YAML
External
- Wikipedia https://en.wikipedia.org/wiki/YAML
- http://yaml.org
- http://docs.ansible.com/ansible/YAMLSyntax.html
- Using YAML for Java Application Configuration https://dzone.com/articles/using-yaml-java-application
- http://www.yaml.org/spec/1.2/spec.html
Overview
YAML is a human-readable data serialization format. YAML syntax was designed to easily map on scalars, list and associative arrays. It is well suited for hierarchical data representation. It does not use enclosures such as quotation marks, brackets, braces and open/close tags, which can be hard fro the human eye to balance in nested hierarchies. Data structure hierarchy is maintained by outline indentation. The specific number of spaces in the indentation is not important as long as parallel elements have the same left justification, and the hierarchically nested elements are indented further.
Strings do not require enclosure in quotations.
The format has support for references, where sections in the document can be referenced, thus eliminating redundancy.
Multiple documents can exist in a single file/stream and they are separated by "---".
An optional "..." can be used at then of the file - useful for signaling an end in streamed communication without closing the pipe.
YAML is a superset of JSON, so a YAML parser should understand JSON. See http://yaml.org/spec/1.2/spec.html#id2759572
Comments
# This is a comment
Scalars
a: 10
YAML auto-detects the data type. A good rule when writing YAML is to quote strings and allow implicit type conversion for everything else.
Devoid of Value
The null word is used to indicate a lack of value. This is typically converted into any native null-like value (e.g., null in Java). Note that a null is different from an empty string and that a mapping entry with some key and a null value is valid and different from not having that key in the mapping.
b: null
Core Data Types
Strings, ints, floats, lists and maps.
a: 123 # an integer b: "123" # a string, disambiguated by quotes c: 123.0 # a float d: !!float 123 # a 'casted' float e: !!str 123 # a 'casted' string
Booleans
true and false must not be enclosed in quotes to be recognized as booleans.
Boolean scalar support:
a: true a: Y a: Yes a: ON b: false b: n
c: "true" # string not boolean
Integer
If a integral value is specified as an unquoted bare word, it is treated as a numeric type:
count: 1
The same quoted value is treated as string:
count: "1" # string not int
count: '1' # string not int
Float
If a float value is specified as an unquoted bare word, it is treated as a numeric type:
size: 32.1
The same quoted value is treated as string.
size: "32.1" # string not float
size: '32.1' # string not float
String
Strings do not require quotation, but it is recommended to quote them, to explicitly specify they are strings and type inference should not be attempted.
The following representations are equivalent for a string:
s1: bare words string s2: "a double-quoted string" s3: 'a single-quoted string'
All forms represented above are named "inline", in that the strings must be rendered on one line.
In the bare word format, characters cannot be escaped.
Double-quoted strings can have specific characters escaped with \. Double quotes can be escaped with \" and line breaks can be escaped with \n.
Single-quoted strings are "literal" strings, they do not use \ to escape characters. The only escape sequence is '' (two single quotes), which is decoded as a single '.
Forced Type Inference
Particular type inference can be forced with the !!type syntax:
age !!str 21 # will be a string, equivalent with "21" port: !!int 80 # will be an integer, equivalent with bare, unquoted 80
Defined Data Types
User-Defined Data Types
Collections
There are two types of collections: lists (or sequences) and associative arrays (or maps).
List (or Sequence)
List elements are designated by hyphen + space, and have the same offset:
- Audi - Mercedes - BMW
A list may contain a combination of "simple" elements and "complex" elements:
- simple
- complex:
key1: val1
key2: val2
Optional In-Line List Format
List elements are enclosed in brackets, and the list elements are separated by comma + space.
cars: ['Audi', 'BMW', 'Chevrolet']
Associative Array (or Map)
Keys are separated from values by a colon + space.
name: Audi color: black capacity: 5
Optional In-Line Associative Array Format
Associative array element are enclosed in braces, and they key: value pairs are separated by comma + space.
car: {brand: Audi, color: black, type: sedan}
Composite Collections
Collections can be combined:
List of Associative Arrays
- country: AU price: 6990000 - country: AT price: 4990000 - country: BE price: 4990000
The elements of an (associative array) list entry have the same indentation relative to each other, including the first one that follows the "-".
Associative Array of Associative Arrays
TODO, test:
command:
command-a:
option-1: value-1
option-2: value-2
command-b:
option-1: value-1
option-2: value-2
Multi-line Support
Literal Style
Multi-line strings can be written using the '|' character followed by a new line. To be considered multi-line content, the first line under the '|'-terminated line must be indented on a level deeper that the line containing the '|' and the subsequent multi-line lines must be indented with the same offset as the first line. Trailing white space is stripped. The new line characters are preserved.
This is correct multi-line (the multi-lines must be indented under 'data'):
data: |
This is a
multi-line
text section
The value of data is equivalent with "This is a\nmulti-line\ntext selection\n".
This is NOT a correct mult-line, because the "mult-lines" are not correctly indented:
data: |
This is not
a correct multi-line
The "|" multi-line operator implies that a trailing newline will be added to the string. In the correct above example, the data value will be equivalent with "This is a\nmulti-line\ntext selection\n" - note the trailing new line. If we want the YAML processor to strip off the trailing newline, we should use "|-" instead of "|":
dataWithoutTrailingNewLine: |-
This is a
multi-line
text section
The dataWithoutTrailingNewLine value is equivalent with "This is a\nmulti-line\ntext selection". Note the lack of trailing newline.
If we want the trailing whitespace to be preserved, we should use "|+" instead of "|":
dataWithTrailingWhitespacePreserved: |+
This is a
multi-line
text section
another: value
The dataWithTrailingWhitespacePreserved value is equivalent with "This is a\nmulti-line\ntext selection\n\n\n".
Folded Style
The '>' character followed by a new line folds all the new lines, after removing trailing white space and new lines. All but the last newline will be converted to space.
data: >
This is another
multi-line
text section
but in the final form
it will be just one long string
without new lines
except the last one
The data value is equivalent with "This is another multi-line text section but in the final form it will be just one long string without new lines except the last one\n"
If we wan to drop the trailing newline instead of preserving it, use ">-" instead of ">":
dataWihtoutTrailingNewLine: >-
Something
else
dataWihtoutTrailingNewLine value is equivalent with "Something else". Note the lack of trailing newline.
Embedding Multiple Documents in One File
It is possible to place more than one YAML documents into a single file. This is done by prefixing a new document with '---' and ending the document with '...' In many cases, either '---' or '...' may be omitted:
---
document: 1
---
document: 2
YAML is a Superset of JSON
Because YAML is a superset of JSON, any valid JSON document should be valid YAML.
JSON can be mixed into YAML, for the benefit of readability:
coffees: [ "Latte", "Cappuccino", "Espresso" ]
is equivalent with:
coffees:
- Latte
- Cappuccino
- Espresso