#13047: Can create bindings of kind Constraint without ConstraintKind, only TypeFamilies -------------------------------------+------------------------------------- Reporter: pggiarrusso | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- 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=constraintKi... #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 https://ghc.haskell.org/trac/ghc/ticket/11715, which might be relevant (or not). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13047 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler