[GHC] #15471: Polymorphism, typed splices and type inference don't mix

[GHC]

#15471: Polymorphism, typed splices and type inference don't mix
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        Reporter:  mpickering        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Template Haskell  |              Version:  8.4.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Well, we have
 {{{
 teset_foo :: forall p. Q (TExp (p -> p))
 }}}
 When we run a top-level splice, as in `$$(test_foo)`, we're going to run
 it at compile time; it's like running it in GHCi; it must be a closed
 expression.  So GHC must pick a type to instantiate `p`; and it chooses
 `Any`.

 After all, `test_foo` could also have type
 {{{
 test_foo :: forall p. Num p => p -> Q (TExp (p -> p))
 }}}
 and now we must pick a `Num` dictionary to pass to it.

 I think what you want is for `test_foo` to have type
 {{{
 test_foo :: Q (TExp (forall p. p->p))
 }}}
 but we can't do that.  Yet anyway.  For a start it'd require impredicative
 polymorphism.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15471#comment:4>
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