[GHC] #8347: Add a Strict LANGUAGE pragma

[GHC]

#8347: Add a Strict LANGUAGE pragma
-------------------------------------+-------------------------------------
        Reporter:  tibbe             |                   Owner:  tibbe
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Compiler          |                 Version:  7.6.3
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:  Phab:D1033
-------------------------------------+-------------------------------------
Changes (by thomie):

 * differential:   => Phab:D1033
 * milestone:   => 7.12.1


Comment:

 From https://ghc.haskell.org/trac/ghc/wiki/StrictPragma:

 > Haskell doesn't allow for ~ patterns in data constructor definitions
 today: we'll add support for such definitions and have it give the current
 lazy behavior.

 From Phab:D1033, I understand that using `~` in data constructor
 definitions without `-XStrictData` will be the error:

     "Lazy annotation (~) without StrictData"

 Maybe it would be useful to be able to compile a module both with and
 without `-XStrictData`, to compare performance differences. In that case
 this new `~` syntax should be under a different language pragma. Has this
 been given consideration?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8347#comment:3>
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