Confusing error message with PolyKinds
Consider the following example code:
import Control.Arrow foo :: a b (m b) foo = arr return
if we typecheck this we rightly get two errors about missing instances (Arrow a, Monad m). However, if we enable
PolyKinds we get the following error message:
T.hs:4:7: Kind incompatibility when matching types: a0 :: * -> * -> * a :: k1 -> k -> * Expected type: a b (m b) Actual type: a0 b0 (m0 b0) Relevant bindings include foo :: a b (m b) (bound at T.hs:4:1) In the expression: arr return In an equation for ‘foo’: foo = arr return
I can sort of see where this is coming from if I think really hard :), so perhaps it's not a bug per se, but it's definitely confusing; and it's a pity that a type error (forgotten type qualifier) is reported as a kind error.