#8177: Roles for type families -------------------------------------+------------------------------------ Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by simonpj): Harump. You are right. `type` families are the very source of pattern matching so ''of course'' `type` families should have argument of role N. You could argue that there might be a parameter of the family that is never matched. Thus: {{{ type family F a@R b@N type instance F a Int = a -> a type instance F a Bool = (a,a) }}} But it's a bit marginal; in most cases you could re-order the args {{{ type family F b@N :: * -> * }}} and similarly for the instances. I think it's the same for `data` families. For example, it'd be utterly wrong to make a newtype coercion between `(D Age)` and `(D Int)`; so `D`'s parameter must clearly be role N. In short, the main merit of this ticket would be to allow ''parametric'' (non-indexed) parameters to appear before ''index'' parameters. That seems a fairly marginal gain. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8177#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler