For my own code (I'm running 5.18.1), I probably would have used state as you've done there: your post got a ++ from me. However, unless the question specifically involves a more recent Perl, or I know the person I'm responding to is familiar with 5.10.0+, I find providing a pre-5.10 (or, more strict, an any Perl5) solution to be a lot less hassle: that would be non-virtuous laziness on my part. :-)

While there may be some subtlety I'm missing, which I'm more than happy to learn about, I really don't see any "tighter encapsulation". In both cases, the hash is only evaluated once and only visible by the expr subroutine. From a visual perspective, the hash is clearly more closely associated with the subroutine: in fact, that would be one of my reasons for preferring it.

There's another issue which I'd forgotten about until I ran a few tests. You can't simply substitute my with state in my %op_func = (...);. If you try this, you get a compilation error:

"Initialization of state variables in list context currently forbidden"

So, you need to code state $op_func = {...}; as you have here. Having to dereference $op_func incurs a minimal overhead which, in general, wouldn't prevent me from using it unless efficiency was of particular concern. I found the %op_func solution to be consistently 2% faster than the $op_func solution: 

$ perl -Mstrict -Mwarnings -E '
    use Benchmark qw{cmpthese};

    {
        my %op_func1 = (
            "+" => sub { $_[0] + $_[1] },
            "-" => sub { $_[0] - $_[1] },
            "*" => sub { $_[0] * $_[1] },
            "/" => sub { $_[0] / $_[1] },
        );

        sub expr1 {
            my ($lhs, $op, $rhs) = split " ", shift;
            $op_func1{$op}->($lhs, $rhs);
        }
    }

    sub expr2 {
        state $op_func2 = {
            "+" => sub { $_[0] + $_[1] },
            "-" => sub { $_[0] - $_[1] },
            "*" => sub { $_[0] * $_[1] },
            "/" => sub { $_[0] / $_[1] },
        };

        my ($lhs, $op, $rhs) = split " ", shift;
        $op_func2->{$op}->($lhs, $rhs);
    }

    my @ops = ("+","-","*","/");

    cmpthese(-1, {
        with_my    => sub { expr1("4 $_ 5") for @ops },
        with_state => sub { expr2("4 $_ 5") for @ops },
    });
'
               Rate with_state    with_my
with_state 112733/s         --        -2%
with_my    114840/s         2%         --

[download]

-- Ken

In reply to Re^3: Array of operators ... by kcott
in thread Array of operators ... by janDD

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.