Can anyone tell me why I get a type error in the following code? I have to define liftA2 twice to avoid the type error. Both times its defined identically but with different type signature. If I use the original liftA2 in place of liftA2' I get: whyerror.lhs:36:25: Ambiguous type variable `a' in the constraint: `Arrow a' arising from use of `>>>' at whyerror.lhs:36:25-27 Possible cause: the monomorphism restriction applied to the following: liftA2' :: forall b a1 b1 c. (a1 -> b1 -> c) -> a b a1 -> a b b1 -> a b c (bound at whyerror.lhs:36:1) unsplit' :: forall a1 b c. (a1 -> b -> c) -> a (a1, b) c (bound at whyerror.lhs:34:1) split' :: forall b. a b (b, b) (bound at whyerror.lhs:33:1) Probable fix: give these definition(s) an explicit type signature or use -fno-monomorphism-restriction ----- whyerror.lhs ----

{-# OPTIONS_GHC -farrows #-} module WhyError where import Control.Arrow

newtype SimpleFunc a b = SimpleFunc { runF :: (a -> b) } instance Arrow SimpleFunc where arr f = SimpleFunc f first (SimpleFunc f) = SimpleFunc (mapFst f) where mapFst g (a,b) = (g a, b) (SimpleFunc f) >>> (SimpleFunc g) = SimpleFunc (g . f)

split :: (Arrow a) => a b (b, b) split = arr (\x -> (x,x)) unsplit :: (Arrow a) => (b -> c -> d) -> a (b, c) d unsplit = arr . uncurry -- arr (\op (x,y) -> x `op` y)

liftA2 :: (Arrow a) => (b -> c -> d) -> a e b -> a e c -> a e d liftA2 op f g = split >>> first f >>> second g >>> unsplit op -- f &&& g >>> unsplit op

f, g :: SimpleFunc Int Int f = arr (`div` 2) g = arr (\x -> x*3 + 1) h = liftA2 (+) f g hOutput = runF h 8

-- XXX I am getting type problems with split, unsplit and liftA2! why? split' = arr (\x -> (x,x)) unsplit' = arr . uncurry -- liftA2' :: (Arrow a) => (b -> c -> d) -> a e b -> a e c -> a e d liftA2' op f g = split' >>> first f >>> second g >>> unsplit' op

plusminus, double, h2 :: Kleisli [] Int Int plusminus = Kleisli (\x -> [x, -x]) double = arr (* 2) h2 = liftA2 (+) plusminus double h2Output = runKleisli h2 8

main = do print hOutput print h2Output

Tim Newsham http://www.thenewsh.com/~newsham/