What is the YAML Norway problem?

Under YAML 1.1, the bare token NO (a country code for Norway) parses as the boolean false, and YES/ON/OFF behave similarly. YAML 1.2's core schema fixes this by only treating true and false as booleans, but many parsers still default to 1.1, so quoting such values is safest.

What is the difference between block and flow style?

Block style uses indentation and dashes for sequences and key: value lines for mappings. Flow style is JSON-like and inline, using square brackets for sequences and curly braces for mappings. YAML is a strict superset of JSON, so flow style is essentially JSON.

What do the pipe and greater-than block scalars do?

A pipe introduces a literal block where newlines are preserved exactly. A greater-than sign introduces a folded block where single newlines become spaces and blank lines become newlines. Chomping indicators control trailing newlines: minus strips them, plus keeps them all.

How do anchors and aliases work?

An anchor marks a node with an ampersand and a name, and an alias references that node later with an asterisk and the same name. Combined with the merge key, anchors let you reuse a mapping across several places without repeating it.

Should I quote my YAML strings?

Quote any scalar that could be misread as a number, boolean, null or date, such as version strings, ports, times, leading-zero IDs and country codes. When in doubt, double-quote it to force a string interpretation.

YAML Syntax Reference

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A tour of YAML 1.2 syntax

YAML is a human-friendly data serialization language used for configuration in tools like Docker Compose, Kubernetes, GitHub Actions and CI pipelines. This reference covers the building blocks — scalars, sequences, mappings, anchors, tags and multi-document streams — alongside the quoting traps that catch almost everyone at least once.

How it works

Structure comes from indentation (spaces only, never tabs). A colon-space makes a mapping entry; a dash-space makes a sequence element:

service:
  name: web
  ports: [80, 443]
  env:
    - DEBUG=1
  base: &defaults
    retries: 3
  worker:
    <<: *defaults
    name: worker

Scalars can be plain, single-quoted (fully literal, only '' escapes a quote) or double-quoted (supports \n, \t and \uXXXX). The literal | and folded > block styles handle multi-line text. Anchors (&name) and aliases (*name) plus the merge key (<<:) let you reuse nodes.

Tips and notes

Use spaces for indentation; a single tab anywhere is a parse error.
Prefer the 1.2 core schema where only true/false are booleans.
Quote times like 22:22, ports, and leading-zero IDs to dodge base-60/octal parsing.
Separate multiple documents in one file with ---, and end optionally with ....