Mutually recursive equality constraints
Given this piece of code:
{-# LANGUAGE TypeFamilies #-}
class C a where
type T a :: *
f1 :: T a ~ () => a
f1 = f2
f2 :: T a ~ () => a
f2 = f1
GHC complains:
Couldn't match expected type `T a ~ ()'
against inferred type `T a1 ~ ()'
When matching the contexts of the signatures for
f1 :: forall a. (T a ~ ()) => a
f2 :: forall a. (T a ~ ()) => a
The signature contexts in a mutually recursive group should all be identical
When generalising the type(s) for f1, f2
Is this a bug? Enabling RelaxedPolyRec fixes the problem. Should TypeFamilies—just like GADTs—imply RelaxedPolyRec?
Trac metadata
Trac field | Value |
---|---|
Version | 6.10.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |