YAML Syntax

YAML is a simple and powerful language. It is designed to make the language easy to read.

Basic Syntax Rules

Characters are case-sensitive.
Indentation is used to represent hierarchical relationships.
Only spaces can be used for indentation.
The number of spaces used for indentation does not matter. Elements of the same level must be aligned on the left side.
Lines that start with a number sign (#) are comments.

YAML supports three types of data structures.

Object: A set of key-value pairs, which is also called mapping, hashes, or dictionary.
Array: A group of values arranged in sequence, which is also called a sequence or list.
Scalar: A single and irreducible value, which is the minimum data unit.

Object

An object is a group of key-value pairs. For key: value, the colon (:) must be followed by a space or newline character. The valid expression is as follows:

animal: pets
plant:
  tree

You can also write multiple key-value pairs into an inline object.

hash: {name: Steve, foo: bar}

However, an error occurs in the following scenario:

foo: somebody said I should put a colon here: so I did
windows_drive: c:

To resolve the issue, you can use single quotation marks (' '), as shown in the following:

foo: 'somebody said I should put a colon here: so I did'
windows_drive: 'c:'

Array

An array is represented by a hyphen (-) and space. The valid expression is as follows:

animal:
- Cat
- Dog
- Goldfish

You can also use the inline representation.

animal: [Cat, Dog, Goldfish]

Objects and arrays can be used in combination to form a composite structure.

languages:
 - Ruby
 - Perl
 - Python
websites: 
 YAML: yaml.org
 Ruby: ruby-lang.org
 Python: python.org
 Perl: use.perl.org

Scalar

Scalar data types include string, Boolean value, integer, floating-point number, null, time, and date.

String:
By default, a string is not enclosed in quotation marks.
```
str: This_is_a_line
```
If a string contains spaces or special characters, the string needs to be enclosed in quotation marks.
```
str: 'content: a string'
```
Both single and double quotation marks can be used. The difference between them is that the former can identify escape characters while the latter cannot convert special characters.
```
s1: 'content:\n a string'
s2: "content:\n a string"
```
If there is a single quotation mark between two single quotation marks, ensure that two single quotation marks are used consecutively to achieve conversion.
```
str: 'labor''s day'
```
Strings can be written into multiple lines. The lines except the first line must be indented with one space. The newline character will be converted to a space.
```
str: This_is
 a_multi_line
```
Integer:
```
int_value: 314
```
Floating-point number:
```
float_value: 3.14
```
Null:
```
parent: ~
```
Time:
The time is in the ISO8601 format.
```
iso8601: 2018-12-14t21:59:43.10-05:00
```
Date:
The date is in the compound ISO8601 format: year-month-day.
```
date: 1976-07-31
```

Special Symbols

---: indicates the start of a YAML file. ...: indicates the end of a YAML file.

---
# A list of delicious fruits
- Apple
- Strawberry
- Mango
...

You can use two exclamation marks (!) to forcibly convert an integer, a floating-point number, or a Boolean value.

strbool: !!str true
strint: !!str 10

For a string in multiple lines, you can use a literal block scalar (|) to start new lines or folded block scalar (>) to fold new lines. The two symbols are often used in the character strings of YAML files.

this: |
 Foo
 Bar
that: >
 Foo
 Bar

The corresponding objects are as follows:

{ this: 'Foo\nBar\n', that: 'Foo Bar\n' }

It is advised to use block scalars (|) in most scenarios.

Comment

YAML supports comments. This is an advantage of YAML compared with JSON.

The comment of YAML starts with a number sign (#), as shown in the following:

languages:
 - Ruby                # Indicates the Ruby language.
 - Go                  # Indicates the Go language.
   --  PythonPy               # Indicates the Python language.