[GHC] #14263: typeKind is quadratic

[GHC]

#14263: typeKind is quadratic
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      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 goldfire):

 I have often wondered if it wouldn't be better to change `Type`s concrete
 representation to avoid this kind of problem structurally. To wit:

 {{{#!hs
 data Type
   = TyVarTy TyVar
   | TyConTy TyCon
   | AppTys Type [Type]    -- INVARIANT 1: List is not empty.
                           -- INVARIANT 2: The head type is not an AppTys
   | ForAllTy ...
   | LitTy ...
   | CastTy ...
   | CoercionTy ...
 }}}

 Note that INVARIANT 1 is needed to avoid confusion between, e.g., `AppTys
 (TyVarTy a) []` and `TyVarTy a`.

 We could perhaps do even better using `Seq Type` instead of `[Type]` as
 the container in `AppTys`, given that we sometimes need access to both
 ends.

 It might be an interesting experiment to see if performance improves with
 this change.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14263#comment:1>
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