On Wed, Sep 17, 2008 at 02:10:56PM +0100, Dougal Stanton wrote:

On Wed, Sep 17, 2008 at 1:17 PM, Manlio Perillo wrote:

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The Python tempfile module, as an example, implements a wrapper around mkstemp function that does exactly this, and the code is portable; on Windows it uses O_TEMPORARY_FILE flag, on POSIX systems the file is unlink-ed as soon as it is created (but note that the code is not signal safe - well, many functions in the Python standard library are not signal safe).

Something like:

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withTempFile :: String -> ((FilePath, Handle) -> IO b) -> IO b withTempFile name action = do tmpdir <- getTemporaryDirectory bracket (openTempFile tmpdir name) (\(fp,h) -> hClose h >> removeFile fp) action

I don't know if this has the safety requirements you mean?

You need to be sure to use the bracket from Control.Exception, not the one from System.IO or IO (which won't work). And you also need special work to make the code safe from signals. But basically, this is the right idea. David

manlio_perillo:

Dougal Stanton ha scritto:

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On Wed, Sep 17, 2008 at 1:17 PM, Manlio Perillo wrote:

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The Python tempfile module, as an example, implements a wrapper around mkstemp function that does exactly this, and the code is portable; on Windows it uses O_TEMPORARY_FILE flag, on POSIX systems the file is unlink-ed as soon as it is created (but note that the code is not signal safe - well, many functions in the Python standard library are not signal safe).

Something like:

...

withTempFile :: String -> ((FilePath, Handle) -> IO b) -> IO b withTempFile name action = do tmpdir <- getTemporaryDirectory bracket (openTempFile tmpdir name) (\(fp,h) -> hClose h >> removeFile fp) action

I don't know if this has the safety requirements you mean?

Yes, this is the ideal solution for Haskell.

But since GHC does not implement such a function, I was curious to know why it don't even implement a function that make sure the temporary file is removed.

Because it is two lines to write, so no one has yet proposed it for the base library. -- Don

On Wed, Sep 17, 2008 at 04:51:07PM +0100, Mitchell, Neil wrote:

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But since GHC does not implement such a function, I was curious to know why it don't even implement a function that make sure the temporary file is removed.

Because it is two lines to write, so no one has yet proposed it for the base library.

Map is 2 lines, but we have that as a library (1 line in Hugs). Otherwise is a whole one lexeme long, but we still provide a library function.

This one is tricky and has lots of edge cases! If its useful, it should be in the libraries.

In particular, the code Don quoted is incorrect depending on which import statements are used. If we assume that the default is the bracket available in Haskell 98, then it is definitely incorrect. It also doesn't behave properly in the presence of signals or keyboard interrupts (i.e. posix or windows systems). Of course, a larger change in the standard libraries is needed in order to solve this particular problem, so I'd want to agree on a solution for that problem first. Which is perhaps a better reason why it's not yet in the standard libraries: it's perhaps not a good idea to put a function with such serious issues into the standard libraries. David

On Wed, Sep 17, 2008 at 10:11:28PM +0200, Manlio Perillo wrote:

David Roundy ha scritto:

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[...] In particular, the code Don quoted is incorrect depending on which import statements are used. If we assume that the default is the bracket available in Haskell 98, then it is definitely incorrect.

It also doesn't behave properly in the presence of signals or keyboard interrupts (i.e. posix or windows systems).

Maybe a withSignalsMasked function, at least on Posix systems (it is not defined in System.Posix.Signals)?

Darcs does something like this, and I obviously think it should be moved into the standard libraries. In my opinion signals ought to throw asynchronous exceptions by default, then we'd have unified handling of exceptional cases.

I have no idea how signals works under Windows.

Windows doesn't have signals per se, but does have something similar for keyboard interrupts, which in darcs we also convert into an asynchronous exception. I'm not clear as to how well it works. Under cygwin shells, ctrl-C does something other than send a windows keyboard interrupt, so darcs doesn't work so well in that environment. David

On 2008 Sep 17, at 18:20, Aaron Denney wrote:

On 2008-09-17, Brandon S. Allbery KF8NH wrote:

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On 2008 Sep 17, at 8:17, Manlio Perillo wrote:

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The Python tempfile module, as an example, implements a wrapper around mkstemp function that does exactly this, and the code is portable; on Windows it uses O_TEMPORARY_FILE flag, on POSIX systems the file is unlink-ed as soon as it is created (but note that the code is not signal safe - well, many functions in the Python standard library are not signal safe).

There are reasons why GHC library does not implement this?

POSIX doesn't guaranteed that open-and-unlink works; HP-UX is a "POSIX" platform on which it doesn't.

Did HPUX's behavior change at some point? This is a standard idiom, and I don't remember having any trouble with it, but I haven't used anything earlier than 9. The manpages for 11 only document being a mount point as cause for EBUSY.

It may have but I recall it being cited as an issue with HP/UX 9 and 10. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH