vty is a terminal interface library. It provides a high-level interface for doing terminal I/O. Vty is supported on GHC versions 7.10.1 and up.

vty and its partner packages are published on Hackage. The vty package works in concert with one or more platform packages to do terminal I/O. Each platform package provides support for terminal I/O on a specific platform. Known platform packages are:

• vty-unix - the Unix terminal backend for Vty
• vty-windows - the Windows terminal backend for Vty
• vty-crossplatform - a package that builds vty-unix or vty-windows based on the build environment

How to use Vty

1. Add a package dependency on vty-unix, vty-windows, or vty-crossplatform, depending on the desired level of platform support. For example, if an application only supports Unix systems, it should depend on vty-unix. But if an application is intended to work anywhere Vty works, then vty-crossplatform is the best choice.
2. Add a package dependency on vty; the core library abstractions, types, and functions are obtained from vty itself. The platform packages do not re-export the core library's modules.
3. Import mkVty from the platform package in step (1) and use that to construct a Vty handle and initialize the terminal.
4. If desired, call Graphics.Vty.Config.userConfig to load the Vty user configuration since this step is not automatic.

Once you've initialized the terminal and have a Vty value, all of the vty package's API is now ready to use to do terminal I/O.

Implementing support for a new platform

Although this shouldn't be necessary to do very often (if ever!), if you would like to implement support for a new platform for Vty, see PLATFORM-HOWTO.md.

Features

• Provides an efficient output algorithm. Output buffering and terminal state changes are minimized.

• Automatically handles window resizes.

• Minimizes repaint area, which virtually eliminates the flicker problems that plague ncurses programs.

• Provides a pure, compositional interface for efficiently constructing display images.

• Automatically supports refresh on Ctrl-L.

• Provides extensible input and output interfaces.

• Properly handles cleanup (but not due to signals).

• Provides a comprehensive test suite.

• Supports "normal" and "extended" (SGR) mouse modes as described at https://invisible-island.net/xterm/ctlseqs/ctlseqs.html#h2-Mouse-Tracking

• Supports bracketed paste mode as described at https://cirw.in/blog/bracketed-paste

• Supports multi-column Unicode characters such as emoji characters. In cases where Vty and your terminal emulator disagree on character widths, Vty provides a tool vty-build-width-table and library functionality to build a width table that will work for your terminal and load it on application startup.

Development Notes

Vty uses threads internally, so programs made with Vty need to be compiled with the threaded runtime using the GHC -threaded option.

Multi-Column Character Support

Vty supports rendering of multi-column characters such as two-column Asian characters and Emoji characters. This section details how to take advantage of this feature, since its behavior will depend on the terminal emulator in use.

Terminal emulators support Unicode to varying degrees, and each terminal emulator relies on a table of column widths for each supported Unicode character. Vty also needs to rely on such a table to compute the width of Vty images to do image layout. Since those tables can disagree if Vty and the terminal emulator support different versions of Unicode, and since different terminal emulators will support different versions of Unicode, it's likely that for some wide characters, Vty applications will exhibit rendering problems. Those rendering problems arise from Vty and the terminal emulator coming to different conclusions about how wide some characters are.

To address this, Vty supports loading custom character width tables that are based on the terminal's behavior in order to eliminate these disagreements. By default, though, Vty will use its built-in Unicode character width table. Since the built-in table is likely to eventually disagree with your terminal, Vty provides an API and a command-line tool to generate and install custom tables.

Custom Unicode width tables based on your terminal emulator can be built by using the API in Graphics.Vty.UnicodeWidthTable. The process works by querying the current terminal environment to obtain its width measurements for the entire supported Unicode range. The results are then saved to a disk file.

Saved width tables can then be loaded in one of two ways:

• Via the library API in Graphics.Vty.UnicodeWidthTable.IO
• By adding a widthMap directive to your Vty configuration file and then invoking mkVty to initialize Vty

The Vty configuration file supports the widthMap directive to allow users to specify which custom width table should be loaded for a given terminal type. This is done by specifying, e.g.,

widthMap "xterm" "/path/to/map.dat"

where the first argument is the value that TERM must have in order for the table to be loaded, and the second argument is the path to the table file itself as generated by the two alternatives listed above. If the Vty configuration file contains multiple matching widthMap directives for the current value of TERM, the last one listed in the file is used.

The tables declared in the configuration file are only ever automatically loaded when applications set up Vty by calling Graphics.Vty.mkVty.

Before a custom table has been loaded, calls to the library's character width functions (e.g. wcwidth) will use the default built-in table. Once a custom table has been loaded, the functions will use the new custom table. Only one custom table load can be performed in a Vty program. Once a custom table has been loaded, it cannot be replaced or removed.

Contributing

If you decide to contribute, that's great! Here are some guidelines you should consider to make submitting patches easier for all concerned:

• Please ensure that the examples and test suites build along with the library by running build.sh in the repository.
• If you want to take on big things, talk to me first; let's have a design/vision discussion before you start coding. Create a GitHub issue and we can use that as the place to hash things out.
• If you make changes, make them consistent with the syntactic conventions already used in the codebase.
• Please provide Haddock documentation for any changes you make.

Further Reading

Good sources of documentation for terminal programming are:

• https://github.com/b4winckler/vim/blob/master/src/term.c
• https://invisible-island.net/xterm/ctlseqs/ctlseqs.html
• https://ulisse.elettra.trieste.it/services/doc/serial/config.html
• https://www.leonerd.org.uk/hacks/hints/xterm-8bit.html
• https://www.unixwiz.net/techtips/termios-vmin-vtime.html
• https://vt100.net/docs/vt100-ug/chapter3.html