data families shouldn't be required to have return kind *, data instances should
I'd like to be able to define
{-# language PolyKinds, KindSignatures, GADTs, TypeFamilies #-}
data family Fix :: (k -> *) -> k
newtype instance Fix f = In { out :: f (Fix f) }But currently this is disallowed:
Fix.hs:2:1: error:
• Kind signature on data type declaration has non-* return kind
(k -> *) -> k
• In the data family declaration for ‘Fix’Ultimately I think the issue here is that data instances should be required to end in kind *, not the families, but this generalization didn't mean anything until we had PolyKinds.
As an example of a usecase, with the above, I could define a bifunctor fixed point such as
newtype instance Fix f a = In2 { out2 :: f (Fix f a) a }Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.0.1 |
| Type | FeatureRequest |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |