forall on non-* types
This code
{-# LANGUAGE RankNTypes, ImpredicativeTypes #-}
x = Nothing :: (forall a. Maybe) Intgives
Couldn't match type `Maybe' with `forall a. Maybe'
Expected type: forall a. Maybe Int
Actual type: Maybe Intwhich does not seem correct. There are two options:
Option A: We allow this.
Option B: We reject this as ill-kinded and allow forall a. T only when T has kind *. I prefer this option; in theory this can break existing code, but I am not aware of any code that uses forall on non-* type (please correct me if I'm wrong), and e.g. this means forall a. a inhabits every kind, for example Bool.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 7.6.3 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |