I am trying to define the left operator for a CPS arrow of type: newtype CPSFunctor ans a b c = CPS ((a c ans) -> (a b ans)) This is the definition given by John Hughes for first: first (CPS f) = CPS $ \k -> arr (\(b,d) -> (f (arr (\c -> (c,d)) >>> k),b)) >>> app So I try and apply this to left and get: left (CPS f) = CPS $ \k -> arr (\z -> case z of Left x -> (f (arr Left >>> k),x) Right x -> (arr Right >>> k,x)) >>> app But when I compile (with ghc-6.0) I get: Inferred type is less polymorphic than expected Quantified type variable `d' is unified with another quantified type variable `b' When trying to generalise the type inferred for `left' Signature type: forall ans a. (ArrowApply a, ArrowChoice a) => forall b1 c1 d1. CPSFunctor ans a b1 c1 -> CPSFunctor ans a (Either b1 d1) (Either c1 d1) Type to generalise: forall b1 c1 d1. CPSFunctor ans a b1 c1 -> CPSFunctor ans a (Either b1 d1) (Either c1 d1) In the instance declaration for `ArrowChoice (CPSFunctor ans a)' Hughes says in his paper that "CPS arrows ... can of course support dynamic choice" - so where am I going wrong with my definition? Any help greatly appreciated... Regards, Keean Schupke.