[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 nomeata):

 I guess the solution to that is adding a type class, such as
 {{{
 class CompressArgs b where
     unpackArgs :: Expr b -> Arity -> [Expr b] -> [Expr b]
     unpackArgs _ _ l = l

     packArgs :: Expr b -> [Expr b] -> [Expr b]
     packArgs _ l = l
 }}}
 or alternatively
 {{{
 class HasType a where
     hasType :: Expr a -> Type
 }}}
 and adding instances for the two type of binders we have (`Var` and
 `TaggedBndr t`). With a few constraints added in various places (four
 modules only), this also compiles. It seems the remaining bit is to solve
 the staging issue: The instance should live in `CoreSyn`, but requires
 `expType` in `CoreUtils`. If that can be solved, the feature could be
 added quite painlessly. Then, in all places where the expression is
 traversed, one can any time make the decision to work on `Apps` directly,
 instead of `App`.

 I expect we’d get the memory performance boost we want (exciting!), but I
 also expect that this encoding/decoding in every traversal will cost
 runtime.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12618#comment:16>
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