[GHC] #14254: The Binary instance for TypeRep smells a bit expensive

[GHC]

#14254: The Binary instance for TypeRep smells a bit expensive
-------------------------------------+-------------------------------------
        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  Typeable
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14337            |  Differential Rev(s):  Phab:D3998,
       Wiki Page:                    |  Phab:D4082, Phab:D4085
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Comment (by simonpj):

 Re the loop, `Type` = `TYPE Lifted`.  So the `TypeRep` for `Type` is a
 `TrApp`, with a cached kind. So
 {{{
 TYPE Lifted :: TYPE Lifted
 }}}
 But that kind has a cached kind:
 {{{
 TYPE Lifted :: TYPE Lifted :: TYPE Lifted
 }}}
 and if the cache field is strict you build an infinite data structure.
 The only way out of this I can see is to
 * define a top level definition the `TypeRep` for `TYPE Lifted`
 * in that definition, do not use `mkTrApp`; instead build an explicit loop
 {{{
 trTYPELifted = TrApp fpr trTYPE trLifted trTYPELifted
 }}}
   note that `trTYPELifted mentions itself directly.
 * In `mkTrApp` spot that case, and return `trTYPELifted`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14254#comment:14>
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