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The builder–builder is a tool for creating (or extending) builder applications, or more simply, builders. Let’s read more about builder–builder!

Well, before we go any further about it, we must know the meaning of a builder. What is it??

A builder is a domain-specific language (DSL) for creating and/or representing hierarchies of related objects using a declarative syntax. By declarative syntax, I mean one that describes the (resultant) hierarchical structure, rather than the means by which it is created. The Cheri builder–builder, then, is a meta-DSL for creating builder DSLs. (However, in light of the above definition, the builder–builder is not, strictly speaking, a meta-builder, or a builder at all, as its syntax is not strictly declarative.)

This is provides an easy way to create builders based on the Cheri builder framework/engine (as are Cheri::Swing, Cheri::Xml and Cheri::Html). Before I bore you with a BNF diagram of Cheri’s particular take on builder syntax (honestly, I was about to), let me move on to an example, which should make things quite clear. It has 3 class information about the store, the authors, and the books.

A Really Simple Builder, about the store:

class Bookstore
  attr :name, true
  attr :address, true
  attr :phone, true
  attr :authors

  def initialize(name=nil)
    @name = name
    @authors = []
  end

  def add(author)
    @authors << author
  end
end

Cheri’s type matching logic works from most-specific class/module (nearest ancestor) to least, so this code will only be invoked if a would-be child object can’t be connected based on a more specific ancestor than Object. [Note that an upcoming version of the builder–builder will provide a shorthand method of specifying warnings/exceptions for unconnected objects.]

Here’s Our completed builder:

BookstoreBuilder = Cheri::Builder.new_builder do
  # specify each class to be built
  build Bookstore
  build Author
  build Book

  # specify how objects of each type connect to other objects
  type Bookstore do
    connect Author, :add_author
  end
  type Author do
    connect Book, :add_book
  end
  # issue warning for unconnected objects
  type Object do
    connect Object do |parent, child, symbol|
      warn "warning: can't connect #{child.class} (#{symbol}) to #{parent.class}"
    end
  end
end

We’ve been working with a very simple model: Bookstore => Author => Book.

For further details visit Cheri project page.

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This entry was posted on Friday, November 23rd, 2007 at 8:11 pm and is filed under Cool, Rails, Ruby, Tips, Tools, Web 2.0. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.