[GHC] #5030: Slow type checking of type-level computation heavy code.

[GHC]

#5030: Slow type checking of type-level computation heavy code.
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        Reporter:  thesz                         |         Owner:
            Type:  bug                           |        Status:  closed
        Priority:  normal                        |     Milestone:  7.2.1
       Component:  Compiler (Type checker)       |       Version:  7.0.2
      Resolution:  fixed                         |      Keywords:
Operating System:  Unknown/Multiple              |  Architecture:  x86
 Type of failure:  Compile-time performance bug  |     Test Case:  T5030
      Blocked By:                                |      Blocking:
 Related Tickets:                                |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"1f09b246d377a0007a953e5a77545d81671d2e36/ghc"
 1f09b246/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="1f09b246d377a0007a953e5a77545d81671d2e36"
 Accept 20% dedgradation in Trac #5030 compile time

 In commit

   31621b12 * A collection of type-inference refactorings.

 I fixed a bug in the on-the-fly unifier.  Usually the
 on-the-fly unifier (TcUnify) defers type function
 applications to the constraint solver.  But in one situation
 it inconsistently did not defer, so a unification happened
 without reducing a type function.  By a fluke this makes
 T5030 (specifcially the definition of cnst) much better.

 It turns out that consistently non-deferring type functions
 makes the test for #3064 go bad.  So somehow the current,
 inconsistent situation was an accidental sweet spot.

 But it's a horrible sweet spot, relying on what was essentially
 a bug.  So I've accepted the worsening (it's an exotic case),
 and opened #12724 to deal with the underlying cause.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5030#comment:12>
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