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Re: Re: Tutorial: Introduction to Object-Oriented Programming

by BrowserUk (Pope)
on Dec 13, 2002 at 17:59 UTC ( #219668=note: print w/ replies, xml ) Need Help??


in reply to Re: Tutorial: Introduction to Object-Oriented Programming
in thread Tutorial: Introduction to Object-Oriented Programming

Thanks Abigail.

And no. I had no insights as to any of the limitations.

Until now I have had no real use for OO in my exploration of Perl, so i have basically avoided it. I have now started something that would benefit from OO which happens to coincide with this thread and your post.

I read a couple of earlier peices on light-weight/inside-out objects, as well as several peices by you and others on the merits of OO in perl, so I just wanted to know what I was getting into. You can see my first attempt as using your method at (my?) problem with re-blessed references(?) though if your interested you'd better look at adrianh's correction of my idiocies at Re: Re: (my?) problem with re-blessed references(?).

I'd be interested to hear your thoughts on using lvalue subs for the accessor/mutators? I've long prefered the syntax of one subname per attribute that returns the value optionally setting it if a value is passed. I hated the whole "should it be set_x/put_x or set_x/get_x debate" that rages still in the Java discussion groups. Another totally pointless, "religious-type", waste of time IMO. I also hate having to look up which convention is used for every attribute of every class I use.

The lvalue sub seems to me to be the logical conclusion to this. One attribute, one name. Name it to use it. Assign to it to set it. In-line assign and use just like any other var. Makes perfect sense to me, but I'm sure that others have a different opinion.


Examine what is said, not who speaks.


Comment on Re: Re: Tutorial: Introduction to Object-Oriented Programming
lvalue considered harmful...
by adrianh (Chancellor) on Dec 14, 2002 at 16:33 UTC
    I'd be interested to hear your thoughts on using lvalue subs for the accessor/mutators?

    Don't like them. No sir! Don't like them one bit.

    For me the problem with lvalue subs is that they expose your object implementation just a little bit too much. Consider:

    use strict; use warnings; my (%Foo, %Bar); sub foo : lvalue { $Foo{+shift} }; sub bar { my $key = shift; $Bar{$key} = shift if @_; $Bar{$key}; }; foo("apples") = 12; foo("pears") = 0; print foo("apples"), "\n"; print foo("pears"), "\n"; bar(bananas => 3); bar(oranges => 18); print bar("bananas"), "\n"; print bar("oranges"), "\n";

    Now, what happens if I want to prevent negative values being assigned to the hashes. With bar() it's trivial.

    sub bar { my $key = shift; if (@_) { die "no negative numbers" if $_[0] < 0; $Bar{$key} = shift; }; $Bar{$key}; };

    With foo... it isn't trivial.

    It's possible with a tied hash - but it's not simple. As soon as you need to do things with the values you are assigning (check them for validity, transform them, etc.) things suddenly get hard. You can end up with a separate tied class for each attribute.

    When you get to this stage "normal" subroutines look good again - but then you're changing your API. Nasty. Best to stick with normal subroutines (be it perls all-in-one or separate setter/getter) from the start.

    I really hope that perl6 takes a lesson from Eiffel and makes class attribute access and method calls look the same - that way all this nonsense can go away. Lexical subs used in this way are really just syntactic sugar for a $hash->{attr} and should be avoided most of the time (IMHO of course :-)


    Updates

      Agreed.

      Though I would argue that the problem is not the concept of lvalue subs that is at fault, but just the current implementation.

      As they stand currently, they are almost completly useless. The fact that there is no way to access the value being assigned prior to it's assignment, and that any attempt to check it after assignment prevents the assignment, is simply bewildering.

      Why the value is not made available to the sub as the last value in @_ I simply don't understand. The current implementation where the assigned value magically overwrites the last available lvalue in the sub if there is one and breaks if there isn't is just simply broken.

      Shame. No wonder nobody uses them.


      Examine what is said, not who speaks.

        One of my favourite languages, Pop-11, does it in quite a nice way. You define a separate subroutine for the lvalue side - known as an updater.

        For example, a function to access and update the second element of a linked list can be written in Pop-11 like this:

        define second(list); hd(tl(list)) enddefine; define updaterof second(value, list); value -> hd(tl(list)) enddefine; vars list; [a b c d] -> list; second(list) => ** b 42 -> second(list); second(list) => ** 42 list => ** [a 42 c d]

        All the advantages of writing separate setter/getter functions without the noise (as I see it) in the function names.

        In Pop-11 lvalues are poorly named, since they appear on the right! Doing value -> something in Pop-11 is the same as something = value in perl.

        I find it reads well (something goes into something else) and it also solves that whole "confusing = with ==" problem.

      Have a look at Attribute::Property on CPAN. It could easily be modified to use inside out objects, the current hype in Perl OO land.
        My impression is that the inside out hype has abated, with Moose mania being the current trend :-)

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