[GHC] #9601: Make the rewrite rule system more powerful

[GHC]

#9601: Make the rewrite rule system more powerful
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        Reporter:  spacekitteh       |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.8.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9137, #10804     |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 I don't know much about `RULES`, let me highlight this idea from
 ticket:9136#comment:1 (that lead to #9137):

 {{{
 forall x y z. (IsLiteral y, IsLiteral z) => (x +# y) -# z = x +# (y -# z)
 forall x y z. (IsLiteral y, IsLiteral z) => (x -# y) -# z = x -# (y +# z)
 ...
 }}}

 This strikes my fancy! It's like a type class (predicate) for syntax

 * Could `IsLiteral` be given a kind?
 * Which constraint can be made available? (`IsEven`, `IsOdd`, ...)
 * Which properties can the compiler make use of?
 * Which properties can the optimizer make use of?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9601#comment:8>
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