[GHC] #15050: ScopedTypeVariables could allow more programs

[GHC]

#15050: ScopedTypeVariables could allow more programs
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        Reporter:  nomeata           |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.1
      Resolution:                    |             Keywords:
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 nomeata):

 > This is a consequence of the fact that a scoped type variable must be a
 variable, never a type. Another way of saying this is that a type variable
 brought into scope in a pattern type annotation simply names an existing
 type variable; it can't, say, bind a type variable to a type.

 TBH, I don’t understand that part of the docs for `ScopedTypeVariables`.
 What does it mean for a type variable to be a type variable? Is there a
 global set of variables in existing? Is it referring to the variables in
 the signature of of the constructor? … I guess the latter could be, but I
 agree with your sentiment that that is pretty odd and unsatisfying,
 especially given the type variables in the type signature of the
 constructor are not in scope here.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15050#comment:5>
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