[GHC] #10541: Expose kind information for type variables with reify

[GHC]

#10541: Expose kind information for type variables with reify
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              Reporter:  spinda      |             Owner:
                  Type:  feature     |            Status:  new
  request                            |         Milestone:
              Priority:  normal      |           Version:  7.10.1
             Component:  Template    |  Operating System:  Unknown/Multiple
  Haskell                            |   Type of failure:  None/Unknown
              Keywords:              |        Blocked By:
          Architecture:              |   Related Tickets:
  Unknown/Multiple                   |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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 The comment on [https://hackage.haskell.org/package/template-
 haskell-2.10.0.0/docs/Language-Haskell-TH-Syntax.html#v:TyVarI TyVarI]
 notes that the attached Type is always {{{VarT theName}}}, and that this
 could potentially be extended. Currently, there isn't a way to access kind
 information for a type variable through the Template Haskell API. It would
 be useful (at least for me) to have this ability here.

 The limitation on the returned Type comes from the
 [https://github.com/ghc/ghc/blob/master/compiler/typecheck/TcSplice.hs#L1060
 ATyVar clause of reifyThing], which zonks the TyVar to a GHC type and then
 uses reifyType to convert it to a TH type. For a TyVarTy,
 [https://github.com/ghc/ghc/blob/master/compiler/typecheck/TcSplice.hs#L1281
 reifyType] always returns {{{VarT theName}}}.

 I can see a few possible ways of exposing kind information here:

 (a) Fill the Type argument to TyVarI with {{{SigT (VarT theName)
 theKind}}}. This could be done either as a special case in place of the
 call to reifyType in reifyThing, or as a modification to reifyType's
 behavior. The question of when SigT should be included is open: all the
 time? when the kind isn't {{{*}}}? when the kind is something that
 involves more than {{{*}}} and {{{->}}}?

 (b) Change TyVarI from {{{TyVarI Name Type}}} to {{{TyVarI Name
 TyVarBndr}}}. This seems like the most straightforward, stable way of
 exposing kind information, but it requires a breaking API change.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10541>
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