[GHC] #10735: Smooth out the differences between `compiler/utils/Pretty.hs` and `libraries/pretty`

[GHC]

#10735: Smooth out the differences between `compiler/utils/Pretty.hs` and
`libraries/pretty`
-------------------------------------+-------------------------------------
        Reporter:  thomie            |                   Owner:  thomie
            Type:  task              |                  Status:  patch
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Compiler          |                 Version:  7.0.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:  #1062, #1176,     |  Differential Revisions:
  #7666, #8809                       |  Phab:D1130, Phab:D1132
-------------------------------------+-------------------------------------

Comment (by thomie):

 There is 1 commit left for parity with pretty-1.1.2.0. I have not landed
 it yet, because I haven't been able to measure the supposed space/time
 improvement yet. The other 2 commits described below seem to suggest this
 commit doesn't actually work:
   "Pretty: improving the space/time performance of vcat, hsep, hcat"

 There are 2 commits left for parity with pretty-1.1.2.1. I have not landed
 them yet, because they seem to cause more allocation in the compiler (see
 discussion in https://github.com/haskell/pretty/pull/9):
    "Resolve foldr-strictness stack overflow bug"
    "Special-case reduce for horiz/vert"

 The commits are here: https://github.com/thomie/ghc/commits/pretty

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10735#comment:19>
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