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YAML AND XML COMPARED 262

XML is intended to be human-readable and self-describing. XML is

human-readable because it is a text format, and it is self-describing

because data is described by elements such as <user>, <username>, elements

and <homepage> in the preceding example. Another option for repre￾senting usernames and home pages would be XML attributes:

<user username="stu" homepage="http://blogs.relevancellc.com"></user>

The attribute syntax is obviously more terse. It also implies seman￾tic differences. Attributes are unordered, while elements are ordered.

Attributes are also limited in the values they may contain: Some char￾acters are illegal, and attributes cannot contain nested data (elements,

on the other hand, can nest arbitrarily deep).

There is one last wrinkle to consider with this simple XML document.

What happens when it travels in the wide world and encounters other

elements named <user>? To prevent confusion, XML allows names￾paces. These serve the same role as Java packages or Ruby modules, namespaces

but the syntax is different:

<rel:user xmlns:rel="http://www.relevancellc.com/sample"

username="stu"

homepage="http://blogs.relevancellc.com">

</rel:user>

The namespace is http://www.relevancellc.com/sample. That would be a

lot to type in front of an element name, so xmlns:rel establishes rel as a

prefix. Reading the previous document, an XML wonk would say that

<user> is in the http://www.relevancellc.com/sample namespace.

YAML is a response to the complexity of XML (YAML stands for YAML

Ain’t Markup Language). YAML has many things in common with XML.

Most important, both YAML and XML can be used to represent and seri￾alize complex, nested data structures. What special advantages does

YAML offer?

The YAML criticism of XML boils down to a single sentence. XML has

two concepts too many:

• There is no need for two different forms of nested data. Elements

are enough.

• There is no need for a distinct namespace concept; scoping is suf￾ficient for namespacing.

To see why attributes and namespaces are superfluous in YAML, here

are three YAML variants of the same configuration file:

YAML AND XML COMPARED 263

Download code/rails_xt/samples/why_yaml.rb

user:

username: stu

homepage: http://blogs.relevancellc.com

As you can see, YAML uses indentation for nesting. This is more terse

than XML’s approach, which requires a closing tag.

The second XML example used attributes to shorten the document to a

single line. Here’s the one-line YAML version:

Download code/rails_xt/samples/why_yaml.rb

user: {username: stu, homepage: http://blogs.relevancellc.com}

The one-line syntax introduces {} as delimiters, but there is no semantic

distinction in the actual data. Name/value data, called a simple map￾ping in YAML, is identical in the multiline and one-line documents. simple mapping

Here’s a YAML “namespace”:

Download code/rails_xt/samples/why_yaml.rb

http://www.relevancellc.com/sample:

user: {username: stu, homepage: http://blogs.relevancellc.com}

There is no special namespace construct in YAML, because scope pro￾vides a sufficient mechanism. In the previous document, user belongs

to http://www.relevancellc.com/sample. Replacing the words “belongs to”

with “is in the namespace” is a matter of taste.

It is easy to convert from YAML to a Ruby object:

irb(main):001:0> require 'yaml'

=> true

irb(main):002:0> YAML.load("{username: stu}")

=> {"username"=>"stu"}

Or from a Ruby object to YAML:

irb(main):003:0> YAML.dump 'username'=>'stu'

=> "--- \nusername: stu"

The leading - – \n: is a YAML document separator. This is optional, and

we won’t be using it in Rails configuration files. See the sidebar on the

next page for pointers to YAML’s constructs not covered here.

Items in a YAML sequence are prefixed with ’- ’:

- one

- two

- three