how to reference a YAML "setting" from elsewhere in the same YAML file?

I have the following YAML:

paths:
  patha: /path/to/root/a
  pathb: /path/to/root/b
  pathc: /path/to/root/c

How can I "normalise" this, by removing /path/to/root/ from the three paths, and have it as its own setting, something like:

paths:
  root: /path/to/root/
  patha: *root* + a
  pathb: *root* + b
  pathc: *root* + c

Obviously that's invalid, I just made it up. What's the real syntax? Can it be done?

Solution 1:

I don't think it is possible. You can reuse "node" but not part of it.

bill-to: &id001
    given  : Chris
    family : Dumars
ship-to: *id001

This is perfectly valid YAML and fields given and family are reused in ship-to block. You can reuse a scalar node the same way but there's no way you can change what's inside and add that last part of a path to it from inside YAML.

If repetition bother you that much I suggest to make your application aware of root property and add it to every path that looks relative not absolute.

Solution 2:

Yes, using custom tags. Example in Python, making the !join tag join strings in an array:

import yaml

## define custom tag handler
def join(loader, node):
    seq = loader.construct_sequence(node)
    return ''.join([str(i) for i in seq])

## register the tag handler
yaml.add_constructor('!join', join)

## using your sample data
yaml.load("""
paths:
    root: &BASE /path/to/root/
    patha: !join [*BASE, a]
    pathb: !join [*BASE, b]
    pathc: !join [*BASE, c]
""")

Which results in:

{
    'paths': {
        'patha': '/path/to/root/a',
        'pathb': '/path/to/root/b',
        'pathc': '/path/to/root/c',
        'root': '/path/to/root/'
     }
}

The array of arguments to !join can have any number of elements of any data type, as long as they can be converted to string, so !join [*a, "/", *b, "/", *c] does what you would expect.

Solution 3:

Another way to look at this is to simply use another field.

paths:
  root_path: &root
     val: /path/to/root/
  patha: &a
    root_path: *root
    rel_path: a
  pathb: &b
    root_path: *root
    rel_path: b
  pathc: &c
    root_path: *root
    rel_path: c

Solution 4:

I've create a library, available on Packagist, that performs this function: https://packagist.org/packages/grasmash/yaml-expander

Example YAML file:

type: book
book:
  title: Dune
  author: Frank Herbert
  copyright: ${book.author} 1965
  protaganist: ${characters.0.name}
  media:
    - hardcover
characters:
  - name: Paul Atreides
    occupation: Kwisatz Haderach
    aliases:
      - Usul
      - Muad'Dib
      - The Preacher
  - name: Duncan Idaho
    occupation: Swordmaster
summary: ${book.title} by ${book.author}
product-name: ${${type}.title}

Example logic:

// Parse a yaml string directly, expanding internal property references.
$yaml_string = file_get_contents("dune.yml");
$expanded = \Grasmash\YamlExpander\Expander::parse($yaml_string);
print_r($expanded);

Resultant array:

array (
  'type' => 'book',
  'book' => 
  array (
    'title' => 'Dune',
    'author' => 'Frank Herbert',
    'copyright' => 'Frank Herbert 1965',
    'protaganist' => 'Paul Atreides',
    'media' => 
    array (
      0 => 'hardcover',
    ),
  ),
  'characters' => 
  array (
    0 => 
    array (
      'name' => 'Paul Atreides',
      'occupation' => 'Kwisatz Haderach',
      'aliases' => 
      array (
        0 => 'Usul',
        1 => 'Muad\'Dib',
        2 => 'The Preacher',
      ),
    ),
    1 => 
    array (
      'name' => 'Duncan Idaho',
      'occupation' => 'Swordmaster',
    ),
  ),
  'summary' => 'Dune by Frank Herbert',
);