January 2008
Beginner to intermediate
446 pages
14h 45m
English
Content preview from The Ruby Programming LanguageBecome an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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Domain-Specific Languages
The goal of metaprogramming in Ruby is often the creation of domain-specific languages, or DSLs. A DSL is just an extension of Ruby’s syntax (with methods that look like keywords) or API that allows you to solve a problem or represent data more naturally than you could otherwise. For our examples, we’ll take the problem domain to be the output of XML formatted data, and we’ll define two DSLs—one very simple and one more clever—to tackle this problem.[*]
Simple XML Output with method_missing
We begin with a simple class named XML for generating XML output. Here’s an
example of how the XML can be
used:
pagetitle = "Test Page for XML.generate"
XML.generate(STDOUT) do
html do
head do
title { pagetitle }
comment "This is a test"
end
body do
h1(:style => "font-family:sans-serif") { pagetitle }
ul :type=>"square" do
li { Time.now }
li { RUBY_VERSION }
end
end
end
end
This code doesn’t look like XML, and it only sort of looks like Ruby. Here’s the output it generates (with some line breaks added for legibility):
<html><head> <title>Test Page for XML.generate</title> <!-- This is a test --> </head><body> <h1 style='font-family:sans-serif'>Test Page for XML.generate</h1> <ul type='square'> <li>2007-08-19 16:19:58 -0700</li> <li>1.9.0</li> </ul></body></html>
To implement this class and the XML generation syntax it supports, we rely on:
Ruby’s block structure
Ruby’s parentheses-optional method invocations
Ruby’s syntax for passing hash literals to methods without curly braces ...
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