[GHC] #13733: Simplify constraints on RULES LHS

[GHC]

#13733: Simplify constraints on RULES LHS
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        Reporter:  nomeata           |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.3
      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 simonpj):

 The problem is that the built-in rule that rewrites
 {{{
 GHC.Classes.$fEq[] @ Integer GHC.Integer.Type.$fEqInteger
 ---->
 GHC.Classes.$fEq[]_$c== @ Integer GHC.Integer.Type.$fEqInteger
 }}}
 is firing before your user-defined rule has a chance to fire.  The basic
 problem is that two overlapping rules are competing, and it's a fluke
 which "wins".  Monkeying around with solving constraints on rule LHSs
 might be a sticking plaster, but it won't tackle the underlying issue.

 Possible solution: make the built-in rule fire in phase 2,1,0, but not in
 the initial "gentle" phase.

 Of course, ''not'' firing the built-in class-op rule means that other
 things won't happen as early as they otherwise might, so I don't know what
 the knock-on effects might be.  But I think this is the path to follow if
 you want to pursue this.

 Something similar happens for the inlining for data constructor wrappers.

 I'm sure there is a ticket about this stuff already.  Maybe more than one.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13733#comment:1>
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