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2shortplanks (968)

2shortplanks
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Mark Fowler has never been the same since he was elected leader of the London Perl Mongers. The strain manifests itself mainly in releasing various [cpan.org] modules [cpan.org] to CPAN, giving talks [2shortplanks.com], and use of the Trelane nick on #london.pm for endless procrastination. Doctors are still seeking a cure.

Journal of 2shortplanks (968)

Friday June 28, 2002
06:11 AM

Slap Dash FileCache

[ #6037 ]
So I need to open a vast number of files. No problem I think to myself, I'll use the FileCache module. So I run my maketry script (which spits out a file with a "#!/usr/bin/perl" line at the top, turns strict and warnings on, and loads a load of standard modules like File::Spec and IO::File and friends, before loading it up in vi) and type (well cut and paste) out the example from the perl cookbook.

$path = "bar";

use FileCache;
cacheout ($path);         # each time you use a filehandle
print $path "output";

And it falls over with the wonderful error message:

Can't use string ("bar") as a symbol ref while "strict refs" in use at
filecache line 22.

So I spend ages trying to work out why the code isn't replacing $path with a ref to a filehandle like it should and it's leaving a bare string in there. I post to London.pm. I ask on IRC. I'm stumped...and then I look at the source. Of course! So FileCache doesn't create reference as store them in $path. It opens a filehandle in the calling package with the same name as the $path. This means that $path in the print statement is treated as a symbolic reference for a variable name and finds the right filehandle.

Okay, this is quite neat, but really really dangerous. use strict quite rightly barfs all over the place when I do this and I have to turn it off.

So it looks like I'm going to have to write my own module. Anyone got any better suggestions? Alternative modules on CPAN to do this?


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  • DIY (Score:2, Insightful)

    by jdporter (36)
    So, fix the module and send the patch to the author.

    And if she isn't responsive, upload the new version yourself!

  • Umm this is really no different than if you took
    FileCache out of the picture, string filehandles
    just don't work under strict refs. They're
    essentially a form of symbolic reference, if
    you know what you're doing it's perfectly
    reasonable to run with no strict 'refs'.

    PS> I patched FileCache for perl 5.8, which still
    has this "problem", but more features. If you
    don't want to get all of 5.8 you can retrieve the
    module from ftp://pthbb.org/pub/pm/FileCache/ [pthbb.org]

    Enjoy
    --
    Were that I say, pancakes?
    • Excellent. This is what I love about Perl - if you waffle on about something for long enough you'll always find someone has already had a go at fixing it ;-)

      As for running without use strict, I'm not sure I trust myself that much ;-) I guess I could just turn it off lexically for the tight loop, but I get really worried about flicking the safety off the loaded gun pointing at my feet.

      Anyway, I've been busy since I posted my journal - I had a go at implementing my own modified version of FileCache whic

      • Not no strict, just no strict refs.

        I'll consider addng a line to the CAVEATS about absolute paths. It's really somehting the user should just keep in mind, again I think of FileCache (not that I am the oringal author) as simply an abstraction over open. It has one sole purpose and is not to catch every tiny faux pas the user might commit ;-)

        As for the roll-over, that wouldn't be an issue. So the file gets closed, and then might soon be reopened. If you look closely, the hash element is deleted when the

        --
        Were that I say, pancakes?

        • Not no strict, just no strict refs.

          Good point. Should have thought about that a bit more.

          I think of FileCache (not that I am the oringal author) as simply an abstraction over open.

          Yeah, though if I use open to open a second file with the same name from a different directory then it opens a different file. FileCache will use the first file it opened.

          However, I don't think I can think of any way to work around this with the current method of creating filehandles in the parent's namespace which

          • An IV (int) will roll over to an NV (double) at overflow point..

            While ++ will not then have exactly the same semantics, it will keep the value increasing, and not have the "random" effect you describe.
            • So does that mean that a ++ will potentially increase the value by more than one, until we reach +inf?

              Interesting - does this mean that ++ and += 1 are not always the same operation?

              • ++ and += 1 are always different ops, try perl -MO=Terse -e '$foo++; $bar += 1;' and note the ops used

                ++ gets folded to PP(preinc) which will do a simple ++SvIVX(sv) if it's a clean IV
                If it's not it calls sv_inc(sv) which does checks for magic and, if the value is an NV, performs SvNVX(sv) += 1.0;. How long the sv retains a value like a tight-jawed integer will be dependant on the implementation of the floating point code

      • If it were to be released for general consumption
        I think a better name would be IO::FileCache.

        One could also subclass FileCache to provide the
        rel2abs masking.
        --
        Were that I say, pancakes?
        • I was thinking more IO::HandleCache. Strictly, I'm dealing with handles not files now - it also avoids confusion with the depricated File::Cache module.


          I need to tidy everything up a bit first and maybe run some benchmarks.


          Fun.

          • It's FileCache not File::Cache.,
            and it isn't exactly caching files
            either. IO::FileCache highlights
            the relationship.
            --
            Were that I say, pancakes?
            • See! It is confusing. I was talking about the File::Cache [cpan.org] module that's been replaced by Cache::Cache.
              • Umm, except that's not CORE now is?
                And you did say CORE earlier, no? :-P

                And what do you mean you were talking
                about? You've been talking about
                cacheout, that's in FileCache...
                --
                Were that I say, pancakes?
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