How to use YAML Aliases

Have you ever had to copy and paste duplicate content in a YAML file and wondered if it is possible to DRY that up? As it turns out, YAML allows you to repeat nodes via aliases.

YAML Aliases allow you to assign a name to a value or block of data and recall the assigned data by its name in the YAML file. Aliases should work for any file written in YAML.

Simple Example

hello: &hello 'hello'
greeting:
  audience: 'world'
  hello: *hello  #greeting.hello has the string value of 'hello'
new_greeting:
  audience: 'room'
  hello: *hello  #new_greeting.hello has the string value of 'hello'

Aliasing blocks of data

Besides string or number values, you can alias a block of data as well:

foo:
  bar: &bar
    qux: 'quxqux'
    baz: 'bazbaz'
greeting:
  audience: 'world'
  bar: *bar  #greeting.bar has the same values as foo.bar.
             #So greeting.bar.baz is 'bazbaz'

Modifying a section of an alias

You can copy an alias and modify a section of it by using the merge key (<<:):

bar: &bar
  qux: 'quxqux'
  baz: 'bazbaz'
greeting:
  audience: 'world'
  bar:
    <<: *bar       # greeting.bar.qux is 'quxqux'
    baz: 'notbaz'  # greeting.bar.baz is 'notbaz'

Defining aliases from modified aliases

You can even modify a section of an alias and define that as a new alias:

bar: &bar
  qux: 'quxqux'
  baz: 'bazbaz'
greeting:
  audience: 'world'
  bar: &newalias
    <<: *bar
    baz: 'notbaz'
new_greeting:
  audience: 'room'
  bar: *newalias  #new_greeting.bar.baz is 'notbaz'

YAML specification

Alias nodes are described in the YAML specification here