[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 nomeata):

 Another view is that the problem is that the user cannot even specify the
 desired rule, namely one that matches
 {{{
 GHC.Classes.$fEq[]_$c== @ Integer GHC.Integer.Type.$fEqInteger
 }}}
 and maybe the problem should be attacked from this angle.

 There is precedent in the way `coerce` works on the LHS of a rule: If it
 appears on the LHS, the compiler assumes the user wants to match any Core-
 level cast (`x |> c`), and thus inlines `coerce`, and even changes the
 free variable of the rule so that it can match unlifted coercions (see
 Note [Getting the map/coerce RULE to work]).

 In that sense, solving constraints on the LHS is not really monkeying or a
 plaster, but it is what what we have to do to allow the user to write
 rules that match a specific instance method.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13733#comment:2>
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