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com-lihaoyi / fansi

Scala/Scala.js library for manipulating Fancy Ansi colored strings
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Fansi 0.5.0 Gitter Chat Patreon

LandingExample

// SBT
"com.lihaoyi" %% "fansi" % "0.5.0"
"com.lihaoyi" %%% "fansi" % "0.5.0" // Scala.js or Scala-Native

// Mill
ivy"com.lihaoyi::fansi:0.5.0"
ivy"com.lihaoyi::fansi::0.5.0" // Scala.js or Scala-Native

Fansi is a Scala library to make it easy to deal with fancy colored Ansi strings within your command-line programs.

While "normal" use of Ansi escapes with java.lang.String, you find yourself concatenating colors:

val colored = Console.RED + "Hello World Ansi!" + Console.RESET

To build your colored string. This works the first time, but is error prone on larger strings: e.g. did you remember to put a Console.RESET where it's necessary? Do you need to end with one to avoid leaking the color to the entire console after printing it?

Furthermore, some operations are fundamentally difficult or error-prone with this approach. For example,

val colored: String = Console.RED + "Hello World Ansi!" + Console.RESET

// How to efficiently get the length of this string on-screen? We could try
// using regexes to remove and Ansi codes, but that's slow and inefficient.
// And it's easy to accidentally call `colored.length` and get a invalid length
val length = ???

// How to make the word `World` blue, while preserving the coloring of the
// `Ansi!` text after? What if the string came from somewhere else and you
// don't know what color that text was originally?
val blueWorld = ???

// What if I want to underline "World" instead of changing it's color, while
// still preserving the original color?
val underlinedWorld = ???

// What if I want to apply underlines to "World" and the two characters on
// either side, after I had already turned "World" blue?
val underlinedBlue = ???

While simple to describe, these tasks are all error-prone and difficult to do using normal java.lang.Strings containing Ansi color codes. This is especially so if, unlike the toy example above, colored is coming from some other part of your program and you're not sure what or how-many Ansi color codes it already contains.

With Fansi, doing all these tasks is simple, error-proof and efficient:

val colored: fansi.Str = fansi.Color.Red("Hello World Ansi!")
// Or fansi.Str("Hello World Ansi!").overlay(fansi.Color.Red)

val length = colored.length // Fast and returns the non-colored length of string

val blueWorld = colored.overlay(fansi.Color.Blue, 6, 11)

val underlinedWorld = colored.overlay(fansi.Underlined.On, 6, 11)

val underlinedBlue = blueWorld.overlay(fansi.Underlined.On, 4, 13)

And it just works:

LandingExample

Why Fansi?

Unlike normal java.lang.Strings with Ansi escapes embedded inside, fansi.Str allows you to perform a range of operations in an efficient manner:

These are tasks which are possible to do with normal java.lang.String, but are tedious, error-prone and typically inefficient. Often, you can get by with adding copious amounts of Console.RESETs when working with colored java.lang.Strings, but even that easily results in errors when you RESET too much and stomp over colors that already exist:

StringError

fansi.Str allows you to perform these tasks safely and easily:

FansiRocks

Fansi is also very efficient: fansi.Str uses just 3x as much memory as java.lang.String to hold all the additional formatting information. Its operations are probably about the same factor slower, as they are all implemented using fast arraycopys and while-loops similar to java.lang.String. That means that - unlike fiddling with Ansi-codes using regexes - you generally do not need to worry about performance when dealing with fansi.Strs. Just treat them as you would java.lang.Strings: splitting them, substringing them, and applying or removing colors or other styles at-will.

Fansi was originally a part of the Ammonite REPL, but is now a standalone zero-dependency library anyone can use if they want to easily and efficiently deal with colored Ansi strings.

Using Fansi

The main operations you need to know are:

fansi.Str

fansi.Attr

fansi.Attrs

Fansi also supports 8-bit 256-colors through fansi.Color.Full and fansi.Back.Full, as well as 24-bit 16-million-colors through fansi.Color.True and fansi.Back.True:

docs/TrueColor.png

Note that Fansi only performs the rendering of the colors to an ANSI-encoded string. Final rendering will depend on whichever terminal you print the string to, whether it is able to display these sets of colors or not.

Digging Deeper

If you want to dig into deeper, there are a few more APIs you can use:

This allows you to perform fast, mutable array operations on the color/character arrays if you know what you're doing and want to perform operations that are inconvenient or slow to do through fansi.Str's immutable API. For example, if you want to do a bunch of work with colored strings and then at-the-end render everything to HTML, you can manually walk over the color/character arrays yourself and decide where to print HTML tags to give the text colors.

fansi.Str currently has a relatively skeletal API: it is slightly smaller than what java.lang.String has, and definitely much less than what is available on scala.RichString's extension methods. Feel free to implement your own custom operations using fromArrays if you can't find what you want on fansi.Str, or send a patch if you think it's arguably general enough to be included in Fansi itself.

For example, if you want to emit the Ansi codes that correspond to the transition from "No Color" to "Red", you can use

fansi.Attrs.emitAnsiCodes(0, fansi.Color.Red.applyMask) // "\u001b[31m"

Or the Ansi codes from "Red" to "No Color"

fansi.Attrs.emitAnsiCodes(fansi.Color.Red.applyMask, 0) // "\u001b[39m"

Or for any other combination of attributes

val attrs = fansi.Color.Red ++ fansi.Back.Blue ++ fansi.Underlined.On
fansi.Attrs.emitAnsiCodes(0, attrs.applyMask) // "\u001b[31m\u001b[44m\u001b[4m"

You can also pass in an errorMode when parsing a string via ansi.Str(...) to tell Fansi how to behave if it finds Ansi escapes it can't handle. You have the options:

Scaladoc

Changelog

0.5.0

0.4.0

0.3.0

0.2.14

0.2.13

0.2.12

0.2.11

0.2.10

0.2.7

0.2.5

0.2.4

0.2.3

0.2.2

0.2.1

0.2.0

0.1.3

0.1.2

0.1.1

0.1.0