In template toolkit: 
my $lazy = do {
  my $result;
  sub {
    unless (defined $result) {
      ... do expensive computation ...
      ... assigning to $result ...
    }
    return $result;
  };
};
my %vars = {
  lazy => $lazy,
  easy => 2 + 2,
};
$template->process('mything.tt', \%vars);

[download]
From the template, you use both "lazy" and "easy" the same way. Except that "lazy" is a coderef. That's what's cool about TT: the call is the same.

I seem to recall a module that does this for you. Yes, Data::Lazy.

-- Randal L. Schwartz, Perl hacker
Be sure to read my standard disclaimer if this is a reply.

In reply to Re: Templating algorithm - pass in variables vs callback? by merlyn
in thread Templating algorithm - pass in variables vs callback? by Tanktalus

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.