[GHC] #12006: Can't infer constraint of pattern synonyms

[GHC]

#12006: Can't infer constraint of pattern synonyms
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        Reporter:  Iceland_jack      |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms
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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Changes (by mpickering):

 * keywords:  TypeSynonyms => PatternSynonyms


Comment:

 The problem here is that the type signature of the builder is fixed by the
 type of the matcher. By fixed, I mean literally fixed, we first typecheck
 the matcher with either the signature or infer the type and then we create
 an `Id` for the builder with precisely the same type.

 So the problem here is that the inferred type for the matcher doesn't
 require the `Num` constraint and so the builder type is `t -> Complex t`
 rather than `Num t => t -> Complex t` as you desire.

 From what I could work out from reading the source, the way it is like
 this is for mainly because of implementation problems. There is some more
 confused discussion on this ticket
 -https://ghc.haskell.org/trac/ghc/ticket/8581 - it is something I hope to
 look at again one day.

 Thank you for all these reports, they are very useful, I am interested in
 any pattern synonyms tickets so feel free to add me to cc to make sure
 that I see them.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12006#comment:1>
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