...you would have to evaluate the right hand side before the left hand side...

I'm not sure that I understand you. The right-hand side of an assignment is always evaluated before the assignment can occur? Except maybe in the case of assigning a constant, but even then the value of that constant is fetched from wherever before it can be assigned to the lvalue.

It's quite possible that I do not fully grasp the purpose or magic that lies behind the implementation of lvalue subs, but any code in an lvalue sub is invoked as a sideeffect of the assignment to an lvalue sub: 

{ my $x; sub x: lvalue { print 'fred'; $x } };
x = 123;
fred

[download]

The problem is that you can't do anything useful with that code as you cannot affect the outcome of the assignment relative to the value being assigned.

Examine what is said, not who speaks.
Silence betokens consent.
Love the truth but pardon error.

In reply to Re^3: Experimenting with Lvalue Subs by BrowserUk
in thread Experimenting with Lvalue Subs by Limbic~Region

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.