#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 goldfire): I would argue that, given the declaration for `D` above, the following instance should also be rejected: {{{ data instance D (Maybe c) = MkD2 c }}} Why should that instance be rejected? Because it pattern-matches on its argument. Before thinking about implementing this (which I agree should be straightforward), we need to agree on the design. Here are a few questions to spur discussion: * What should the default role be for families? Is it different for data and for type families? (I would prefer N for both.) * Am I right that the instance above should be rejected? * Consider {{{ type family Flip x@R y@R type instance Flip x y = (y, x) }}} Is that accepted? I think "yes". Here are some proposed rules: * If a parameter is at role N, its use is unrestricted in instances. * If a parameter is at role R, it cannot be matched against and it must only appear in R (or P) positions in the RHS of instances. * If a parameter is at role P, it cannot be matched against and it must only appear in P positions in the RHS of instances. Thoughts? Let's have the conversation here, and I will put the resolution on the wiki along with writing the implementation. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8177#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler