Dear Monks,

Having nothing better to do, I wondered whether that "last expression" that is returned by non-empty subroutines according to perlsub is well-defined. So I tested my understanding of that corner of the docs with these one-liners:

    % perl -wle 'sub foo { 1 for @_ } print 2 if defined foo()'
    % perl -wle 'sub foo { 1 for @_ } print 2 if defined foo(3)'
    2
    % perl -wle 'sub foo { 1 for @_ } print 2 if foo(3) == 0'   
    2
    % perl -wle 'sub foo { 1 for @_ } print 2 if foo(3) eq ""'
    2

[download]

and I am puzzled. I guess the difference between the first and the rest have to do with the underlying implementation, though I don't actually know it. From a conceptual point of view I wouldn't have been able to predict that behaviour.

Then, IF the same value is returned always in the rest of the cases, I don't understand how can it be at the same time defined, == 0, and eq "" (edited: and not issue the warning Argument "" isn't numeric in numeric eq (==) ...).

So, is "the last expression" well-defined albeit not always apparent? Are those one-liners just showing unespecified behaviour (which would be a valid answer to my original question)?

-- fxn

PS1: To make things more interesting someone in MagNet#perl pointed out that if you change the 1 with a 43 something different a warning is issued:

    % perl -wle 'sub foo { 43 for @_ } print 2 if defined foo()'
    Useless use of a constant in void context at -e line 1.

[download]

PS2: I have posted this question in a few places before, but got no definitive answer yet.

In reply to "last expression" quiz by fxn

Title:
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, details, 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, summary, 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.