[GHC] #14873: The well-kinded type invariant (in TcType)

[GHC]

#14873: The well-kinded type invariant (in TcType)
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        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  merge
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 I've wondered about this, too. If we had `TcType` separate from `Type`,
 then we this would be very easy to find.

 I suppose we could smoke it out: put an `ASSERT` in the `typeKind` case
 for `TyVarTy` that the tyvar isn't a `TcTyVar`. Do the same with `eqType`.
 Then we'd catch (almost) all the cases.

 I wonder what the performance implications would be.

 (+) We don't have to zonk unnecessarily just to maintain the invariant.

 (-) When we do zonk, we don't memorialize this in the type -- only in the
 kind.

 (-) Is monadic code slower than pure code?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14873#comment:13>
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