From: Phlip <phlip2005@...>
Date: 2007-08-16T03:19:44+09:00
Subject: Re: Where can I find eval rules for ruby?

Daryoush Mehrtash wrote:

> I am trying to understand Domain Specific Language implementation in Ruby,
> there are lot of examples such as
> http://www.infoq.com/articles/eval-options-in-ruby, but I can't find a
> definitive guide on the interpreters rule for evaluating a line of DSL
> String in instance_eval call.

DSL is a state of mind, not a language inside a string inside a language.

The state of mind is that you intend to write in one language style,
inside another language. For example, Rake is a _light_ form of rule
solver, so you can write it in a format paralleling a Makefile:

  task :rule_1 => [:rule_2, :rule_C] do
     evaluate_rule_1
  end

To write in a declarative mode, write object methods with store their
commands in an engine, and then process the engine. Here's a little
Flea script to draw a spiral:

egg = startOvum(18, 45, 100)
ax = egg.newRootAxiom('A')
b  = egg.newAxiom('B')

ax.longer(6).right.color(1).gosub(b)
b.tube.shorter(0.8).left.link(b)
render egg

The commands like longer, right, color, etc are turtle graphics
describing a shape. render then outputs the shape.

All the ActiveRecord association commands - has_many, belongs_to, etc
- qualify as DSLs too. The goal is to give your user-programmer a
clear but deep interface, so they can write the command they need, in
the relevant context they need, and the engine on the inside hides the
complexity.

-- 
 Phlip
 http://www.oreilly.com/catalog/9780596510657/
 ^ assert_xpath
 http://tinyurl.com/yrc77g  <-- assert_latest Model