package ocolor

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Print with style in your terminal using Format's semantic tags


Dune Dependency






This package provides a nice way to use ANSI escape codes thanks to Format's semantic tags. This mode is compositional: ending a style restore the previous one, instead of destroying everything. This package also allows using directly ANSI escape codes (with Printf).

Note that this library does not intend to handle anything else than ANSI escape codes (in particular, not the old Windows style of styling). Moreover, it aims to be as pure as possible, so insensitive to the environment. As a consequence, there is no mechanism to detect terminal's settings. However, some configuration is possible, but must be done manually.

Published: 28 Aug 2023



OColor is an OCaml library which help to format nicely using ANSI escape codes.

Subset of ANSI

With OColor, you can print... colors. That's the least you can expect. But there is more. It also support a lot of style such as bold, underline, italic, faint, overline.... And 4, 8 and 24- bits colors. In fact, OColor is probably capable of more than your terminal allows. However, it does not support every SGR code (the family of ANSI sequences that does styling). The supported subset was minutely chosen with a combination of "Do I understand what it does?", "Do I know someone that understand it?", "Is it supported by at least one terminal my coworker or I use or know?", "Will I use it at least once?", "How much time do I have?"....

The general philosophy is that there must be at least what you need, but use with care, OColor does not detect if your terminal support what you're asking it. However, most terminals will simply ignore unknown sequences, so it's probably not so bad anyway. Portability is user's problem.

About colors

In ANSI, there are 3 ways to describe colors.

  • 4 bits: the old one, 16 available colors that are "named" by their number, the real rendered color depend on the terminal configuration.

  • 8 bits: provides an access to 4 bits colors, an rgb (6^3) cube and a 24 steps grayscale. The former depend on the terminal, the two latter are absolute.

  • 24 bits: true rgb colors (256^3). Obviously absolute.

OColor has a quite nice feature: you can tell it which is the better kind of colors you support and will convert colors appropriately. In fact, it only perform downgrade.

Nowadays, most terminal support 24 bits colors, but not all. Some support only 4 and 8 bits, and, worse, sometimes, only 4 bits. So when printing a color, OColor look if it is available given the configuration (that have to be set manually) and, if not, find the closest allowed color. The metric is the euclidean distance in the CIELAB colorspace (under the standard illuminant D65). Should be good enough.

A more pragmatic problem: when downgrading a 24 into a 4 bits colors, you have to know how these 4 bits colors are rendered. This is configurable as well. This is especially important because it may vary a lot between terminals with white and black background.


There is two way to use OColor: with Printf or Format. Format is good. Format is better. Format will win the final battle! Oh, and before we go in the details, most of the mechanism is exposed for the developper, even things that are not used in the library actually, so it should be possible to get what you want of OColor.


To format with style with Format, we use semantic tags. Semantic tags are a bit like boxes: you can open them, with a tag (a string annotating it) and close them. They are handled by user-provided functions, and disabled by default.

We can open a semantic tag, with tag "foo", with a format containing @{<foo>, and we close the last opened tag with @}. There is another way to do it:

val Format.pp_open_tag : Format.formatter -> string -> unit
val Format.pp_close_tag : Format.formatter -> unit -> unit

Thus, we have access to an almost-well-parenthesized structure (because closing a tag when none are open is legal and does nothing), that we can use to print with styles in a compositional way. Opening a tag will apply a new style, and closing it will cancel the effect, restoring the previous style, if any.

A simple example:

Ocolor_format.printf "Normal @{<red>This is red @{<green> Here is green@} Red again@} Normal"


Ocolor_format.printf "Normal @{<red>This is red @{<ul;bold> Here is red, bold and underlined@} Only red again@} Normal"

To make styling work, you'll have to use Ocolor_format.printf or use standard Format functions with a formatter that has been prettified with Ocolor_format functions. So, everything, especially %a will work as expected: called with a prettified formatter, tags will be interpreted, otherwise, text will be printed without formatting. Here is an example of usage with Format functions.

(* Non prettified formatter does not style. *)
Format.printf "Normal @{<red>This is normal@} Normal";
(* Format function with a predefined prettified formatter *)
Format.fprintf Ocolor_format.raw_std_formatter "Normal @{<red>This is red@} Normal"; (* Exactly as Ocolor_format.printf *)
(* Format function with a arbitrary formatter *)
Ocolor_format.prettify_formatter (fmt: Format.formatter); (* We assume fmt is defined somewhere *)
Format.fprintf fmt "Normal @{<red>This is red@} Normal";
(* Prettified formatter from a buffer *)
let fmt2 : Format.formatter = Ocolor_format.raw_formatter_of_buffer (Buffer.create 64) in
(* Could have be done with:
let fmt2 : Format.formatter = Format.formatter_of_buffer (Buffer.create 64) in
Ocolor_format.prettify_formatter fmt2;
Format.fprintf fmt2 "Normal @{<red>This is red@} Normal"

Prettifying an already prettified formatter will reset the internal state but not reset the output. So it will stay dirty until the whole styling is recomputed, for instance, by closing the bold style. Since old styles are forgotten, there are no way to close them gracefully. So, don't do that.

Ocolor_format also define a non standard formatter type that contains a standard Format.formatter and some information about the state. It is useful only for introspection: get the current style from the formatter. Otherwise, only the standard (underlying) formatter is required.

More details in ocolor_format.mli.


Printf has no semantic tags, so it does styling the old way: applying style linearly, and forgetting the past. That's dangerous. Use with care.

When you apply a style, the previous style is totally forgotten. Thus, if you want to restore the previous style, you have to do it manually. And of course, a reset will really reset everything. Thus, this is dangerously non compositional.

I recommend to use it only when printing self-contains lines, only at the toplevel of the printing process (not in a printer that will be called by a %a).

More details in


For now, when a formatter is prettified, tag-marking functions are overwritten and tag-marking is enabled. It is yet not currently supported to call both new and previous tag-marking functions, since it does not seem useful. However, if needed, we could let the user decide if we should replace tag-marking functions, call the previous ones before or after, or call after if styling fails (unrecognized tag).


OColor is under MIT Licence. In short, feel free to use it in pretty much every way you want. I don't really care a lot.

You can also consider it as a mere example of what semantic tags are good to.

Dependencies (3)

  1. cppo build & >= "1.6.5"
  2. dune >= "1.6.3"
  3. ocaml >= "4.02.3"

Dev Dependencies (1)

  1. ounit2 with-test & >= "2.0.8"

Used by (4)

  1. catala >= "0.9.0"
  2. docfd >= "3.0.0"
  3. simlog
  4. speed




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