[GHC] #11276: GHC hangs/takes an exponential amount of time with simple program

22 Dec 2015

      #11276: GHC hangs/takes an exponential amount of time with simple program
-------------------------------------+-------------------------------------
           Reporter:  mpickering     |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           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:
-------------------------------------+-------------------------------------
 This was discovered when trying to compile xml-conduit. Here is the
 standalone test case with a few comments indicating how to make it
 compile.

 The program compiles with ghc-7.10.2 but fails with HEAD.

 {{{#!hs
 {-# LANGUAGE RankNTypes #-}
 module Hang where
 import Control.Monad
 import Data.Char

 data Event
   = EventBeginDocument
   | EventEndDocument
   | EventBeginDoctype
   | EventEndDoctype
   | EventInstruction
   | EventBeginElement
   | EventEndElement
   | EventContent Content
   | EventComment
   | EventCDATA

 data Content
   = ContentText String
   | ContentEntity String

 peek :: Monad m => Consumer a m (Maybe a)
 peek = undefined

 type Consumer i m r = forall o. ConduitM i o m r

 tag :: forall m a b c o . Monad m =>
     ConduitM Event o m (Maybe c)
 tag = do
     _ <- dropWS
     return undefined
   where
 -- Add this and it works
 --    dropWS :: Monad m => ConduitM Event o m (Maybe Event)
     dropWS = do
 -- Swap these two lines and it works
 --        let x = undefined
         x <- peek
         let isWS =
                 case x of
                     -- Remove some of these and it works
                     Just EventBeginDocument -> True
                     Just EventEndDocument -> True
                     Just EventBeginDoctype{} -> True
                     Just EventEndDoctype -> True
                     Just EventInstruction{} -> True
                     Just EventBeginElement{} -> False
                     Just EventEndElement{} -> False
                     Just (EventContent (ContentText t))
                         | all isSpace t -> True
                         | otherwise -> False
                     Just (EventContent ContentEntity{}) -> False
                     Just EventComment{} -> True
                     Just EventCDATA{} -> False
                     Nothing -> False
         if isWS then dropWS else return x

 -- Inlined Instances

 instance Functor (ConduitM i o m) where
     fmap f (ConduitM c) = ConduitM $ \rest -> c (rest . f)

 instance Applicative (ConduitM i o m) where
     pure x = ConduitM ($ x)
     {-# INLINE pure #-}
     (<*>) = ap
     {-# INLINE (<*>) #-}

 instance Monad (ConduitM i o m) where
     return = pure
     ConduitM f >>= g = ConduitM $ \h -> f $ \a -> unConduitM (g a) h

 instance Monad m => Functor (Pipe l i o u m) where
     fmap = liftM
     {-# INLINE fmap #-}

 instance Monad m => Applicative (Pipe l i o u m) where
     pure = Done
     {-# INLINE pure #-}
     (<*>) = ap
     {-# INLINE (<*>) #-}

 instance Monad m => Monad (Pipe l i o u m) where
     return = pure
     {-# INLINE return #-}

     HaveOutput p c o >>= fp = HaveOutput (p >>= fp)            c
 o
     NeedInput p c    >>= fp = NeedInput  (p >=> fp)            (c >=> fp)
     Done x           >>= fp = fp x
     PipeM mp         >>= fp = PipeM      ((>>= fp) `liftM` mp)
     Leftover p i     >>= fp = Leftover   (p >>= fp)            i

 newtype ConduitM i o m r = ConduitM
     { unConduitM :: forall b.
                     (r -> Pipe i i o () m b) -> Pipe i i o () m b
     }

 data Pipe l i o u m r =
     HaveOutput (Pipe l i o u m r) (m ()) o
   | NeedInput (i -> Pipe l i o u m r) (u -> Pipe l i o u m r)
   | Done r
   | PipeM (m (Pipe l i o u m r))
   | Leftover (Pipe l i o u m r) l
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11276>
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