Re: [GHC] #14394: Inferred type for pattern synonym has redundant equality constraint

8 Nov 2017

      #14394: Inferred type for pattern synonym has redundant equality constraint
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |             Keywords:
      Resolution:                    |  PatternSynonyms, TypeInType
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 Simon Peyton Jones ):

 In [changeset:"2c2f3cea93733e0c6dd04e1d891082652dcf5ea1/ghc" 2c2f3ce/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="2c2f3cea93733e0c6dd04e1d891082652dcf5ea1"
 Minimise provided dictionaries in pattern synonyms

 Trac #14394 showed that it's possible to get redundant
 constraints in the inferred provided constraints of a pattern
 synonym.  This patch removes the redundancy with mkMinimalBySCs.

 To do this I had to generalise the type of mkMinimalBySCs slightly.
 And, to reduce confusing reversal, I made it stable: it now returns
 its result in the same order as its input.  That led to a raft of
 error message wibbles, mostly for the better.
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14394#comment:1
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