[GHC] #15628: Higher-rank kinds

[GHC]

#15628: Higher-rank kinds
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.6.1-beta1
      Resolution:  duplicate         |             Keywords:
                                     |  ImpredicativeTypes
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 RyanGlScott):

 * keywords:   => ImpredicativeTypes
 * status:  new => closed
 * resolution:   => duplicate


Comment:

 Replying to [comment:2 Iceland_jack]:
 > adding a `Proxy` induces an existential variable `x` seemingly without
 introducing quantification

 It //does// introduce quantification, and it's precisely because GHC can't
 support impredicative polymorphism at the moment. When you write:

 {{{#!hs
 newtype Bar = Bar (forall (f :: forall xx. xx -> Type). Proxy f -> ())
 }}}

 Then if we display all arguments visibly, this is what you are trying to
 write:

 {{{#!hs
 newtype Bar = Bar (forall (f :: forall xx. xx -> Type). Proxy @(forall xx.
 xx -> Type) f -> ())
 }}}

 You can't instantiate `Proxy`'s first argument with `forall xx. xx ->
 Type` since it is a polytype, and that would be impredicative. Therefore,
 GHC has to pick //something// to instantiate `xx` with in order to avoid
 impredicativity. Since we're in a data type, GHC bails out by creating an
 existentially quantified variable `x` and using that to instantiate the
 higher-rank kind:

 {{{#!hs
 newtype Bar = forall x. Bar (forall (f :: x -> Type). Proxy @(x -> Type)
 (f @x) -> ())
 }}}

 But `Bar` is a newtype, and therefore can't have existentials, leading to
 your error.

 As simonpj notes in comment:1, this ticket is really asking for
 impredicative polymorphism. There is already a raft of tickets about this
 topic, so I'll opt to close this one.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15628#comment:3>
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