[GHC] #8185: Change role annotation syntax

[GHC]

#8185: Change role annotation syntax
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        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:           |
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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>
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