#9236: hGetContents leads to late/silent failures -------------------------------+------------------------------------------- Reporter: dfeuer | Owner: Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.8.2 Component: | Keywords: libraries/base | Architecture: Unknown/Multiple Resolution: | Difficulty: Easy (less than 1 hour) Operating System: | Blocked By: Unknown/Multiple | Related Tickets: Type of failure: Other | Test Case: | Blocking: | -------------------------------+------------------------------------------- Changes (by dfeuer): * failure: None/Unknown => Other * cc: hvr, ekmett (added) * component: Compiler => libraries/base * difficulty: Unknown => Easy (less than 1 hour) * version: 7.6.3 => 7.8.2 Old description:
A common newbie error: {{{ #!haskell withFile "file" ReadMode hGetContents >>= putStr }}}
The problem, of course, is that the result of `withFile "file" ReadMode hGetContents` isn't forced until `putStr` is executed, at which point the file has already been closed. The Haskell report doesn't specify what it will find, and, at least in 7.6.3, it finds the string empty. The behavior that seems most correct to me would be to guarantee that if a file has not been completely read (or read up to an I/O error) when it is closed, then its contents should be reported as something like `"Four score and seven years ag"++error "Forcing a suspended computation led to an attempted read from a handle that was already closed."` Since the current system apparently puts an `[]` as a temporary marker while waiting to see what the rest of the list will be, I imagine that the fix is as simple as putting the error thunk there instead.
New description: A common newbie error: {{{ #!haskell withFile "file" ReadMode hGetContents >>= putStr }}} The problem, of course, is that the result of `withFile "file" ReadMode hGetContents` isn't forced until `putStr` is executed, at which point the file has already been closed. The Haskell report doesn't specify what it will find, and, at least in 7.6.3, it finds the string empty. The behavior that seems most correct to me would be to guarantee that if a file has not been completely read (or read up to an I/O error) when it is closed, then its contents should be reported as something like `"Four score and seven years ag"++error "Forcing a suspended computation led to an attempted read from a handle that was already closed."` My suggested fix is in a comment below. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9236#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler