[GHC] #8848: Warning: Rule too complicated to desugar

[GHC]

#8848: Warning: Rule too complicated to desugar
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        Reporter:  carter            |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.8.1-rc2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"41ba7ccb742278de0abf32cb7571c71b150997a3/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="41ba7ccb742278de0abf32cb7571c71b150997a3"
 Improve the desugaring of RULE left-hand-sides (fixes Trac #8848)

 I've added detailed comments with
   Note [Decomposing the left-hand side of a RULE]

 The result is a noticeable improvement.  Previously

  * we rejected a perfectly decent SPECIALISE (Trac #8848)

  * and for something like
       f :: (Eq a) => b -> a -> a
       {-# SPECIALISE f :: b -> [Int] -> [Int] #-}
    we ended up with
       RULE  f ($fdEqList $dfEqInt) = f_spec
    whereas we wanted
       RULES forall (d:Eq [Int]). f d = f_spec
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8848#comment:1>
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