[GHC] #14683: Hole-y partial type signatures lead to slow compile times

17 Jan 2018

      #14683: Hole-y partial type signatures lead to slow compile times
-------------------------------------+-------------------------------------
           Reporter:  harpocrates    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This came up while investigating the compile times of Happy grammars:
 https://github.com/simonmar/happy/issues/109. GHC 8 and later take a lot
 longer to compile grammars with types that rely on `PartialTypeSignatures`
 and have lots of wildcards. Example:

 {{{#!hs
 happyReduce_6
   :: ()
   => Happy_GHC_Exts.Int#
   -> L Token
   -> Happy_GHC_Exts.Int#
   -> Happy_IntList
   -> HappyStk (HappyAbsSyn _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _)
   -> ParserMonad (HappyAbsSyn _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _)
 }}}

 There is an example repo (https://github.com/wiz/too-happy) in that thread
 containing a grammar whose generated code has lots of wildcards.

 On my laptop, the project takes about 25 seconds to compile on 7.10.3.
 With 8.0.2, 8.2.2, and HEAD it takes on the order of 4 minutes.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14683
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