[GHC] #11376: Inconsistent specified type variables among functions and datatypes/classes when using -XTypeApplications

[GHC]

#11376: Inconsistent specified type variables among functions and datatypes/classes
when using -XTypeApplications
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        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |             Keywords:
      Resolution:                    |  TypeApplications
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Replying to [comment:19 goldfire]:

 > With `:type`, though, we don't want to instantiate, because doing so
 might change specified variables to invisible ones. Or in cases like
 `forall a. (a ~ Int) => ...`, instantiating and then solving will actually
 get rid of a variable.

 I'm not sure I agree.  `:type` takes an ''expression'' and ''infers'' its
 most general type. Sometimes that expression is a bare function name, but
 not always.

 In contrast `:info` takes a name, and tells you about it, with its un-
 instantiated type.

 So in cases like `forall a. (a~Int) => Int -> a -> Int` I think it would
 be absolutely acceptable to get rid of the variable in `:type`.  Moreover,
 you can't consistently ''prevent'' it happening.  Why should `:type f` and
 `:type (f 3)` behave so differently (when `f` has the type given earlier
 in this para)?

 In short, I suspect that using `Note [Instantiate when inferring a type]`
 for `:type` would be a jolly good thing.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11376#comment:20>
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