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Re^4: Use method/function signatures with Perl

by Ovid (Cardinal)
on Dec 06, 2004 at 16:03 UTC ( #412677=note: print w/ replies, xml ) Need Help??


in reply to Re^3: Use method/function signatures with Perl
in thread Use method/function signatures with Perl

I'm not sure I understand your question. My code doesn't actually care about the specific implementation. Can you give me an example of what you're thinking?

Cheers,
Ovid

New address of my CGI Course.

  • Comment on Re^4: Use method/function signatures with Perl

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Re^5: Use method/function signatures with Perl
by dragonchild (Archbishop) on Dec 06, 2004 at 16:16 UTC
    Let's say I have a subroutine defined as
    sub foo (ARRAY $arr) { ... }

    And, I have a class Bar. Today, it is defined as

    package Bar; sub new { bless [], shift }

    This means that

    my $bar = Bar->new; foo( $bar );

    will work just fine. But, what happens if the implementation of Bar changes from arrayref to hashref? This is a completely internal change, but one that will break the code listed above.

    Personally, I would define it to be

    if ($signature) { if (blessed $x) { return TRUE if $x->isa( $signature ); } elsif (ref $x) { return TRUE if ref $x eq $signature; } else { die "$x doesn't match $signature\n"; } }

    In other words, if it's been blessed, you only consider the class - not the underlying implementation.

    Being right, does not endow the right to be rude; politeness costs nothing.
    Being unknowing, is not the same as being stupid.
    Expressing a contrary opinion, whether to the individual or the group, is more often a sign of deeper thought than of cantankerous belligerence.
    Do not mistake your goals as the only goals; your opinion as the only opinion; your confidence as correctness. Saying you know better is not the same as explaining you know better.

      Now I understand what you're asking. You're actually dealing with some interesting stuff here, but I think that most of the concern is a theoretical non-issue, but in practice it is an issue, but I think we're looking at it slightly differently. For example, your initial code doesn't work:

      sub foo(ARRAY $bar) {...} # later foo(Bar->new);

      That breaks immediately because &foo is expecting an ARRAY, not an object of class Bar. It's an easy fix:

      sub foo(Bar $bar) {...}

      Now it doesn't matter how someone implements Bar internally. However, there is one (false) objection and one very real objection. The false objection is this:

      sub foo(Bar $bar) { print $bar->[3]; }

      Obviously, violating encapsulation is going to cause problems regardless of whether or not method signatures are implemented. The real problem, however, and I think this is what you're getting at, lies in this:

      my $parser = HTML::TokeParser->new(\$html); sub foo(ARRAY $token) {...} foo($parser->get_token) {...}

      HTML::TokeParser returns array references. However, what if someone wants to use HTML::TokeParser::Simple? That module blesses the references. As a result, it is no longer a drop-in replacement. I am forced to alter the above code. (And this just highlighted a bug in my code that I'll have to fix ASAP -- darn.)

      Cheers,
      Ovid

      New address of my CGI Course.

        That breaks immediately because &foo is expecting an ARRAY, not an object of class Bar.

        In other words, because you're (mis)using ref (which we agreed you shouldn't), you already implement the scheme I suggested. However, there's going to be the user (and it's usually someone like me ... :-) that would assume that since I'm using an array to represent my Bar class, it should fit into either an ARRAY or Bar hole.

        It may not be a code issue as much as a documentation issue. That said, there is still an issue. :-)

        Being right, does not endow the right to be rude; politeness costs nothing.
        Being unknowing, is not the same as being stupid.
        Expressing a contrary opinion, whether to the individual or the group, is more often a sign of deeper thought than of cantankerous belligerence.
        Do not mistake your goals as the only goals; your opinion as the only opinion; your confidence as correctness. Saying you know better is not the same as explaining you know better.

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