[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):

 Using a pattern synonynm works always; the problem is that the code should
 be generic in the binder, but then we have no way of knowing the binder’s
 type. But for what it’s worth: It seems to compile with this change:
 {{{
 data Expr b
   = Var   Id
   | Lit   Literal
 --  | App   (Expr b) (Arg b)
   | Apps  (Expr b) Arity [Expr b]
   | Lam   b (Expr b)
   | Let   (Bind b) (Expr b)
   | Case  (Expr b) b Type [Alt b]       -- See #case_invariant#
   | Cast  (Expr b) Coercion
   | Tick  (Tickish Id) (Expr b)
   | Type  Type
   | Coercion Coercion
   deriving Data

 unpackArgs :: Expr b -> Arity -> [Expr b] -> [Expr b]
 unpackArgs _ _ l = l -- do something smarter here

 packArgs :: Expr b -> [Expr b] -> [Expr b]
 packArgs _ l = l  -- do something smarter here

 popArg :: Expr b -> Maybe (Expr b, Expr b)
 popArg (Apps e a xs) = case unpackArgs e a xs of
     [x] -> Just (e, x)
     xs  -> Just (Apps e (a-1) (packArgs e (init xs)), last xs)
 popArg _ = Nothing

 pattern App e1 e2 <- (popArg -> Just (e1, e2))
   where App e1 e2 | (f, args) <- collectArgs e1
                   = Apps f (length args +1) (packArgs f (args ++ [e2]))
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12618#comment:15>
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