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  • #12680

Closed
Open
Opened Oct 10, 2016 by Edward Z. Yang@ezyangDeveloper

Permit type equality instances in signatures

David Christiansen requested this, and I want to record it so I don't forget. What he wants to be able to do is say things like this in signatures:

signature A where

data T
data S
type family F a

instance T ~ S
instance F Int ~ S

This is currently rejected by GHC because you are not allowed to define instances of type equality. But really all this is saying is that the type equality must hold when we eventually implement the module; easy to check.

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Version 8.1
Type FeatureRequest
TypeOfFailure OtherFailure
Priority low
Resolution Unresolved
Component Compiler (Type checker)
Test case
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Reference: ghc/ghc#12680