From: "David A. Black" <dblack@...>
Date: 2007-10-26T02:44:45+09:00
Subject: Re: Need help understanding metaclassing

Hi --

On Fri, 26 Oct 2007, Chris Czub wrote:

> I've been reading lots of Ruby tutorials trying to figure out
> metaclassing - the one that got me closest was Why?s poignant guide to
> Ruby, but I'm still struggling over some of the Ruby-specific syntax.
>
> I attached a class I've been writing that needs to be highly extensible,
> and it's loaded with comments I wrote to try to understand each part of
> it. If any part of the comments are wrong, it would be nice to have my
> misconceptions cleared up. I've only been using Ruby for a day and, wow.
>
> Here's the part I'm having the hardest time figuring out, and Why?s
> guide kind of skipped over it:
>
>      metaclass.instance_eval do
>        define_method( a ) do |val|
>          @attribs ||= {}
>          @attribs[a] = val
>        end
>      end
>
> So, I understand that the block inside the instance_eval will be
> executed in the instance of the object(meaning when I create a new
> object, right?)
>
> However, I don't understand the next part. define_method is a method
> call to the method that is defined further down, right? But what is the
> " do |val|" after the method call, and what is the point of the two
> lines after that?
>
> If anyone could explain this to me, I would be greatly indebted.

The do/end sequence is a code block. Have a look at some of the
tutorials -- they'll definitely talk about code blocks. When you pass
a code block to define_method, the code block serves as the method
body for the method you're defining.

The lines after that assign an empty hash to @aattribs (except if
@attribs is already set, the assignment is skipped; that's what ||=
does), and sets the value of @attribs[a] to val. val is the block
parameter, and will therefore also be a method parameter for the new
method.


David

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