[GHC] #15809: Use level numbers for generalisation

[GHC]

#15809: Use level numbers for generalisation
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           Reporter:  simonpj        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1
           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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 GHC's type inference engine goes to quite a bit of effort to pin a ''level
 number'' on every type variable (both skolems and unification variables).

 '''Idea''': when generalising a type or kind, use those level numbers to
 decide which variables to generalise.

 This could completely replace the "global tyvars" field of the `TcLclEnv`
 (the `tcl_tyvars` field), which is very tiresome to maintain.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15809>
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