[GHC] #12618: Add saturated constructor applications to Core

[GHC]

#12618: Add saturated constructor applications to Core
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        Reporter:  simonpj           |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 simonpj says:
 > A simple once-and-for-all analysis on the DataCon will establish how to
 do the matching, which type args to retain, etc.

 So a `DataCon` will have this info stored in it? It might be non-trivial!
 For example:

 {{{
 data T a where
   MkT :: F a -> T a
 }}}

 If `F` is not injective, we would need to store the choice for `a`. Even
 if it is injective, it may be more convenient to store the choice for `a`.

 And then there are examples like

 {{{
 data T2 a where
   MkT2 :: Maybe (Either Bool a -> a) -> T2 a
 }}}

 where the relationship between the constructor field's type and the choice
 for `a` is non-trivial. However, perhaps a use of `tcMatchTy` or one of
 its friends when constructing the `DataCon` is enough to sort this out.

 If we successfully do this for data constructors, it should not be hard to
 do the same for poly-kinded type constructors. I'm specifically thinking
 about the redundant `RuntimeRep` arguments to unboxed-tuple type
 constructors.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12618#comment:21>
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