[GHC] #14993: QuantifiedConstraints and principal types

[GHC]

#14993: QuantifiedConstraints and principal types
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints, wipT2893
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):

 Aha! I'd forgotten OutsideIn(X) entertains implication constraints.

 If you're looking for explanation/documentation, section **4.2.1**
 (starting "costs to the programmer", "costs to the type inference engine")
 explain the rationale, and it still stands. Simon's saying quantified
 constraints must be explicitly given by a signature, never inferred.
 That's in line with many exotic type system features; and makes perfect
 sense.

 Would be worth testing the examples from section **2.4** by giving an
 explicit signature same as what the paper says would be inferred. Does
 that get `-XQuantifiedConstraints` to accept them?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14993#comment:3>
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