[GHC] #8213: Bad error message when using lazy IO to read from closed handle

2 Sep 2013

      #8213: Bad error message when using lazy IO to read from closed handle
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       Reporter:  nh2               |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Runtime System    |           Version:  7.6.3
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  Documentation bug
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 Today I accidentally wrote this code:

 {{{
 filesEqual :: FilePath -> FilePath -> IO Bool
 filesEqual f1 f2 = do
   equal <- withBinaryFile f1 ReadMode $ \f1 ->
     withBinaryFile f2 ReadMode $ \f2 ->
       c1 <- BSL.hGetContents h1
       c2 <- BSL.hGetContents h2
       return (c1 == c2)
   return $! equal
 }}}

 The problem is that I should have done the forcing before the files are
 closed:

 {{{
 return $! (c1 == c2)
 }}}

 I got the incredibly unhelpful error message

 {{{
 myprogram: myfile.txt: illegal operation
 }}}

 It took me 2 hours and strace to find out what was going on, especially
 because around every file open() there are multiple things that map top
 IllegalOperation (e.g. the ENOTTY from the ioctl that checks whether the
 device is a TTY).

 Can we give a better error message here, at least mentioning that the
 problem has to do with closed (and GC'd) handles?

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8213
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