How is metaprogramming in Ruby better?

[Joseph Rose]

Aloha,

I have recently picked up an interest in Ruby and I am curious to learn about metaprogramming. What makes it different then regular programming practices?

Thanks.

[Katrina Owen]

Better? It does seem that asking which is better is a bit like you are comparing raspberries and steak. Both are great, useful, and healthy, but have different (and perhaps complimentary) uses. I don't think I'd want to choose one or the other exclusively.

[Paolo Perrotta]

It's difficult to explain how metaprogramming works and how it's useful in just a few sentences (unless you already know about it, and then it feels very easy and natural).
Instead, I can point you to the Introduction chapter from Metaprogramming Ruby. You can download it freely here: http://media.pragprog.com/titles/ppmetr/introduction.pdf .
Enjoy!

[David Newton]

Metaprogramming is programming about programming: it's code that writes code, but in a different way than compile-time generation like most Java code generation. Because Ruby is highly dynamic, things like getters and setters can be written by a method that writes additional methods:

class Foo
    attr_accessor :bar
end

This ends up generating a class that looks like this:

class Foo
    def bar()
        @bar
    end

    def bar=(newBar)
        @bar = newBar
    end
end

A trivial example, yes, but it's important to understand when and how this is happening. This allows very simple programmatic generation of instance methods *at runtime*. Now think about something like this:

class Foo < ActiveRecord::Base
    validates_presence_of :title
    validates_length_of :title, :minimum => 1
end

Now, with an absolute minimum of intrusiveness, we've defined some instance requirements, at runtime, in code, using a mini-validation DSL. validates_presence_of and validates_length_of are methods that generate behavior.

One thing I worked on was a test framework for web pages. Driving Selenium, I was able to code and test pages like this:

class LoginPage
    has_textfield :id
    has_password :password
    has_button :login
end

class TestLoginPage
    def test_login_ok
        id.type "foobar"
        password.type "bazplugh"
        login.click

        assert_url_is home_page
    end
end

Like Lisp macros, metaprogramming can be difficult to explain, but the power and clarity it can provide is *huge*.