#8185: Change role annotation syntax ----------------------------+---------------------------------------------- Reporter: | Owner: goldfire goldfire | Status: new Type: feature | Milestone: 7.8.1 request | Version: 7.7 Priority: normal | Keywords: Component: | Architecture: Unknown/Multiple Compiler | Difficulty: Moderate (less than a day) Resolution: | Blocked By: Operating System: | Related Tickets: Unknown/Multiple | Type of failure: | None/Unknown | Test Case: | Blocking: | ----------------------------+---------------------------------------------- Comment (by goldfire): In a conversation with several interested parties this morning, we came up with the following leading proposals: * Separate declaration, such as {{{ type roles Map nominal representational data Map a b = ... }}} Variants: Always use `type`? Or use `data` or `newtype` or `class`. * Use a `where` clause: {{{ data Map a b = ... where roles Map nominal representational }}} The role annotation would appear in the existing `where` clause for GADT-style declarations and for classes. A new `where` clause would be permitted for non-GADT-style data/newtype declarations. Variant: drop the redundant `Map`. It was decided that using a `{-# PRAGMA #-}` is a bad idea because roles are properly part of the language, not something outside it. Thus, whatever we do will not be backwards-compatible, and that desideratum is discarded. Though not brought up in the conversation, I would like to propose an alternative syntax for an inline annotation: {{{ data Map (a ::*, {nominal}) (b :: {representational}) = ... }}} Here, the role annotation is but in `{ }` along with a kind annotation. They are, of course, optional. A role annotation would also be accepted without a kind annotation -- the role would be distinguished by the `{ }`. Variant: add the word `role` in the braces. In any case, I would like to make a decision on this by early next week, to give me time to implement the change before 9/14. Thoughts? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8185#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler