Ruby, Rails and DSLs

Recently I have been looking to the Ruby on Rails framework.
I am not currently working on the kind of web application Rails
is suitable for, so I have no concrete use for the technology.
However, I think it's really a nice tool and it *does* show you
how simple web applications can be. Java EE *is* overly complicated.

What I find even more interesting is the Ruby language itself and
what you can do with it, especially with regards to building DSLs.
Take a look at typical Rails code: it looks much more like declarative
programming, or said differently, Rails comes close to a DSL for
building web apps, not "just" a framework.

For example, you can write the following code:

class Part < ActiveRecord::Base
  belongs_to :whole
end

This piece of code defines a model class (model in the sense
of MVC) called part. It is made persistent by the ActiveRecord
framework. To enable this functionality, you have to extend from
the ActiveRecord::Base class. So far so good. The interesting thing
is the belongs_to statement. What does this to?

It's purpose is easily explained. It states that this class (Part)
belongs to (i.e. is a part of) the Whole class (that class has to be
defined somewhere ... I don't show it here).

What happens technically is quite interesting. The belongs_to statement
is actually a method call. An alternative notation is as follows:

class Part < ActiveRecord::Base
  belongs_to( "whole" )
end

So, as a part of the class definition, you call a method? Yes,
that's actually possible. Where does it come from? It is defined
as a class method (static, for Java folks) in the Base class from
which we inherit. So the extension mechanism makes the class methods
available in the super class accessible to the currentl class
definition.

But it's even more interesting what happens inside the method.
The belongs_to method adds additional methods to the current
class! The implementation of the belong_to method calls the
define_method operation, which is a kind of "meta method" that
you can use to dynamically modify the program you're currently
writing. So, belong_to adds the following methods:

whole returns the whole to which this part belongs
whole=(whole) assigns this part to a new whole
whole.nil? is a whole assigned?
etc. etc.

So why is this important? It is important because you can use
the belongs_to just like a declaration. It looks as if it were
part of the Ruby language. Compare these two lines of code:

class Part
belongs_to :whole

The first line is core syntax of Ruby. The second line calls
an application defined method. However, the two lines look quite alike.
This means that you can basically define your own syntactical
elements. And *that* means, you can nicely build DSLs by "extending"
the Ruby language. Instead of generating code (using an external
code generator) you use Ruby's reflection mechanisms mechanisms to
do all that dynamically.

Nice.  permalink