[GHC] #13401: GHCi gives conflicting information about visible type application

[GHC]

#13401: GHCi gives conflicting information about visible type application
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        Reporter:  crockeea          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  GHCi              |              Version:  8.0.1
      Resolution:                    |             Keywords:
                                     |  TypeApplications
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 RyanGlScott):

 What is happening here is that `:type` is regeneralizing the type of the
 expression `foo`. In the regeneralized type, `a` is now properly invisible
 (i.e., not available for type application), which is why `:type foo` will
 give you `forall {a}. a` regardless of whether you declared foo with `foo
 = undefined` or `foo :: a; foo = undefined`.

 If you wish to see the type of `foo` that preserves the visibility of its
 type parameters, you need to use the `:type +v` GHCi command (which is
 currently only available in GHC HEAD):

 {{{
 λ> let foo = undefined
 λ> :type +v foo
 foo :: forall {a}. a
 λ> let foo :: a; foo = undefined
 λ> :type +v foo
 foo :: forall a. a
 }}}

 So I don't think there's a GHC bug here—just slightly misleading
 documentation at worst. I think including a mention of `:type +v` in the
 users' guide section on `-XTypeApplications` might dispel this confusion.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13401#comment:2>
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