Beefy Boxes and Bandwidth Generously Provided by pair Networks
Perl-Sensitive Sunglasses

comment on

( #3333=superdoc: print w/replies, xml ) Need Help??

As ar0n already replied, the constant pragma does create a subroutine. If you're curious, this is an inlined subroutine with a null prototype (one of the few places where a prototype is really useful). Looking in the constant module, the following code is where the constant is actually created:

{ no strict 'refs'; my $full_name = "${pkg}::$name"; $declared{$full_name}++; if (@_ == 1) { my $scalar = $_[0]; *$full_name = sub () { $scalar }; } elsif (@_) { my @list = @_; *$full_name = sub () { @list }; } else { *$full_name = sub () { }; } }

What the null prototype does is allow your constant to behave like a Perl built-in:

use constant FOO => 7; print FOO + 1;

That prints 8, just like you would expect. Contrast that to this:

sub FOO {7}; print FOO + 1'

That will print 7. Why? Because Perl will interpret the +1 as being an argument to &FOO, which is silently discarded as it is not used. Yuck! With the null prototype, Perl knows that nothing coming after FOO can be an argument, so everything parses as you expect. It's also nice to note, from the docs, that as of Perl 5.004, Perl will replace all instances of your constant with the returned value, thus saving you the overhead of a subroutine call.


Join the Perlmonks Setiathome Group or just click on the the link and check out our stats.

In reply to (Ovid -- constant internals) by Ovid
in thread Accessing Constant Hashes by enoch

Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":

  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.
  • Log In?

    What's my password?
    Create A New User
    and the web crawler heard nothing...

    How do I use this? | Other CB clients
    Other Users?
    Others exploiting the Monastery: (5)
    As of 2020-10-20 12:00 GMT
    Find Nodes?
      Voting Booth?
      My favourite web site is:

      Results (210 votes). Check out past polls.