Different saturations of the same polymorphic-kinded type constructor aren't seen as apart types
{-# LANGUAGE TypeFamilies, PolyKinds #-}
module A where
data family D :: k
type family F (a :: k) :: *
type instance F (D Int) = Int
type instance F (f a b) = Char
type instance F (D Int Bool) = Char
The last equation, even though a specialization of the middle one, trips up the equality consistency check:
a.hs:9:15: error:
Conflicting family instance declarations:
F (D Int) = Int -- Defined at a.hs:9:15
F (D Int Bool) = Char -- Defined at a.hs:10:15
|
9 | type instance F (D Int) = Int
| ^
So GHC is able to infer that D Int ~/~ f a b
because D ~/~ f a
, but for some reason the same reasoning doesn't work for D ~/~ D a
.
Trac metadata
Trac field | Value |
---|---|
Version | 8.6.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |