[GHC] #12364: Demand analysis for sum types

[GHC]

#12364: Demand analysis for sum types
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        Reporter:  nomeata           |                Owner:
            Type:  task              |               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):

 Work in progress on branch wip/T12354, just a quick experiment that might
 be able to tell us if this is worth pursuing.

 But it is possibly less useful than I thought:
 {{{
 let foo a b = ... either return Left foobar .... or return Right foobaz
 in ... case foo a b of Left x [Demand=1*U] -> ... ; Right y [Demand=1*U]
 ->
 }}}
 Because the demand analyzer is a backwards analysis, the information on
 the usage of `foo` does not propagate into the definition of `foo` and
 into `foobar` and `foobaz`.

 For functions returning a product, a product demand is simply assumed
 `[Product demands for function body]` (but even there, not with single-use
 information). So I doubt that this is a direction worth pursuing.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12364#comment:3>
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