Can create bindings of kind Constraint without ConstraintKind, only TypeFamilies
This code compiles with GHC 8.0.1 (in either variant, with or without the definition of ApplyCtx Int in Foo Int. I think it shouldn't according to the docs in http://ghc.readthedocs.io/en/master/glasgow_exts.html?highlight=constraintKinds#the-constraint-kind. If this is intended, I guess documenting it is acceptable.
{-# LANGUAGE TypeFamilies #-}
module Foo where
import GHC.Exts (Constraint)
class Foo a where
type ApplyCtx a :: Constraint
type ApplyCtx a = ()
instance Foo Int where
type ApplyCtx Int = Show Int -- Commenting this out makes no difference.
f :: ApplyCtx Int => Int
f = 0
Googling found https://mail.haskell.org/pipermail/glasgow-haskell-users/2016-February/026151.html, but seems a different issue (and a non-bug). Also found #11715 (closed), which might be relevant (or not).
Trac metadata
Trac field | Value |
---|---|
Version | 8.0.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |