Eta reduce type synonyms when possible
If you can eta reduce a type synonym, it is always better:
-- Bad
type F a = Maybe a
instance Functor F -- illegal, partially applied type synonym!
-- Good
type F = Maybe
instance Functor F -- ok with type synonym instancesSo... GHC should do this automatically for users. Ordinarily this is not terribly important (thus the priority); in the context of #12680 it is a modestly helpful affordance which allows a user to implement data Map a using type Map a = HashMap a (signature matching requires the type synonym to be nullary.)
Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.1 |
| Type | FeatureRequest |
| TypeOfFailure | OtherFailure |
| Priority | lowest |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | goldfire |
| Operating system | |
| Architecture |