#9123: Need for higher kinded roles -------------------------------------+------------------------------------- Reporter: simonpj | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: 7.12.1 Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Project (more Type of failure: | than a week) None/Unknown | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by simonpj): At the nub of this is the following observation. If a function has type {{{ forall m. (forall a b. Coercible a b => Coercible (m a) (m b)) => ...blah... }}} then that, in effect, restricts `m` to type constructors whose first argument has nominal role -- exactly the problem posed in the Description of this ticket. Very good observation. But the inferred context for `deriving` clauses is deliberately restricted. If you say {{{ newtype T m a = MkT (m a) deriving( C ) }}} (again from the Description) GHC will insist on a simple context, i.e. a class applied to type variables. You want to ''infer'' a rather complicated context. I don't know how to do that. You could perhaps ''declare'' it like this, using "standalone deriving" {{{ deriving instance (C m, forall a. Coercible a b => Coercible (m a) (m b)) => C (T m) }}} but GHC users might not be so happy with that whenever they say `deriving( Monad )`. Or perhaps {{{ type RepArg1 m = forall a b. (Coercible a b => Coercible (m a) (m b)) deriving instance (C m, RepArg1 m) => C (T m) }}} And now we are close to [wiki:Roles2]. (But we still don't have `deriving( Monad )` unannotated. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9123#comment:30 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler