[GHC] #14873: GHC HEAD regression (piResultTy)

[GHC]

#14873: GHC HEAD regression (piResultTy)
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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 simonpj):

 Richard, I'm beginning to wonder if the pain of `Note [The well-kinded
 type invariant]` is worth the gain.  It's all very delicate.

 Here's a possible alternative

 1. Define `tcTypeKind :: TcType -> TcM TcKind` which zonks, if necessary,
 on the fly.  This would be dead easy to write.

 2. Call `tcTypeKind` instead of `typeKind` in ... well, in various places.
 Anywhere in the type checker where un-zonked kinds might be floating
 about.

 For (2) there are two difficulties

 * Which calls, exactly, would need to be in the monad?
 * Are there any calls that don't have the `TcM` monad conveniently to
 hand?  For example `eqType` calls `typeKind`, so we'd really need a
 `tcEqType` variant that calls `tcTypeKind`.

 Advantages: (a) redundant Refls disappear much earlier; this is Good (b)
 less delicate invariants.

 What do you think?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14873#comment:10>
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