YAML 

YAML: YAML Ain't Markup Language™

Origin

flowchart TB
    SGML["Standard Generalized Markup Language (SGML)"] --> XML
    XML["Extensible Markup Language (XML)"] --> SML
    SML["Simple Markup Language (SML)"] --> YAML
    YAML["YAML Ain't Markup Language (YAML)"] -..-|happened to have similar syntax| similar
    subgraph similar[similar object notation]
        direction TB
        JSON
        Python
        NewtonScript
    end

Names

  • YML - The Why Markup Language (source: mail thread from 1999)
  • Originally “Yet Another Markup Language”
  • Nowadays “YAML Ain’t Markup Language” (recursive acronym)

Version differences

FeatureYAML 1.1YAML 1.2
Booleans “on/off” & “yes/no”✅❌
Octals “0700”✅❌
Octals “0o700”❌✅
Base-60 (sexagesimal) “190:20:30”✅❌
Underscores in numbers “1_2_3”✅❌

Nice website comparing v1.1 & v1.2: https://perlpunk.github.io/yaml-test-schema/schemas.html

Weird syntax

Null

a: null
b: !!null null
c:              # empty also means null
d: ~            # tilde also means null

Sets

a: { x, y, z }
b: { x:, y:, z: }
c: { x: null, y: null, z: null }
d:
  ? x
  ? y
  ? z

Merge keys

https://yaml.org/type/merge.html

Introduced in v1.1 

a: &a
  foo: 1
b: &b
  bar: 2
c: &c
  foo: 3
d:
  <<: *a
  moo: 3
e:
  <<: *a
  foo: 4
  <<: *c
f:
  <<: [*a, *c]
 
# Result
a:
  foo: 1
b:
  bar: 2
c:
  foo: 3
d:
  foo: 1
  moo: 3
e:
  foo: 3
f:
  foo: 1

Cool facts

July 31, 2023