From: Mathieu Bouchard <matju@...>
Date: 2005-04-24T23:25:57+09:00
Subject: Re: [ANN] Article: Seeing Metaclasses Clearly

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On Sun, 24 Apr 2005, gabriele renzi wrote:

> Mathieu Bouchard ha scritto:
>> On Thu, 21 Apr 2005, gabriele renzi wrote:
>> Because of backwards-compatibility with C++/Java/Perl/Python programmers=
=2E
>> Most backwards-compatibility issues occur wrt the _programmers_ and not =
so=20
>> much wrt the _programmes_.
> Python programmers for sure can define singleton methods directly in a ob=
ject=20
> without singleton classes,

I avow I don't know enough Python for that.

> I guess perl folks too, since an object is mostly=20
> an hash (I may be wrong, not really good at perl).

No: in Perl, a hash can contain anonymous subs (that is like Ruby procs),=
=20
but the method lookup is a different thing. For example, suppose you have=
=20
a variable $x holding a pointer to an object that is made out of a hash.=20
Then calling a proc that is an instance variable will look like:

$x->{hello}();

and that is also valid syntax for calling an anonymous sub in any Perl=20
hash, even if it doesn't have a class attached to it. But that mechanism=20
does not support inheritance at all. What is called a method-call in Perl=
=20
is:

$x->hello;

where hello is a _named_ sub in the package that has been assigned to the=
=20
$x hash using the bless() builtin.

In Ruby, this is the same difference as between those two expressions:

x.foo
x.instance_eval{@foo}[]

and clearly the latter isn't calling a method foo in x, and can't perform=
=20
anything inheritance-related either.

> Actually, only Java's anonymous classes seem somewhat similar to singleto=
n=20
> ones.

They are not. They are a special case of inner classes, and inner classes=
=20
blend the concepts of class, delegation, and closure, all together, but=20
nothing in Java is singleton. But still I am curious: could you please=20
provide a snippet in which you emulate singleton behaviour using Java's=20
anonymous classes?



  ,-o---------o---------o---------o-. ,----.                            |
  | The Diagram is the Program (TM) | |  ,-o----------------------------o-.
  `-o-----------------------------o-' |  | Mathieu Bouchard (Montr=E9al QC)=
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