[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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Changes (by ekmett):

 * cc: ekmett@… (added)


Comment:

 Simon,

 While I can respect the fact that other pragmas are forgettable without
 changing the compilability of the program, the big issue to me here is
 that by introducing Roles we're forcing users to choose between backwards
 compatibility and compatibility with other compilers, correctness under
 GHC and doing the right thing and putting on annotations when they need
 invariants.

 With a ```{-# ROLE #-}``` pragma users can at least write code that could
 work with other compilers without requiring an ```#ifdef``` block around
 every container type that provides invariants.

 I expect these to be a lot more common than Richard seems to, given that
 almost any sort of `Map`, `Set`, `Fingertree` or `Heap`-like data
 structure makes use of some invariant about its contents. I've been
 compiling a mental checklist of my own packages that will break when this
 goes in and it isn't a small list.

 It seems to me that this means any package that provides basically any
 sort of container is going to have to compile with `#ifdef`, which is a
 really ugly consequence of the current syntax.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8185#comment:9>
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