Excerpts from Thomas Hartman's message of Mon Mar 23 09:08:41 +0100 2009:
I got bitten by a bug (well, I call it bug) in bracketCD from HSH/MissingH demonstrated by the following code
bracketCD is very useful for sysadminny one-offs, I use it all the time, but..... I suspect that unless people are very careful, this behavior will affect other users of bracketCD, in potentially very subtle and tricky ways.
1) -- is there a more elegant way to deal with lazy io than the hack below? (putStrLn the last character)
Yes without changing the bracketCD function you can use a strict 'return' function to help you avoid leaks. return' :: (Monad m, NFData sa) -> sa -> m sa return' x = rnf x `seq` return x
2) -- should MissingH function bracketCD be fixed, and if so how?
Not completely while staying in the full 'IO' monad. Have a look at the strict-io package [1] that goes in this direction. myBracketCWD :: (NFData sa, Show sa) => FilePath -> SIO sa -> IO sa myBracketCWD fp action = do oldcwd <- getCurrentDirectory setCurrentDirectory fp a <- SIO.run action rnf a `seq` setCurrentDirectory oldcwd return a The same function could be more deeply in the 'SIO' monad by returning in the 'SIO' monad. However this would require to first wrap getCurrentDirectory and setCurrentDirectory in the 'SIO' monad. Best regards, [1]: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/strict-io -- Nicolas Pouillard