A quick reference to YAML

YAML is designed to be human-readable and maps directly to native data types like lists, dictionaries, and strings. Here is a brief summary of the essential syntax:

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1. Basic Structures

• Key-Value Pairs: Created with a colon and a space.

  key: value

• Comments: Begin with a hash symbol.

  # This is a comment

• Document Separators: Use --- to start a new document and ... to end one (optional).

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2. Collections (Arrays and Objects)

• Lists (Sequences): Use a dash followed by a space for each item.

  YAML

  - Item 1
  - Item 2
  

• Dictionaries (Mappings): Nested using indentation (usually 2 spaces).

  YAML

  parent:
    child1: value
    child2: value
  

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3. Multiline Strings (Scalars)

• Literal Block (|): Preserves newlines and trailing whitespace.

• Folded Block (>): Folds newlines into spaces (converts a block into a single paragraph).

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4. Data Types

YAML automatically detects types, but you can explicitly cast them using !!.

• Strings: Can be unquoted, single-quoted (literal), or double-quoted (supports escape sequences like \n).

• Numbers: Integers (12) and Floats (3.14).

• Booleans: true, false, yes, no.

• Nulls: null or ~.

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5. Advanced Features

• Anchors (&) and Aliases (*): Used to duplicate or reference data to avoid repetition.

  YAML

  default: &base
    adapter: postgres
  development:
    <<: *base
  

• Complex Keys: Use ? for keys that are themselves objects or lists.

• "<<" : is the merge operator, which indicates the section to be replaced with the alias indicated by "*" after "<<". 

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Quick Reference Table

Feature	Syntax Example	Notes
Mapping	name: Gemini	Requires space after :
List	- apple	Requires space after -
Inline List	[1, 2, 3]	JSON-style syntax
Inline Map	{a: 1, b: 2}	JSON-style syntax
Literal String	`	`
Folded String	>	Newlines become spaces