FAQ 7.17 What's the difference between dynamic and lexical (static) scoping? Between local() and my

Discussion in 'Perl Misc' started by PerlFAQ Server, Jan 6, 2011.

  1. This is an excerpt from the latest version perlfaq7.pod, which
    comes with the standard Perl distribution. These postings aim to
    reduce the number of repeated questions as well as allow the community
    to review and update the answers. The latest version of the complete
    perlfaq is at http://faq.perl.org .

    --------------------------------------------------------------------

    7.17: What's the difference between dynamic and lexical (static) scoping? Between local() and my()?

    "local($x)" saves away the old value of the global variable $x and
    assigns a new value for the duration of the subroutine *which is visible
    in other functions called from that subroutine*. This is done at
    run-time, so is called dynamic scoping. local() always affects global
    variables, also called package variables or dynamic variables.

    "my($x)" creates a new variable that is only visible in the current
    subroutine. This is done at compile-time, so it is called lexical or
    static scoping. my() always affects private variables, also called
    lexical variables or (improperly) static(ly scoped) variables.

    For instance:

    sub visible {
    print "var has value $var\n";
    }

    sub dynamic {
    local $var = 'local'; # new temporary value for the still-global
    visible(); # variable called $var
    }

    sub lexical {
    my $var = 'private'; # new private variable, $var
    visible(); # (invisible outside of sub scope)
    }

    $var = 'global';

    visible(); # prints global
    dynamic(); # prints local
    lexical(); # prints global

    Notice how at no point does the value "private" get printed. That's
    because $var only has that value within the block of the lexical()
    function, and it is hidden from called subroutine.

    In summary, local() doesn't make what you think of as private, local
    variables. It gives a global variable a temporary value. my() is what
    you're looking for if you want private variables.

    See "Private Variables via my()" in perlsub and "Temporary Values via
    local()" in perlsub for excruciating details.



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    PerlFAQ Server, Jan 6, 2011
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