[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:  #11376, #11975    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 It's not that the information that `:type` gives is wrong, it's just not
 what you're asking for. `:type foo` gives you the visibility of `foo` as
 an expression post-typechecking. This can be quite handy when you have,
 say:

 {{{
 λ> let foo :: a; foo = undefined
 λ> :type foo @Int
 foo @Int :: Int

 λ> :type foo
 foo :: forall {a}. a
 λ> let goo = foo
 λ> :type goo
 goo :: forall {a}. a
 λ> :type goo @Int
 }}}

 Notice that you can't use VTA with `goo`, since its type (which is the
 same as `foo`, post-typechecking) doesn't have any visible type variables.
 This is precisely what `:type foo` warned us about, and I'd hate to
 cripple a legitimately useful feature for the benefit of a degenerate
 case.

 My suggestion would be to include a blurb in the users' guide section that
 gives an example of asking GHCi for the visibility of a top-level
 expression (using `:type +v`), and then show the pitfalls of using
 `:type`. Would you be satisfied with that?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13401#comment:6>
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