Flow Style

YAML is best known for its indentation based block style. Each entry, a sequence item, a key value pair or a scalar, is on its own line.

There are many use cases where a more compact style is helpful. in YAML it's called Flow Style.

There are Flow Style Collections and Flow Scalars.

This chapter is about Collections, while Flow Scalars are part of the Quoting Chapter.

Sequences

Let's say you define a list of perl versions in a CI configuration file:

---
perl:
- 5.8
- 5.10
- 5.12
- 5.14
- 5.16
- 5.18
- 5.20
- 5.22
- 5.24
- 5.26
- 5.28
- 5.30

You can write that in flow style to save space. A sequence is enclosed in square brackets [], and entries are separated by commas ,:

---
perl: [5.8, 5.10, 5.12, 5.14, 5.16, 5.18, 5.20, 5.22, 5.24, 5.26, 5.28, 5.30]

That looks similar to JSON, and in fact, JSON is a subset of YAML 1.2. The difference is that you don't need to quote the strings, just like in block style.

If the line gets too long, you can wrap it:

---
perl: [5.8, 5.10, 5.12, 5.14, 5.16, 5.18, 5.20, 5.22,
  5.24, 5.26, 5.28, 5.30]

You can also put the brackets on their own lines:

---
perl: [
    5.8, 5.10, 5.12, 5.14, 5.16, 5.18, 5.20, 5.22,
    5.24, 5.26, 5.28, 5.30
  ]

Note that even the last closing bracket must be indented (more than the parent block style item), so the following would be invalid:

---
perl: [
  5.8, 5.10, 5.12, 5.14, 5.16, 5.18, 5.20, 5.22,
  5.24, 5.26, 5.28, 5.30
]

You might notice that many YAML parsers accept it, though. To be compatible with all parsers, you should indent it.

You can also use comments inside flow style collections:

---
perl: [ # Test all important perl versions
  5.8, 5.10, 5.12, 5.14, 5.16, 5.18, 5.20, 5.22,
  5.24, 5.26, 5.28, 5.30
  ]

Mappings

Let's say you have a list of coordinates. Each coordinate set is a mapping:

---
- x: 3
  y: 4
  z: 5
- x: 5
  y: 4
  z: 3

In Flow Style, a mapping is enclosed in curly braces {}, and commas , separate the key value pairs from each other:

---
- {x: 3, y: 4, z: 5}
- {x: 5, y: 4, z: 3}

Or even more compact:

---
[ {x: 3, y: 4, z: 5}, {x: 5, y: 4, z: 3} ]

Quoting

The rules for quoting are a bit more strict than in block style.

--- # Block
no: prob{lem,
also: f]ine

--- # Flow
{
  please: 'quo{te,',
  me: 't]oo'
}

See the Quoting Chapter for details about which character sequences require quoting additional to block style rules.

Nested

Inside of flow style collections you can only use flow collections and flow scalars.