YAML Syntax

YAML (YAML Ain't Markup) is a human friendly data serialization language.

Flextype uses YAML because it's as close to plain English as data serialization and configuration formats get. It has no curly braces, it allows you to omit quotation marks for strings in most cases, and it relies on indentation for structure, which makes it incredibly readable compared to other languages, such as JSON and XML.

YAML is used extensively in Flextype for its configuration files, fieldsets, and also in entries settings.

Basic Rules

There are some rules that YAML has in place to avoid issues related to ambiguity in relation to various languages and editing programs. These rules make it possible for a single YAML file to be interpreted consistently, regardless of which application and/or library is being used to interpret it.

  • YAML files should end in .yaml whenever possible in Flextype.
  • YAML is case sensitive.
  • YAML does not allow the use of tabs. Spaces are used instead as tabs are not universally supported.


Values in YAML’s key-value pairs are scalar. They act like the scalar types in languages like Perl, Javascript, and Python. It’s usually good enough to enclose strings in quotes, leave numbers unquoted, and let the parser figure it out.

But that’s only the tip of the iceberg. YAML is capable of a great deal more.

Key-Value Pairs and Dictionaries

The key-value is YAML’s basic building block. Every item in a YAML document is a member of at least one dictionary. The key is always a string. The value is a scalar so that it can be any datatype.

So, as we’ve already seen, the value can be a string, a number, or another dictionary.

Numeric types

YAML recognizes numeric types. We saw floating point and integers above. YAML supports several other numeric types.

foo: 12345
bar: 0x12d4


YAML strings are Unicode. In most situations, you don’t have to specify them in quotes.

foo: this is a normal string

If we want escape sequences handled, we need to use double quotes.

foo: "this is not a normal string\n"
bar: this is not a normal string\n

YAML processes the first value as ending with a carriage return and linefeed. Since the second value is not quoted, YAML treats the \n as two characters.

YAML will not escape strings with single quotes, but the single quotes do avoid having string contents interpreted as document formatting.

String values can span more than one line. With the fold (greater than) character, you can specify a string in a block. But it’s interpreted without the newlines.

foo: >
  this is not a normal string it
  spans more than
  one line

The block (pipe) character has a similar function, but YAML interprets the field exactly as is. So, we see the newlines where they are in the document.

foo: |
  this is not a normal string it
  spans more than
  one line


You enter nulls with a tilde or the unquoted null string literal.

foo: ~
bar: null


YAML indicates boolean values with the keywords True, On and Yes for true. False is indicated with False, Off, or No.

foo: True
bar: False
light: On
TV: Off


You can specify arrays or lists on a single line.

items: [ 1, 2, 3, 4, 5 ]
names: [ "one", "two", "three", "four" ]

Or, you can put them on multiple lines.

  - 1
  - 2
  - 3
  - 4
  - 5
  - "one"
  - "two"
  - "three"
  - "four"

The multiple line format is useful for lists that contain complex objects instead of scalars.

  - things:
      thing1: huey
      things2: dewey
      thing3: louie
  - other things:
      key: value

An array can contain any valid YAML value. The values in a list do not have to be the same type.


We covered dictionaries above, but there’s more to them.

Like arrays, you can put dictionaries inline. We saw this format above. It’s how python prints dictionaries.

foo: { thing1: huey, thing2: louie, thing3: dewey }

Resources and Further Documentation

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