Re: [GHC] #13035: GHC enters a loop when partial type signatures and advanced type level code mix

27 Dec 2016

      #13035: GHC enters a loop when partial type signatures and advanced type level code
mix
-------------------------------------+-------------------------------------
        Reporter:  xcmw              |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 FWIW, I don't think this is causing the typechecker to loop, but rather it
 causes it to take an extremely long time to complete. I discovered this by
 trying to minimize the test case by commenting some lines in the
 definition of `RecipeT`:

 {{{#!hs
 type RecipeT
     = Name         =: String
     $ Author       =: String
     {-
     $ Image        =: String
     $ Description  =: String
     $ CookTime     =: Maybe Int
     $ PrepTime     =: Maybe Int
     $ TotalTime    =: Maybe Int
     $ Yield        =: String
     -}
     $ Nutrition    =: NutritionRec
     {-
     $ Tags         =: [String]
     $ Url          =: String
     -}
     $ GNone
 }}}

 When I compiled this, it lagged noticeably, but it completed in finite
 time:

 {{{
 $ time /opt/ghc/8.0.1/bin/ghc -fforce-recomp Wat.hs[1 of 1] Compiling Wat
 ( Wat.hs, Wat.o )

 Wat.hs:140:28: warning: [-Wpartial-type-signatures]
     • Found type wildcard ‘_’ standing for ‘'['Author]’
     • In the type signature:
         g :: MyRec RecipeFormatter _
     • Relevant bindings include
         g :: MyRec RecipeFormatter '['Author] (bound at Wat.hs:141:1)

 real    0m1.973s
 user    0m1.940s
 sys     0m0.028s
 }}}

 I then uncommented one more line (`Image := String`):

 {{{#!hs
 type RecipeT
     = Name         =: String
     $ Author       =: String
     $ Image        =: String
     {-
     $ Description  =: String
     $ CookTime     =: Maybe Int
     $ PrepTime     =: Maybe Int
     $ TotalTime    =: Maybe Int
     $ Yield        =: String
     -}
     $ Nutrition    =: NutritionRec
     {-
     $ Tags         =: [String]
     $ Url          =: String
     -}
     $ GNone
 }}}

 And that contributed to the compilation time significantly (almost
 doubling it):

 {{{
 $ time /opt/ghc/8.0.1/bin/ghc -fforce-recomp Wat.hs[1 of 1] Compiling Wat
 ( Wat.hs, Wat.o )

 Wat.hs:140:28: warning: [-Wpartial-type-signatures]
     • Found type wildcard ‘_’ standing for ‘'['Author]’
     • In the type signature:
         g :: MyRec RecipeFormatter _
     • Relevant bindings include
         g :: MyRec RecipeFormatter '['Author] (bound at Wat.hs:141:1)

 real    0m3.635s
 user    0m3.564s
 sys     0m0.068s
 }}}

 Similarly, uncommented out more and more lines seems to approximately
 double the compilation time for each line.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13035#comment:2
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler