Goals

• Simple and ergonomic formatting of strings, especially where multiple styles are applied to same line.
• Focus on colours and text styling
• Support full colour range on modern terminals

Non-goals

• Any extended "Terminal UI" kind of features, we're just doing text styling (but hopefully it should fit in fine with Format or Fmt's existing box and table features etc)
• Maximum performance: if you are formatting high volumes of logs you may like to look at the alternative below. (Performance should be ok but it's not benchmarked and at the end of the day we have to parse the string tags)

See also

• ANSITerminal
• OColor
• Fmt

These OCaml libs provide support for styling console text with ANSI colours, and some also offer other features useful for formatting and interactivity in the terminal. In contrast, Spectrum focuses only on coloured text styling but offers deeper colour support. Hopefully it's complementary to the stdlib and other libs you may be using.

Installation

opam install spectrum

The main spectrum package includes everything you need for terminal color formatting. The implementation is split into several packages:

• spectrum - main runtime and user-facing API
• spectrum_capabilities - standalone terminal color capability detection
• spectrum_palette_ppx - PPX extension for generating palette modules from JSON palette definitions
• spectrum_palettes - pre-generated palette modules (Basic and Xterm256)
• spectrum_tools - color conversion utilities and query functions

All of these are installed automatically as dependencies when you install spectrum.

Quick start

Spectrum.Simple.printf "@{<green>%s@}\n" "Hello world 👋";;

The pattern is @{<TAG-NAME>CONTENT@}.

Tag names match the 256 xterm color names and are case-insensitive. You can also specify colours directly with hex codes, RGB, or HSL values:

Spectrum.Simple.printf "@{<#f0c090>%s@}\n" "Hex color";;
Spectrum.Simple.printf "@{<rgb(240 192 144)>%s@}\n" "RGB color";;
Spectrum.Simple.printf "@{<hsl(60 100 50)>%s@}\n" "HSL color";;

Styles like bold, italic, underline, dim, strikethru, and overline are also supported. Tags can be nested and combined with comma-separated compound tags:

Spectrum.Simple.printf "@{<bold,bg:red,yellow>%s@}\n" "Compound tag";;
Spectrum.Simple.printf "@{<green>%s @{<bold>%s@} %s@}\n" "Hello" "world" "there";;

For efficient repeated printing, prepare a formatter once:

let reset = Spectrum.prepare_ppf Format.std_formatter in
Format.printf "@{<green>%s@}\n" "Hello world 👋";
reset ();;

Automatic color quantization

Spectrum automatically detects terminal capabilities and quantizes colors accordingly. If you specify an RGB color like #FF5733 or rgb(255 87 51), Spectrum will:

• On True_color terminals: output the exact RGB values using 24-bit ANSI codes
• On Eight_bit terminals: quantize to the nearest xterm-256 color using perceptually accurate LAB color space distance
• On Basic terminals: quantize to the nearest ANSI-16 color using the same perceptual matching

Documentation

Documentation is generated with odoc:

opam install spectrum --with-doc

Or online at: anentropic.github.io/ocaml-spectrum

Changelog

See CHANGELOG.md.