[GHC] #14356: "Main: thread blocked indefinitely in an MVar operation" in fixIO

16 Oct 2017


      #14356: "Main: thread blocked indefinitely in an MVar operation" in fixIO
-------------------------------------+-------------------------------------
           Reporter:  nickkuk        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Runtime        |           Version:  8.2.1
  System                             |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect result
  Unknown/Multiple                   |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I'm not sure whether such behavior is expected, but this program
 {{{
 import System.IO

 main = fixIO (\x -> return 1) >>= print
 }}}
 prints "1"; and this
 {{{
 import System.IO

 main = fixIO (\(x, _) -> return (1, print x)) >>= print . fst
 }}}
 prints "Main: thread blocked indefinitely in an MVar operation".

 Second program arises from something like
 {{{
 {-# LANUGAGE RecursiveDo #-}

 main = mdo
   ...
   x <- return 1
   let f = do
     ...
     print x
     ...
   ...
   return f
 }}}
 It is not necessary to call f somewhere in mdo to get "Main: thread
 blocked indefinitely in an MVar operation".

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14356
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