Re: [GHC] #15143: Passing an IO value through several functions results in program hanging.

17 May 2018

      #15143: Passing an IO value through several functions results in program hanging.
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        Reporter:  Burtannia         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by danilo2):

 Hi! I think I might get similar problem. However, no minimal example here
 (unless you will need some). The code is pretty simple, I'm building a
 "fold-like" function with type classes:

 {{{
 class Monad m => FoldableLayer t m layer where
     buildLayerFold :: ∀ layout. Layer.Cons layer layout -> m (Result t) ->
 m (Result t)

 class Monad m => LayerFoldableBuilder__ (active :: Bool) t m layer where
     buildLayerFold__ :: SomePtr -> m (Result t) -> m (Result t)

 instance {-# OVERLAPPABLE #-} Monad m
       => LayerFoldableBuilder__ 'False t m layer where
     buildLayerFold__ = \(!_) (!a) -> !a ; {-# INLINE buildLayerFold__ #-}

 instance (Monad m, Layer.StorableLayer layer m, FoldableLayer t m layer)
       => LayerFoldableBuilder__ 'True t m layer where
     buildLayerFold__ = \(!ptr) (!mr) -> do
         !layer <- Layer.unsafePeekWrapped @layer ptr
         !r     <- mr -- | Performance
         buildLayerFold @t @m @layer layer $! pure r
     {-# INLINE buildLayerFold__ #-}
 }}}

 The problem is that if I change the last two lines to

 {{{
 buildLayerFold @t @m @layer layer mr -- mr instead of pure r
 }}}

 The performance is 2 times slower while evaluating it. The `m` is a
 `State` over `IO`. Basically I am building here an action and this code
 change should not affect anything - because this action is just a function
 composition and will be evaluated eventually. I was initially thinking
 this might be related to strictness on the result, but that is apparently
 not the case. Even if I change the code to:

 {{{
 !_ <- mr
 buildLayerFold @t @m @layer layer mr
 }}}

 I get the same slow results (2 times slower than the fast version). I
 don't see any reason why would it happen and this bug could be related.
 Thanks @iamrecursion for showing it to me!

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15143#comment:5
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