[GHC] #5296: Add explicit type applications

[GHC]

#5296: Add explicit type applications
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        Reporter:  dsf               |                Owner:  goldfire
            Type:  feature request   |               Status:  closed
        Priority:  low               |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  7.0.3
  checker)                           |
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  GHC rejects       |            Test Case:
  valid program                      |  typecheck/should_compile/Vta{1,2}
                                     |  typecheck/should_fail/VtaFail
      Blocked By:  1897              |             Blocking:  10770
 Related Tickets:  #4466             |  Differential Rev(s):  Phab:D1681
       Wiki Page:                    |
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Comment (by andrewthad):

 This might be a little too late, but I wanted to raise an about the
 relationship this has with AllowAmbiguousTypes. I haven't tried it out
 yet, but based on an earlier comment in this thread, I think that turning
 on VisibleTypeApplication will also turn on AllowAmbiguousTypes. I don't
 think that this is necessary, and I think that there are situation where
 it is detrimental the library users. Let's say that I have a `Rec` from
 `vinyl` and I'm using `rget` from `Data.Vinyl.Lens`. Today, if I have
 `myRec :: Rec Identity '[Char,Bool,Int]`, I can write `rget (Proxy ::
 Proxy Int) myRec` to get the `Int` value out. In the future, it may be
 desirable to write a function `rget2` that can take the argument by
 VisibleTypeApplication instead. So, we would call `rget2 @Int myRec`
 instead.

 Here's the issue that I'm getting at. What does an end user have to enable
 to be able to use `rget2`? At the definition site, in the case of `rget2`,
 we actually don't need AllowAmbiguousTypes, but let's pretend that we did
 (I should have picked a different example). When a user turns on
 VisibleTypeApplication to be able to use a function that will otherwise
 have ambiguous type variable instantiation, I don't think that they should
 get AllowAmbiguousTypes turned on as well. If I understand correctly,
 AllowAmbiguousTypes is only needed to define these functions, not to use
 them.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5296#comment:29>
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