[GHC] #9515: Deprecate -XExplicitForAll

[GHC]

#9515: Deprecate -XExplicitForAll
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              Reporter:  dfeuer      |            Owner:
                  Type:  feature     |           Status:  new
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.8.3
             Component:  Compiler    |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by dfeuer):

 Replying to [comment:2 kosmikus]:
 > If the idea is to avoid confusion, and the `ExplicitForall` flag is
 currently implied by others, then shouldn't `RankNTypes`,
 `LiberalTypeSynonyms`, and `ExistentialQuantification` all imply
 `ScopedTypeVariables` instead? But that's somewhat non-obvious ...
 >
 > I personally think that while the current situation is unfortunate, the
 proposed solution isn't ideal either. I'd keep everything as it currently
 is, until we get to a point where we can agree that `ScopedTypeVariables`
 should be enabled by default.

 I think it would be great if `ScopedTypeVariables` (or conceivably
 something even better) were eventually implied by others, or even if it
 came to be the default behavior in the next Report. I think the first step
 in any case has to be phasing out the formally useless `ExplicitForAll`;
 once it's sufficiently dead, that syntactic space will be entirely free
 for a more useful replacement. You were wise to note the problem that
 `ExplicitForAll` is currently implied by other flags; I think the interim
 solution is to add a warning when those flags are used ''without''
 `ScopedTypeVariables`: "`XxxxXxxxxx` currently implies `ExplicitForAll`,
 which is deprecated. To avoid this warning, add `ScopedTypeVariables` and
 rename type variables as necessary."

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9515#comment:3>
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