[GHC] #14937: QuantifiedConstraints: Reify implication constraints from terms lacking them

[GHC]

#14937: QuantifiedConstraints: Reify implication constraints from terms lacking
them
-------------------------------------+-------------------------------------
        Reporter:  Bj0rn             |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14822, #15635    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 The magical built-in function doesn't seem like strong enough medicine.
 Today, you asked for

 {{{#!hs
 reifyImplication :: forall a b. (forall r. a => (b => r) -> r) -> (forall
 r. ((a => b) => r) -> r)
 }}}

 But tomorrow, you might need

 {{{#!hs
 reifyImplication1 :: forall a b. (forall r x. a x => (b x => r) -> r) ->
 (forall r. ((forall x. a x => b x) => r) -> r)
 reifyImplication2 :: forall a b. (forall r x y. a x y => (b x y => r) ->
 r) -> (forall r. ((forall x y. a x y => b x y) => r) -> r)
 }}}

 and the day after that, you may want

 {{{#!hs
 reifyImplicationQ :: forall a b y. (forall r x. a x y => (b x y => r) ->
 r) -> (forall r. ((forall x. a x y => b x y) => r) -> r)
 }}}

 and so on. It would be possible to offer a whole set of these primitives,
 but it would be a bit of a mess and never complete.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14937#comment:10>
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