[GHC] #10697: Change template-haskell API to allow NOUNPACK, lazy annotations

[GHC]

#10697: Change template-haskell API to allow NOUNPACK, lazy annotations
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        Reporter:  RyanGlScott       |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Template Haskell  |              Version:  7.10.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #5290, #8347      |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * cc: adamse (added)


Comment:

 It turns out that even without considering `NOUNPACK`, `Strict` and
 `TemplateHaskell` don't play well together at the moment. This code:

 {{{#!hs
 {-# LANGUAGE StrictData, TemplateHaskell #-}

 import Language.Haskell.TH

 data T a = T !a a ~a

 $(return [])

 main :: IO ()
 main = putStrLn $(reify ''T >>= stringE . show)
 }}}

 produces this output (prettied up a bit):

 {{{#!hs
 TyConI (DataD [] Main.T [KindedTV a_1627391747 StarT]
   [NormalC Main.T [ (IsStrict, VarT a_1627391747)
                   , (NotStrict,VarT a_1627391747)
                   , (NotStrict,VarT a_1627391747)
                   ]] [])
 }}}

 Which is pretty darn misleading, since the second argument is actually
 strict!

 That being said, I'm not exactly sure what it ''should'' output here. One
 might could argue is should be `(IsStrict, NotStrict, IsLazy)`, but then
 again, the definition of `NotStrict` depends on whether `StrictData` is
 enabled or not.

 If we output `(Strict, NotStrict, IsLazy)`, then would splicing the TH
 code for that datatype into a module without `StrictData` fail (since
 `IsLazy` implies a laziness annotation)? Perhaps TH should check whether
 `StrictData` is on and splice accordingly.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10697#comment:3>
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