[GHC] #14609: Per-instance UndecidableInstances

[GHC]

#14609: Per-instance UndecidableInstances
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        Reporter:  ryanreich         |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      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 AntC):

 Replying to [ticket:14609 ryanreich]:
 > Can (or why can't) this also be done for UndecidableInstances?

 It can and should; and there's almost certainly a proposal somewhere; or a
 comment on a ticket on (say) overlaps or FunDeps.

 See also this
 [https://people.cs.kuleuven.be/~tom.schrijvers/portfolio/haskell2017a.html
 recent paper]

 It's a symptom of Functional Dependencies not getting much love
 recently/perhaps getting phased out in favour of Type Families.

 And a great deal of nervousness that GHC's implementation of
 UndecidableInstances is
 [https://ghc.haskell.org/trac/ghc/ticket/10675#comment:15 bogus] -- which
 to me argues we should encourage a per-instance pragma, to limit the
 bogusness.

 Perhaps we should try to root out the bogusness: [https://github.com/ghc-
 proposals/ghc-proposals/pull/56 properly]

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14609#comment:1>
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