#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): A post on #8672 seems more relevant here: Simon PJ says: Richard, two questions. First (Q1), can't a data instance have a representational role inferred? Eg this ought to work. The definition of `g` is currently rejected. {{{ data family T a data instance T [b] = MkT b newtype Age = MkAge Int g :: T [Age] -> T [Int] g x = coerce x }}} Second question (Q2). Are role annotations allowed on data family declarations, where they could indicate which parameters are the indices: {{{ type role T representational nominal -- Only second param can be an index data family T a b data instance T a Bool = ... -- Allowed data instance T Bool b = ... -- Rejected }}} (And similarly for type families.) Similarly are role annotations allowed on data instance declarations, where they would be useful in exactly the same way that they are on data decls {{{ data family T a type role T [a] (a:nominal) data instance T [a] = MkT a }}} Here we are running out of syntax! This isn't supported now because I think all parameters are automatically nominal (see Q1 for why they shouldn't be). Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8177#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler