YAML Ain't Markup Language

T. Pitner, L. Bártek, A. Rambousek. L Grolig FI MU Brno 2020

YAML: What is it?
● “YAML is a human friendly data serialization standard for all programming languages.” https://yaml.org/ YAML home https://en.wikipedia.org/wiki/YAML Wikipedia YAML ● Specification (not a formal standard endorsed by ISO or W3C) https://yaml.org/spec/1.2/spec.html

YAML: Purpose, Goal
● “YAML Ain’t Markup Language” (abbreviated YAML) is a data serialization language designed to be human-friendly and work well with modern programming languages for common everyday tasks.” ● Yet Another Markup Language ● .yaml or .yml file extensions

YAML: Design principles
● It uses Unicode characters, ○ some provide structural information ○ rest containing the data itself, so it is a markup. ● YAML achieves a unique cleanness ● Markup is minimal, content is maximal => low overhead and natural look ● For example: ● spaces (indentation) may be used for structure, ● key: value pairs ● dashes - for “bullet” lists

YAML: Design priorities
● ● ● ● ● ● ● “YAML is easily readable by humans. YAML data is portable between programming languages. YAML matches the native data structures of agile languages. YAML has a consistent model to support generic tools. YAML supports one-pass processing. YAML is expressive and extensible. YAML is easy to implement and use.”

YAML: Data types
● Three basic primitives ● mappings (hashes/dictionaries), ● sequences (arrays/lists) and ● scalars (strings/numbers) Everything else is a combination of above - and it is enough.

YAML: Usage
YAML was specifically created to work well for common use cases such as: ● ● ● ● ● ● configuration files, log files, interprocess messaging, cross-language data sharing, object persistence, and debugging of complex data structures.

YAML: Interfaces
● allows incremental (event-driven) interfaces ● one-pass interfaces ● thus enables processing of large documents (e.g. transaction logs) or ● continuous streams (e.g. feeds from a production machine)

YAML: Typing YAML documents
● Motivated by Internet Mail (RFC0822) ● C-style escape sequences. This enables ASCII encoding of non-printable or 8-bit (ISO 8859-1) characters such as “\x3B”. Non-printable 16-bit Unicode and 32-bit (ISO/IEC 10646) characters are supported with escape sequences such as “\u003B” and “\U0000003B”. ● A single line break is folded into a single space, ● while empty lines are interpreted as line break characters. ● This technique allows for paragraphs to be word-wrapped without affecting the canonical form of the scalar content.

YAML vs JSON
● ● ● ● YAML’s foremost design goals are human readability and support for serializing arbitrary native data structures. Extremely readable files, but is more complex to generate and parse YAML can therefore be viewed as a natural superset of JSON, offering improved human readability and a more complete information model. ● Every JSON file is also a valid YAML file.

YAML: Collections - Sequence

YAML: Map String -> Number

… and comments after #

YAML: Map String -> Sequence

YAML: Sequence of 2 maps

YAML: Sequence of sequences

YAML: Map of Maps

YAML: Basic syntactic rules
● Entire Unicode character set, except for some control characters, and may be encoded in UTF-8, UTF-16 and UTF-32. ● Whitespace indentation is used for denoting structure; ● tab characters are not allowed. ● Comments begin with the number sign (#), can start anywhere on a line and continue until the end of the line. ● List members are denoted by a leading hyphen (-) with one member per line. ● A list can also be specified by enclosing text in square brackets ([ ]) with each entry separated by commas.

YAML: Associative arrays
● associative array entry is key: value ● ?key: value allows the key to contain leading dashes, square brackets, etc., without quotes.

● associative array can also be enclosed in JSON-style: { key: value, key2: value2,... }, with

YAML: Scalar values
● Strings are unquoted or in double or single quotes " ' ● Within double-quotes: C-style escape sequences \ may be used ● Octal escape is \0. ● Block scalars (longer texts) are delimited with indentation ● optional modifiers to preserve | or fold > newlines.

YAML: Big structures
● ● ● ● ● Multiple documents within a stream Start of document --- and optional … end of document Nodes can be named using ampersand & and referenced with asterisk * label (type or tag) using !! followed by a string, which can be expanded into a URI.