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warning: tied vars may evaluate twice in logical expressions

by danger (Priest)
on Sep 27, 2001 at 01:29 UTC ( #114940=perlmeditation: print w/replies, xml ) Need Help??

Logical AND and OR may call FETCH on tied variable more than once. Here is an example using Tie::Cycle, which cycles through a list of values on each access:

#!/usr/bin/perl -w use strict; use Tie::Cycle; tie my $cycle, 'Tie::Cycle', [0,1,2,3]; for(1..7){ print $cycle, " "; } # printed thus far: 0 1 2 3 0 1 2 print "\n", '-' x 10, "\n"; # logical OR returns left expression if TRUE otherwise right # expression: my ($x,$y) = (0,42); my $z; $z = $x || $y; print "\$z is $z\n"; # prints: $z is 42 (as expected) $z = $y || $x; print "\$z is $z\n"; # prints: $z is 42 (as expected) print '-' x 10,"\n"; # next use of $cycle should get 3: my $ook = $cycle || 42; print "\$ook is $ook\n"; # prints: $ook is 0

And, of course, one wouldn't expect any logical expression of the form (EXPR || 42) to evaluate to 0. In that last nugget, $cycle is evaluated and found to be 3 which is true but instead of just returning that value, perl fetches $cycle's value once again. This only happens when the expression is a bare variable, not when it is part of any other expression: That is, had I said  my $ook = $cycle + 0 || $z;, then the expression would have returned 3 (ie, 3 + 0). The same (mis)behaviour can be shown with logical AND as well. In either case, it appears only to be triggered on a short circuit.

This *seems* unrelated to a similar bug (double evaluation of tied variables in interpolated strings) I mentioned in this node.

Replies are listed 'Best First'.
Re: warning: tied vars may evaluate twice in logical expressions
by blakem (Monsignor) on Sep 27, 2001 at 04:07 UTC
    merlyn mentioned what I believe to be a similar bug, while writing the code to one of his most recent articles. I don't think the article explaining this code is online yet, but it relates to traversing a file system.

    To see the bug in action, save the code to something like tiefinder.pl. Uncomment the 'DEBUG: fetching' line and invoke something like ./tiefinder.pl /tmp You should notice the DEBUG line getting printed twice as often as you might expect.

    I don't have an answer for you (and at the time, neither did merlyn) but perhaps this can help you zero in on the problem.

    -Blake

Re: warning: tied vars may evaluate twice in logical expressions
by japhy (Canon) on Sep 27, 2001 at 07:12 UTC
    I think I'll take a crack at fixing the bug.

    _____________________________________________________
    Jeff[japhy]Pinyan: Perl, regex, and perl hacker.
    s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;

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