String test2 = proc xs -> do let y = func1 xs z <- job1 -< xs (|(repeat' 100) (do i <- StaticArrow (arr . const) -< () StaticArrow (\i -> job3 (i * 2)) -< xs !! i + y + z)|)