in reply to Use of uninitialized value in substr

How many other ops do you know which grovel through the optree at runtime to find out the type of a child op in order to perform different behaviors?

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Re^2: Use of uninitialized value in substr
by BrowserUk (Patriarch) on May 03, 2010 at 19:26 UTC

    But there is no need for ops to "grovel through the optree at runtime".

    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]
      While his "at runtime" is a straw man, it's not as simple as you imply it is. A lot more than undef returns PL_sv_undef. For example, `foo` (that you insisted is completely unrelated to undef) also returns PL_sv_undef. 
      #! perl -slw
use 5.010;
use strict;
use Inline C => Config => BUILD_NOISY => 1;
use Inline C => <<'END_C',  NAME => 'uCmp', CLEAN_AFTER_BUILD => 0;

bool uCmp( SV* t ) {
    return (bool)( t == (&PL_sv_undef) );
}

END_C

print 'explicit undef: ', uCmp( undef         ) ? 1 : 0;
print '`foo`:          ', uCmp( scalar(`foo`) ) ? 1 : 0;

__END__
explicit undef: 1
Can't exec "foo": No such file or directory at a.pl line 14.
`foo`:          1

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[d/l]
[select]

        And you think that a programmer who

        1. Invokes a non-existant command,
        2. wrapped in scalar,
        3. as the fourth argument to substr

        And gets the target substring replaced by nothing--without a warning that the fourth parameter was undefined--is going to ignore the 

        Can't exec "foo": No such file or directory at a.pl line 14.

        [download]

        warning and throw their hands in the wondering what went wrong?

        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        "Science is about questioning the status quo. Questioning authority".
        In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]
[d/l]
        What am I missing here? Perl can decide what to do at compile time. In the case of substr perl could just replace undef with '' behind the scenes. Though personally I think '' is easier to understand (and easier to type).

        Elda Taluta; Sarks Sark; Ark Arks
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