The "Guess the Version" Game

What version of Perl 5 is necessary to run this example?

say $message;

It depends. The obvious answer is any version newer than 5.10, at least if you use feature ':say'; or use 5.010;.

It can also be Perl 5.6 or 5.8, if you use Perl6::Say.

What version of Perl 5 is necessary to run this example?

sub foo
{
    state $foo = shift;
    say $foo;
}

foo( 100 );
foo( 200 );
foo( 400 );
foo( 800 );

Again, the obvious answer is any version newer than 5.10, but I can imagine having written code something like this for Perl 5.6 or 5.8:

my $foo;
my @lexicals;

sub state :lvalue
{
    push @lexicals, \(my $foo);
    ${ $lexicals[-1] } = shift;
    return $lexicals[-1];
}

The semantics aren't exactly the same, but that's the point. The syntax is similar enough that you can't reliably read the code and determine to which Perl 5 version the original author wrote!

This problem comes up over and over when someone proposes adding a new keyword to Perl 5. The likelihood that someone wrote Perl 5 code with this syntax is vanishingly low:

class Foo extends Bar {
    ...
}

... but it's not entirely impossible.

There is no general purpose heuristic by which the Perl 5 parser can determine the appropriate version of the Perl 5 syntax to use to parse any given piece of code. (That's not a problem specific to Perl 5, but I'm only talking about the Perl language family now.)

No one wants to break existing code. Yet if there's value to adding new features, there are only a few possibilities to find a balance between adding new features which may conflict with existing programs and not adding new features.

The current Perl 5 approach prefers to maintain existing programs unchanged. It's possible to take code written for Perl 1 (and last modified in 1987) and run it unmodified on Perl 5.11.2, released last month. I repeat, unmodified.

In other words, given a Perl 5 program which does not specifically declare the intended version of the Perl 5 syntax and (more important) keyword set, the Perl 5 parser prefers the oldest available keyword set. (Of course, any syntax which was impossible in older versions and is now available in newer versions may be available... but that doesn't require modifications to existing programs.)

That's not likely to change soon, if ever in Perl 5, and I can understand that policy especially given its two-decade history.

That does suggest changing how the Perl 5 community thinks about writing new code and modifying existing code under active development. I'll write more about that next time.

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This page contains a single entry by chromatic published on December 14, 2009 3:38 PM.

Perl and the Multiversion Grammar was the previous entry in this blog.

Safety in (Version) Numbers is the next entry in this blog.

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