#8213: Bad error message when using lazy IO to read from closed handle -------------------------------------+------------------------------------- Reporter: nh2 | Owner: Type: bug | Status: infoneeded Priority: normal | Milestone: Component: Core | Version: 7.6.3 Libraries | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: Incorrect | Related Tickets: result at runtime | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Changes (by thomie): * cc: core-libraries-committee@… (added) * failure: Documentation bug => Incorrect result at runtime * status: new => infoneeded * component: Runtime System => Core Libraries Old description:
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?
New description: Today I accidentally wrote this code: {{{ import System.IO import Data.ByteString.Lazy as BSL filesEqual :: FilePath -> FilePath -> IO Bool filesEqual f1 f2 = do equal <- withBinaryFile f1 ReadMode $ \h1 -> withBinaryFile f2 ReadMode $ \h2 -> do c1 <- BSL.hGetContents h1 c2 <- BSL.hGetContents h2 return (c1 == c2) return $! equal main = filesEqual "test.hs" "test.hs" >>= print }}} 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 {{{ test: test.hs: 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? -- Comment: I can not reproduce the problem. nh2: which OS were you using? Can you try again? On Linux, I get the following (nice) error message: {{{ $ ghc-7.6.3 test.hs ... $ ./test test: test.hs: hGetBufSome: illegal operation (handle is closed) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8213#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler