Your question does not ask for a Python solution, but here is one using PyYAML.

PyYAML allows you to attach custom constructors (such as !include) to the YAML loader. I've included a root directory that can be set so that this solution supports relative and absolute file references.

Class-Based Solution

Here is a class-based solution, that avoids the global root variable of my original response.

See this gist for a similar, more robust Python 3 solution that uses a metaclass to register the custom constructor.

import yaml
import os

class Loader(yaml.SafeLoader):

    def __init__(self, stream):

        self._root = os.path.split(stream.name)[0]

        super(Loader, self).__init__(stream)

    def include(self, node):

        filename = os.path.join(self._root, self.construct_scalar(node))

        with open(filename, 'r') as f:
            return yaml.load(f, Loader)

Loader.add_constructor('!include', Loader.include)

An example:

foo.yaml

a: 1
b:
    - 1.43
    - 543.55
c: !include bar.yaml

bar.yaml

- 3.6
- [1, 2, 3]

Now the files can be loaded using:

>>> with open('foo.yaml', 'r') as f:
>>>    data = yaml.load(f, Loader)
>>> data
{'a': 1, 'b': [1.43, 543.55], 'c': [3.6, [1, 2, 3]]}

Includes are not directly supported in YAML as far as I know, you will have to provide a mechanism yourself however, this is generally easy to do.

I have used YAML as a configuration language in my python apps, and in this case often define a convention like this:

>>> main.yml <<<
includes: [ wibble.yml, wobble.yml]

Then in my (python) code I do:

import yaml
cfg = yaml.load(open("main.yml"))
for inc in cfg.get("includes", []):
   cfg.update(yaml.load(open(inc)))

The only down side is that variables in the includes will always override the variables in main, and there is no way to change that precedence by changing where the "includes: statement appears in the main.yml file.

On a slightly different point, YAML doesn't support includes as its not really designed as as exclusively as a file based mark up. What would an include mean if you got it in a response to an AJAX request?

For Python users, you can try pyyaml-include.

Install

pip install pyyaml-include