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.
Trac metadata
Trac field | Value |
---|---|
Version | 7.8.2 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |