[GHC] #15480: Rename SigTv

[GHC]

#15480: Rename SigTv
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           Reporter:  nomeata        |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.5
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 After #15050 and [changeset:"4d91cabcd5e3c603997d9876f6d30204a9b029c6/ghc"
 4d91cabc/ghc] it seems odd to call `SigTv` for the kind of type variables
 that used to be used in type signatures in patterns, but are now used in
 other places (kind signatures, partial type signatures).

 Also, `SigTv` may be associated with sigma. At some point I assumed that
 `SigTv` are type variables that may unify with polytypes, while `TauTv`
 are those that unify with monotypes.

 As a service to our future self, I’d like to rename `SigTv`. Suggestions?

 Best one I have is `TvTv` or `TyVarTv` (for a type variable that stands
 for a type variable).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15480>
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