Yossarian

The insanity of terminal emulation

Yossarian is a headless terminal emulator. It receives input, as a stream of bytes, and interprets it to maintain the state of a terminal, of some size. In particular, most standard any many common escape sequences will be interpreted to modify the terminal's content (including colors and styles), cursor position, and hyperlinks.

Features

• interprets ANSI control sequences, notably CSI and SGR codes
• terminals of any dimensions can be emulated
• the graphic rendition of each character cell is tracked independently
• hyperlinks are also tracked for each cell
• take snapshots of the terminal

Availability

Getting Started

All terms and types are defined in the yossarian package:

import yossarian.*

Create a new Pseudo-Terminal

An instance of Pty, representing a pseudo-terminal (PTY), is the main entry-point to Yossarian's features. We can create one by specifying its width and height:

val pty = Pty(80, 24)

This represents a screen size of 80×24 characters, white-on-black text, with a cursor in the top-left corner, and no interesting text styles.

Updating PTY state

Input may be provided to the Pty instance by supplying it as a Text value to the consume method. For example,

val pty2 = pty.consume(t"Hello world\n")

This will construct a new Pty with the words Hello world on its virtual screen, and move the cursor to the start of the next line. This is an immutable operation, so the original state of pty will be unchanged.

Accessing PTY state

Changing the state of the PTY is not useful unless we can inspect its state! We can access much of that state through pty.buffer, and instance of ScreenBuffer.

A ScreenBuffer represents a rectangular region of characters in the pseudo-terminal, which may not be the entire terminal window, and provides the following methods:

• width and height, to get the buffer's dimensions
• char(x, y), to get the character at a particular position
• style(x, y), to get the Style instance for a position
• link(x, y), to get the link text applied to the position
• line, to get a new ScreenBuffer of the entire screen as a single line
• render, to get the textual content of the screen
• styles, to get an array of the Styles for each character in the buffer
• find(text), to get a smaller ScreenBuffer whose content matches the search text

A Style value provides information on the visual style of a single character in a ScreenBuffer. It includes the properties, bold, italic, blink (blinking text), faint, conceal (invisible text), strike (strike-through), underline, reverse (inverted colors), foreground (color) and background (color).

Interactivity

A pseudoterminal sometimes needs to produce output to communicate with the process controlling it. For example, if the PTY receives the escape codes to query its size, it needs to respond through some channel.

The stream method of Pty will provide a stream of Text output from the pseudo-terminal, which the controlling process and read and interpret accordingly.

Status

Yossarian is classified as embryotic. For reference, Soundness projects are categorized into one of the following five stability levels:

• embryonic: for experimental or demonstrative purposes only, without any guarantees of longevity
• fledgling: of proven utility, seeking contributions, but liable to significant redesigns
• maturescent: major design decisions broady settled, seeking probatory adoption and refinement
• dependable: production-ready, subject to controlled ongoing maintenance and enhancement; tagged as version 1.0.0 or later
• adamantine: proven, reliable and production-ready, with no further breaking changes ever anticipated

Projects at any stability level, even embryonic projects, can still be used, as long as caution is taken to avoid a mismatch between the project's stability level and the required stability and maintainability of your own project.

Yossarian is designed to be small. Its entire source code currently consists of 441 lines of code.

Building

Yossarian will ultimately be built by Fury, when it is published. In the meantime, two possibilities are offered, however they are acknowledged to be fragile, inadequately tested, and unsuitable for anything more than experimentation. They are provided only for the necessity of providing some answer to the question, "how can I try Yossarian?".

1. Copy the sources into your own project

   Read the fury file in the repository root to understand Yossarian's build structure, dependencies and source location; the file format should be short and quite intuitive. Copy the sources into a source directory in your own project, then repeat (recursively) for each of the dependencies.

   The sources are compiled against the latest nightly release of Scala 3. There should be no problem to compile the project together with all of its dependencies in a single compilation.

2. Build with Wrath

   Wrath is a bootstrapping script for building Yossarian and other projects in the absence of a fully-featured build tool. It is designed to read the fury file in the project directory, and produce a collection of JAR files which can be added to a classpath, by compiling the project and all of its dependencies, including the Scala compiler itself.

   Download the latest version of wrath, make it executable, and add it to your path, for example by copying it to /usr/local/bin/.

   Clone this repository inside an empty directory, so that the build can safely make clones of repositories it depends on as peers of yossarian. Run wrath -F in the repository root. This will download and compile the latest version of Scala, as well as all of Yossarian's dependencies.

   If the build was successful, the compiled JAR files can be found in the .wrath/dist directory.

Contributing

Contributors to Yossarian are welcome and encouraged. New contributors may like to look for issues marked beginner.

We suggest that all contributors read the Contributing Guide to make the process of contributing to Yossarian easier.

Please do not contact project maintainers privately with questions unless there is a good reason to keep them private. While it can be tempting to repsond to such questions, private answers cannot be shared with a wider audience, and it can result in duplication of effort.

Author

Yossarian was designed and developed by Jon Pretty, and commercial support and training on all aspects of Scala 3 is available from Propensive OÜ.

Name

Yossarian was the protagonist in Joseph Heller's Catch 22, in which he desires to be declared insane in order to be excused from flying combat missions. But in doing so, he must request an evaluation, which only a sane person would do, and would thus be considered proof of sanity. This library makes it possible to evaluate the sanity of terminal output.

In general, Soundness project names are always chosen with some rationale, however it is usually frivolous. Each name is chosen for more for its uniqueness and intrigue than its concision or catchiness, and there is no bias towards names with positive or "nice" meanings—since many of the libraries perform some quite unpleasant tasks.

Names should be English words, though many are obscure or archaic, and it should be noted how willingly English adopts foreign words. Names are generally of Greek or Latin origin, and have often arrived in English via a romance language.

Logo

The logo shows an abstract depiction of some rows of content in a console.

License

Yossarian is copyright © 2024 Jon Pretty & Propensive OÜ, and is made available under the Apache 2.0 License.