Introduction to YAML

A serialization language for configuration files.

YAML stands for YAML Ain't Markup Language, but it originally stood for Yet Another Markup Language.
YAML is a human-readable data serialization language that is often used for writing configuration files.
Serialization is a process where one application or service that has different data structures and is written in a different set of technologies can transfer data to another application using a standard format.
serialization is about translating, converting, and wrapping up a data structure in another format.
XML, JSON, and YAML are all used for creating configuration files and transferring data between applications.
YAML is often used as a format for configuration files, but its object serialization abilities make it a viable replacement for languages like JSON.
YAML files use a .yml or .yaml extension.
YAML is an official strict superset of JSON despite looking very different from JSON.
YAML is also a superset of JSON, so JSON files are valid in YAML. However, note that newlines and indentation actually mean something in YAML, as opposed to JSON, which uses brackets and braces.
Like JSON, Key/value pairs are the building blocks of YAML documents.
YAML files use Python-style indentation to determine the structure and indicate nesting. Tab characters are not allowed for indentation by design, to maintain portability across systems, so whitespaces—literal space characters—are used instead.
Whitespace is part of YAML's formatting. Unless otherwise indicated, newlines indicate the end of a field. You structure a YAML document with indentation. The indentation level can be one or more spaces. The specification forbids use of tabs.
A comment begins with pound or hash symbol #.
No multiline comments.
Three dashes --- are used to signal the start of a document
Three dots ... can be used to signal the end of a document (it is optional)
YAML supports multiple documents in a single file, and compliant parsers will recognize each set of three-dashes as the beginning of a new one.
The structure of a YAML file is a map or a list, and it follows a hierarchy depending on the indentation, and how you define your key values.
In YAML, there is an emphasis on indentation and line separation to denote levels and structure in data. The indentation system is quite similar to the one Python uses.
YAML doesn't use symbols such as curly braces-, square brackets-[], or opening or closing tags - just indentation.
Although YAML auto-detects the data types in a file, you can specify the type of data you want to use.
To explicitly specify the type of data, use the !! symbol and the name of the data type before the value: date: !!str 2022-11-11 or fave_number: !!float 1

From JSON to YAML

{
  "doe": "a deer, a female deer",
  "ray": "a drop of golden sun",
  "pi": 3.14159,
  "xmas": true,
  "french-hens": 3,
  "calling-birds": ["huey", "dewey", "louie", "fred"],
  "xmas-fifth-day": {
    "calling-birds": "four",
    "french-hens": 3,
    "golden-rings": 5,
    "partridges": {
      "count": 1,
      "location": "a pear tree"
    },
    "turtle-doves": "two"
  }
}

using https://www.json2yaml.com/ site the above JSON is converted to following YAML

---
doe: a deer, a female deer
ray: a drop of golden sun
pi: 3.14159
xmas: true
french-hens: 3
calling-birds:
  - huey
  - dewey
  - louie
  - fred
xmas-fifth-day:
  calling-birds: four
  french-hens: 3
  golden-rings: 5
  partridges:
    count: 1
    location: a pear tree
  turtle-doves: two

A YAML collection be:
- a sequence (equivalent to a list or array)
- a mapping (equivalent to a dictionary, hashes or object)
A sequence in YAML coressponds to JS array or python list.
Each item (element) of a sequence is written on a separate line and should start with a dash (-) followed by a space and then the item's value.
And each item in the list should be on the same level of indentation.

- HTML
- CSS
- JavaScript

A YAML mapping allows you to list keys with values.

Employee:
  name: John Doe
  age: 23
  country: USA

or a sequence (array) of employees (note each employee is a mapping or object/dictionary):

Employees:
  - name: John Doe
    department: Engineering
    country: USA
  - name: Kate Kateson
    department: IT support
    country: United Kingdom