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Chapter 04

4.1 More Blocks

Blocks play a vital role in Fancy. They're used for many different things that all have one thing in common: Allow seperation of concerns in a natural way and providing the possibility to write code close to your problem domain. This means writing DSLs in Fancy is a natural and easy thing to do, due to the keyword-based message syntax.

4.2 Partial Blocks

Fancy has the concept of Partial Blocks. Partial Blocks behave like regular Blocks but have their argument ommitted. They always only take one argument which is implicit and that argument gets prefixed into any message send within the Block as its receiver. They allow Ruby-style simple DSLs while not relying on instance_eval, as many Ruby DSLs do, thus preserving things like the binding of self within the Block. Examples for libraries using instance_eval heavily are Sinatra and parts of Ruby on Rails.

One good example is Fancy's built-in HTML Generator Class which can be used for writing Webapps, for example. I wrote a simple url-shortener webapp with Sinatra for Fancy. See here. You'll also find a link to a Screencast there.

Let's have a look at what I mean.

4.2.1 Without Partial Blocks

require: "html"

html = HTML new: |h| {
  h html: |h| {
    h head: |h| {
      h title: "Using HTML Class"
    }
    h body: |h| {
      h div: { id: "header" } with: |h| {
        h h1: { class: "title" } with: "My Title"
      }
      h div: { id: "footer" } with: |h| {
        h h3: "The End."
      }
    }
  }
}

4.2.2 And the same code with Partial Blocks

require: "html"

html2 = HTML new: @{
  html: @{
    head: @{
      title: "Using HTML Class"
    }
    body: @{
      div: { id: "header" } with: @{
        h1: { class: "title" } with: "My Title"
      }
      div: { id: "footer" } with: @{
        h3: "The End."
      }
    }
  }
}

Both code snippets will produce this output:

<html>
  <head>
    <title>
      Using HTML Class
    </title>
  </head>
  <body>
    <div id="header">
      <h1 class="title">
        My Title
      </h1>
    </div>
    <div id="footer">
      <h3>
        The End.
      </h3>
    </div>
  </body>
</html>

Since Partial Blocks preserve the meaning of self, you can use the HTML generator class for your view code and don't need to use a seperate template language, as all instance variables will be bound nicely, depending on where you put that code.