Type family with higher-rank result is too accepting
GHC accepts this garbage:
type family F2 :: forall k. k -> Type
data SBool :: Bool -> Type
data Nat
data SNat :: Nat -> Type
type instance F2 = SBool
type instance F2 = SNatThe family F2 should have an arity of 0, meaning that only one instance is possible -- and the RHS of that instance must have kind forall k. k -> Type. In other words, even accepting only one of the instances above is hogwash.
This is from ticket:11719#comment:161416, but you don't have to read that to understand this.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.6.1 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |