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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.
git clone git://github.com/jtdaugherty/vty.git
cabal install vty
Supports a large number of terminals, i.e., vt100, ansi, hurd, linux,
screen, etc., or anything with a sufficient terminfo entry.
Automatically handles window resizes.
Supports Unicode output on terminals with UTF-8 support.
Provides an efficient output algorithm. Output buffering and terminal state changes are minimized.
Minimizes repaint area, which virtually eliminates the flicker problems that plague ncurses programs.
Provides a pure, compositional interface for efficiently constructing display images.
Automatically decodes keyboard keys into (key,[modifier]) tuples.
Automatically supports refresh on Ctrl-L.
Supports a keypress timeout after for lone ESC. The timeout is customizable.
Provides extensible input and output interfaces.
Supports ANSI graphics modes (SGR as defined in
console_codes(4)) with a type-safe interface and graceful fallback for terminals with limited or nonexistent support for such modes.
Properly handles cleanup (but not due to signals).
Provides a comprehensive test suite.
Supports "normal" and "extended" (SGR) mouse modes as described at http://invisible-island.net/xterm/ctlseqs/ctlseqs.html#h2-Mouse-Tracking
Supports bracketed paste mode as described at http://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-tableand library functionality to build a width table that will work for your terminal and load it on application startup.
Vty uses threads internally, so programs made with Vty need to be
compiled with the threaded runtime using the GHC
For the most part, Vty uses
terminfo to determine terminal protocol
with some special rules to handle some omissions from
Windows is not supported.
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 running Vty's built-in tool,
vty-build-width-table. The tool
works by querying the current terminal emulator to obtain its width
measurements for the entire supported Unicode range. The
results are then saved to a disk file. These custom tables
can also be generated programmatically by using the API in
Saved width tables can then be loaded in one of two ways:
- Via the library API in
- By adding a
widthMapdirective to your Vty configuration file and then invoking
mkVtyto 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
for the current value of
TERM, the last one listed in the file is
The tables declared in the configuration file are only ever
automatically loaded when applications set up Vty by calling
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
Without using a custom width table, users of Vty-based applications are likely to eventually experience rendering problems with with wide characters. We recommend that developers of Vty-based applications either:
- Provide the
vty-build-width-tabletool and documentation for running it and updating the Vty configuration, or
- Have the application invoke the Vty library's table-building functionality and load the table at startup without using the Vty configuration.
The best option will depend on a number of factors: the user audience, the amount of risk posed by wide character rendering, the terminal emulators in use, etc.
If you decide to contribute, that's great! Here are some guidelines you should consider to make submitting patches easier for all concerned:
- 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.
Terminals have numerous quirks and bugs, so mileage may vary. Please report issues as you encounter them and provide details on your terminal emulator, operating system, etc.
STOP, TERM and INT signals are not handled.
The character encoding of the terminal is assumed to be UTF-8 if unicode is used.
Terminfo is assumed to be correct unless there is an override configured. Some terminals will not have correct special key support (shifted F10 etc). See
Configfor customizing vty's behavior for a particular terminal.
Vty uses the
TIOCGWINSZioctl to find the current window size, which appears to be limited to Linux and BSD.
Good sources of documentation for terminal programming are: