Re: [GHC] #14046: “Illegal type synonym family application in instance” is too strict in the presence of functional dependencies

[GHC]

#14046: “Illegal type synonym family application in instance” is too strict in the
presence of functional dependencies
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        Reporter:  lexi.lambda       |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #3485             |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 > I could be convinced by an example of the sort that SPJ alluded to, in
 which an instance of the form Multi X (Y a) behaves differently from (b ~
 Y a) => Multi X b,

 It affects type inference:
 {{{
    instance (a ~ Int) => C a
    instance (a ~ Bool) => C a
 vs
    instance C Int
    instance C Bool
 }}}
 GHC's type inference engine does not support backtracking. If it did, it
 could try out the former two instances one at a time, and see which one
 "worked".  With the latter two, only one matches.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14046#comment:8>
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