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Postings from March 2008
On Thu, March 27, 2008 11:28 am, Peter Scott wrote:
> I just found the discussion at
> http://groups.google.com/group/perl.perl5.porters/browse_thread/thread/540
> 1ccdcbd6d4fa1/
> the hard way. It appears to have petered out and I would like to reopen
> it.
>
> Posit that you are on a large networked filesystem where many user home
> directories are segmented by first letter, e.g., /home/a/alfred,
> /home/x/xavier, etc. And a user wishes to vet a list of putative
> usernames to see if they have home directories. A natural solution is:
>
> print "$_ not found\n" unless glob "/home/?/$_" for @putative_users;
>
> Elegant, yes. Correct, no. Because when glob() matches the first
> username, on the next call, it will ignore its argument and return undef
> to indicate that it has no more matches for the previous username.
>
> Imagine that user is then trying to use the debugger that you have taught
> them to use, on an expanded version of this so they can inspect $_
> before the glob call, and they find that when they type
>
> DBG> x glob "/home/?/$_"
>
> it produces a different result from what the program itself does when
> they take the next step. It's somewhat embarrassing and not a little
> convoluted to explain to them that this is their fault.
Carried to extremes, that argument would banish all context-dependent
behavior.
> I think that scalar glob iteration should be reset when the value of its
> expression changes. This is what Schwern and chromatic espoused in the
> thread I cite above.
I'm afraid to look in that thread lest I disagree with myself, but at
this time, I feel like this would be sweeping the problem under the
carpet. In point of fact, reusable code which uses scalar-context glob
without making sure to exhaust the iterator is buggy. Suppose you have
this code instead:
sub direxists {
print "checking for $_[0]\n";
print "$_[0] not found\n" unless glob "/home/?/$_[0]"
}
Your proposed fix would make it work except in the case of being
called with the same parameter twice in a row (where the directory
does exist), an even harder problem to debug.
I'd rather see a warning for scalar-context glob not in a while()
condition, or something like that. (And even that is buggy if the
while loop may be prematurely terminated with last or an exception.)
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