formatter

A formatter is a software component, library, or tool that converts data from its internal representation into a prescribed presentation format. It separates the data from its display, enabling consistent rendering across contexts such as user interfaces, files, or logs. Formatters can operate on text, numbers, dates, code, or structured data, and may be locale-aware to accommodate internationalization.

Common categories include text formatters, which control indentation, line wrapping, and alignment; numeric and date/time formatters, which apply locale-specific patterns for decimals, thousands separators, currencies, and date formats; code formatters, which reformat source code to a standardized style; data formatters, which produce readable or machine-readable representations of structured data such as JSON, XML, CSV, or YAML; and log formatters, which assemble timestamps, severity levels, and messages into consistent log lines. User interface formatters render values for display, often including localization.

Implementation approaches vary. Many languages provide format strings or template-based systems that place values into fixed positions. Locale-aware formatters rely on locale data to ensure culturally appropriate representations. Code formatters are typically pluggable tools integrated into development workflows and editors. Examples include Python’s format specification mini-language and the string.Formatter class, Java’s Formatter and String.format, JavaScript’s Intl.NumberFormat and Intl.DateTimeFormat, and specialized code formatters like clang-format or Prettier.

Considerations for using formatters include consistency, performance, locale and accessibility requirements, and error handling. See also: pretty printing, localization, data serialization, string formatting.