[GHC] #3064: Very long compile times with type functions

[GHC]

#3064: Very long compile times with type functions
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        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  low               |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  6.10.1
  checker)                           |
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Compile-time      |            Test Case:
  performance bug                    |  perf/compiler/T3064
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
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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/3064#comment:24>
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