[GHC] #9872: Runing type functions is too slow

[GHC]

#9872: Runing type functions is too slow
-------------------------------------+-------------------------------------
              Reporter:  simonpj     |            Owner:
                  Type:  bug         |           Status:  closed
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
            Resolution:  fixed       |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------

Comment (by jstolarek):

 I mentioned on ghc-devs that I witnessed exponential compile times for
 type families. I just built the latest HEAD and I see that the problem is
 gone. With my test case I get the following dependency between input size
 and compile time:

 ||= Input size =||= GHC 7.8.4 RC1 =||= GHC HEAD ||
 || 64  || 1.2s  || 0.6s ||
 || 128 || 11.1s || 1.3s ||
 || 256 || 1m41s || 3.7s ||

 Compiling with GHC 7.8.4 required passing `-ftype-function-depth=1000`
 (perhaps a smaller value would work, but the default one was too low).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9872#comment:19>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler