Allow type synonym family application in instance head if it has no free variables
GHC rejects the following program:
{-# LANGUAGE TypeFamilies, FlexibleInstances #-}
module F where
type family F a
type instance F Int = Int -> Int
instance Show (F Int) where
show _ = "fun"But this doesn't seem fundamentally problematic to me: at the instance declaration, we know that F Int will reduce, and after that reduction there isn't any problem with the instance.
Here's a less restrictive instance head test: first, we reduce type families as much as possible. Then, if there are still leftover non-reducing type families, reject the instance.
After having typed this up, I have realized that I don't actually need this feature. But I'm filing it here for posterity.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.0.1 |
| Type | FeatureRequest |
| TypeOfFailure | OtherFailure |
| Priority | lowest |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |