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Re^6: undef/defined &{$name} while "strict refs"

by anazawa (Scribe)
on Aug 21, 2012 at 07:58 UTC ( #988630=note: print w/replies, xml ) Need Help??

in reply to Re^5: undef/defined &{$name} while "strict refs"
in thread undef/defined &{$name} while "strict refs"

When we execute my code above, the script throws an exception because &{ 'run' } violates "strict refs". Khen1950fx explained how to avoid this problem (I appreciate his suggestion).

On the other hand, when we comment out &{ 'run' }, my script doesn't output error messages. This result shows the following doesn't violate "strict refs":
my $walkable = defined &{ 'walk' }; undef &{ 'walk' };
It seems each line uses symbolic references. I don't understand why this is possible.

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Re^7: undef/defined &{$name} while "strict refs"
by Anonymous Monk on Aug 21, 2012 at 08:25 UTC

      No, here is the relevant quote from the documentation for the strict pragma:

      strict refs This generates a runtime error if you use symbolic references (see per +lref). ... There is one exception to this rule: $bar = \&{'foo'}; &$bar; is allowed so that goto &$AUTOLOAD would not break under stricture.

      and here is code demonstrating the problem:

      >perl -Mstrict -e "sub walk {} sub run {} my $walkable = defined(&{'wa +lk'}); my $runnable = &{'run'};" Can't use string ("run") as a subroutine ref while "strict refs" in us +e at -e line 1.

      So, the question is: How does the documented exception for use strict 'refs', which allows use of a reference to a symbolic sub, explain the observed fact that when defined is applied to the sub itself (not a reference), no runtime error is raised?

      Or, conversely: If the expression &{'walk'} is legal as an argument to defined, why is the expression &{'run'} illegal as the right-hand-side of an assignment?

      How (and why) does Perl distinguish between these two cases?

      Athanasius <°(((><contra mundum

        Ooooooh, well that could be called a bug

        But if you ask me, both 'run' and 'walk' being string constants, well, I don't see why either &{'run'} or &{'walk'} should be illegal

        Athanasius clarified what I wanted to say. Thanks a lot :) By the way, the following is equivalent to the code in question:
        no strict 'refs'; my $walkable = defined *{ 'walk' }{CODE};
        In this case, "no strict refs" is required and it seems reasonable. Should we cosider defined &{ 'walk' } not to be a recommended expression?

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