toml

TOML, short for Tom's Obvious, Minimal Language, is a configuration file format designed for human readability and unambiguous parsing. Introduced by Tom Preston-Werner in 2013, it presents a minimal alternative to JSON, XML, and INI, with a focus on clear syntax and predictable data mapping to hierarchical structures. The name is commonly treated as a backronym and a reflection of the project’s emphasis on clarity.

The syntax centers on key-value pairs, tables, and arrays. A line such as key = value assigns a scalar, string, boolean, or datetime. Strings can be quoted with double or single quotes, and multiline forms are available. TOML supports integers, floats, booleans, and date-time values in RFC 3339 format. Tables group related keys using square brackets, and dotted keys express nested objects. Arrays are written as [1, 2, 3], and arrays of tables use double brackets, for example [[products]].

Inline tables provide compact grouping of fields using curly braces, as in owner = { name = "Alice", id = 1 }. Comments begin with a hash character and continue to the end of the line, provided they are not inside string literals. TOML is designed to be easy to serialize and deserialize, with a clear mapping to native data structures in most programming languages, supporting deterministic round-tripping between text and data.

Usage and ecosystem: TOML is widely adopted for software configuration. Notably, Rust’s package manager Cargo uses Cargo.toml files. Many languages offer libraries for parsing and generating TOML, aiding tooling, reproducible builds, and cross-project configuration. While less permissive than some formats, TOML’s readability and strong tooling support contribute to its ongoing relevance in configuration management.