BSD-3-Clause licensed by AUTHORS
Maintained by Jonathan Daugherty ([email protected])
This version can be pinned in stack with:vty-5.28.2@sha256:8cffce1f4f92eec70b5c3763bb6437852fd3fa7546869496f7667a5abc3f07d7,20859

Build Status

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.

Install via git with:

git clone git://

Install via cabal with:

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

  • Supports bracketed paste mode as described at

  • 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.

Platform Support

Posix Terminals

For the most part, Vty uses terminfo to determine terminal protocol with some special rules to handle some omissions from terminfo.


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 Graphics.Vty.UnicodeWidthTable.Query.

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.

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-table tool 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.

Known Issues

  • 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 Config for customizing vty’s behavior for a particular terminal.

  • Vty uses the TIOCGWINSZ ioctl to find the current window size, which appears to be limited to Linux and BSD.

Further Reading

Good sources of documentation for terminal programming are:



Bug fixes:

  • Added a package dependency on semigroups for the vty-build-width-table tool on older GHCs (#185)


Bug fixes:

  • installUnicodeWidthTable: use throwIO, not throw


This release improves Vty’s support for multi-column Unicode characters and provides greater compatibility with a wider array of terminal emulators. The following sections summarize the relevant changes, but an overview of the new functionality is motivated and detailed in the new “Multi-Column Character Support” README section. For historical context, please also consider reading over #175.

API changes:

  • New modules were added:
    • Graphics.Vty.UnicodeWidthTable.Types
    • Graphics.Vty.UnicodeWidthTable.IO
    • Graphics.Vty.UnicodeWidthTable.Query
    • Graphics.Vty.UnicodeWidthTable.Install
  • The Config type got a new field, allowCustomUnicodeWidthTables, that controls whether mkVty will attempt to load a Unicode width table if specified in the configuration.

Configuration file changes:

  • A new syntax was added to support specifying Unicode width tables on a per-TERM basis. The syntax is widthMap <TERM> <PATH>. See the documentation for Graphics.Vty.Config for details. Since prior versions of this library will silently ignore any configuration file lines they cannot parse, this change to user configuration files is at least non-breaking for older versions of Vty.

Other changes:

  • The mkVty function now automatically attempts to load a custom Unicode width table if one is specified in the configuration, provided allowCustomUnicodeWidthTables is not set to Just False. See the documentation for Graphics.Vty.mkVty for details.
  • Vty now includes a command line tool, vty-build-width-table, that queries the terminal emulator to construct a custom Unicode width table and optionally update the Vty configuration file to use it. Programs that want to use that tool’s functionality may also do so via the API exposed in the various modules listed above.


  • Added Graphics.Vty.Config.getTtyEraseChar to support querying the kernel for the current terminal’s settings to obtain the character assigned by the stty erase command. That can then be added to the Vty configuration’s input map to map to KBS (backspace) if desired.


  • Resolved various import warnings (thanks @glguy)
  • Removed the MonadIO constraint from the Output type’s fields and removed MonadFail uses (PR #177, thanks @glguy)
  • Clarified documentation for ANSI colors (thanks Colby Jenn)
  • Graphics.Vty.Attributes no longer re-exports Graphics.Vty.Attributes.Color
  • The Graphics.Vty.Attributes.Color module is now exposed (thanks Colby Jenn)
  • Raised upper bound for microlens to 0.4.12 (thanks Artyom Kazak)
  • Changed from using System.Posix.Env.getEnv to System.Environment.lookupEnv (thanks Jonathan Osser)
  • Added Graphics.Vty.Image functions for dealing with character width computations on Text values instead of Strings:
    • safeWctwidth
    • safeWctlwidth
    • wctwidth
    • wctlwidth


  • Avoided a conflict with a Microlens 0.4.10 operator and added an upper bound on Microlens of 0.4.11.


  • The Vty type got a new field, isShutdown, that returns whether the Vty handle has had its ‘shutdown’ function called (thanks Ian Jeffries)
  • Vty’s shutdown function is now thread-safe.


  • The “shutdown” method of Vty handles is now idempotent (#159)


  • Add Generic and NFData instances for some types
  • Image: remove custom Show instance, add derived Show and Read instances
  • Updated Travis build settings (thanks Eric Mertens)


  • Fixed a bug where italics did not combine properly with other display modes (#155, thanks Eric Mertens)


  • Added support for italicized output when terminfo supports it. This takes the form of a new Style, “italic”. Note that most terminfo descriptors do not report capabilities for italics, so support for this will be very spotty.
  • Updateed text/string function documentation to indicate that escapes are not permitted in their inputs.


  • Added Graphics.Vty.Attributes.Color240.color240CodeToRGB function (thanks Brent Carmer)
  • Added nextEventNonblocking function (field) to Vty type (#87)


  • Picture and Background now provide Eq instances (thanks Jaro Reinders)
  • #145: vty builds with microlens 0.4.9 (thanks Daniel Wagner)
  • #142: note requirement of threaded RTS


API changes:

  • Split up Monoid instances into Monoid and Semigroup for newer GHCs (thanks Ryan Scott)


API changes:

  • Cursor now provides an Eq instance (thanks Jaro Reinders)


API changes:

  • URL hyperlinking (via ‘withURL’) is now optional and disabled by default due to poor support on some common terminals. A new ‘Mode’ constructor, ‘Hyperlink’, has been added to enable this feature. To change the hyperlinking mode, use ‘setMode’ on the ‘outputIface’ of a Vty handle.


Bug fixes:

  • Reset the hyperlink state on line endings to avoid run-on hyperlinks


API changes:


  • withStyle now ignores zero arguments, leaving attribute styles untouched if the input style is the null style


API changes:

  • Add support for terminal focus events. This change adds a new mode usable with setMode, Focus, that requests that the terminal send events on focus lose/gain. This change also adds two new Event constructors, EvLostFocus and EvGainedFocus.
  • No longer enable UTF8 mouse event encoding. This encoding was not working properly with, and using the other modes (SGR, etc.) work.
  • Graphics.Vty.Attributes: escape backticks in Haddock comment (fixes #131)


API changes:

  • Added support for mouse wheel events while in mouse mode. The Button type got two new constructors as a result: BScrollUp and BScrollDown. Thanks to [email protected] for this contribution!

Bug fixes:

  • charFill now clamps negative arguments to zero (thanks Eric Mertens!)


Package changes:

  • Documentation files are now marked accordingly (thanks Michal Suchánek)

Bug fixes:

  • translateX/Y: fix negative translations


Package changes:

  • Discontinued support for GHC versions prior to 7.10.1.
  • Removed instructions and configuration for Stack builds since they are no longer supported.
  • Clarified README mention of (lack of) Windows support (contributors wanted, though!)
  • Removed dependency on data-default (see below).

API changes:

  • Moved color definitions from Attributes to Color module.
  • In lieu of data-default (Default) instances for Attr and Config, use ‘defAttr’ and the new ‘defaultConfig’ (or ‘mempty’) instead of ‘def’.
  • Graphics.Vty.Output no longer re-exports Graphics.Vty.Output.Interface.
  • Removed Graphics.Vty.Prelude module and moved DisplayRegion and its accessors to Graphics.Vty.Image.
  • Graphics.Vty.Image no longer re-exports Graphics.Vty.Attributes.
  • Graphics.Vty.Picture no longer re-exports Graphics.Vty.Image.


  • addMaybeClippedJoin: instead of raising an exception when the join is totally clipped, just reduce the clip amount and continue
  • addMaybeClipped: skip blit of joins when their primary dimension is zero
  • ‘string’ and related text functions no longer treat an empty string as an empty image (thanks Chris Penner). This means that now it is possible to use ‘str “”’ as a non-empty image with height 1.



  • Replaced ‘wcwidth’ with a call to the utf8proc library’s character width function, which is much more up to date (by several Unicode versions) and returns the right width for a much larger set of characters.
  • Added a bundled version of the utf8proc C library.


  • Fix mouse event offsets in mouse-up events


  • Mouse events were modified so that the upper-left corner of the window is (0,0) rather than (1,1).


  • Add Generic instance for Image
  • nextEvent: stop trying to refresh on a resize event (fixes segfault on refresh with normal cursor positioning mode)
  • Remove redundant clause from clipForCharWidth (thanks Eric Mertens)
  • Update maintainer


  • Vty now raises a VtyConfigurationError exception when the TERM evironment variable is missing (thanks Eric Mertens)
  • Graphics.Vty.Config got an explicit export list to avoid accidentally exporting internal types (thanks Eric Mertens)


  • Add absolute cursor positioning mode AbsoluteCursor to Cursor. This mode provides greater control over cursor positioning by bypassing the logical positioning provided by default. Rather than positioning the cursor by looking at the widths of characters involved, this constructor lets you provide a physical row and column instead. This is useful in more sophisticated programs. (thanks Eric Mertens)
  • Added a new Generic-derived config parser (thanks Eric Mertens)
  • Fixed the MShift case in the configuration file parser (thanks Eric Mertens)
  • Fixed wcwidth import and matched safeWcswidth to its documented behavior. Previously vty_mk_wcwidth was being imported with the wrong type causing the -1 return value to be mapped to the wrong Int value. Additionally safeWcswidth was using the unsafe character width function and only ensuring that the final result was non-negative. (thanks Eric Mertens)


  • Vty now only emits UTF8 charset sequences in terminals without a preexisting UTF8 declaration to avoid emitting garbage sequences (fixes #89)


  • Added new Output methods supportsBell and ringTerminalBell to find out whether the output device has an audio bell and to ring it (see #102)


  • Fixed “refresh” to work as advertised (see #104)


  • API change: EvPaste input event now provides paste data as a raw ByteString rather than a String to allow the application to decode how best to decode it


  • ModeDemo: added an explicit Control.Applicative import for older GHCs


  • Mouse and paste modes are now off by default.
  • The Config type got new fields: mouseMode and bracketedPasteMode. These determine whether these modes are enabled initially (for terminals that support them).
  • Added a Mode type for modal terminal features (mouse events, bracketed paste mode) that is used with new Output interface functions:
    • supportsMode :: Mode -> Bool tells whether the device supports a mode
    • setMode :: Mode -> Bool -> IO () turns a mode on or off
    • getModeStatus :: Mode -> IO Bool tells you whether a mode is on or off
  • Added a new demo program, ModeDemo.hs, to demonstrate usage of modes


  • Added support for normal and extended mouse modes in Xterm-like terminals via the MouseDown and MouseUp Event constructors
  • Added support for bracketed paste mode in Xterm-like terminals via the EvPaste event constructor
  • Added derived Show instances for Event and Button (thanks Felix Hirn)
  • Now TERM values containing “screen” will automatically use the XtermColor driver rather than just TerminfoBased


  • Replaced lens dependency with microlens, microlens-mtl, microlens-th dependencies. Issue #90
    • Thanks Jonathan Daugherty
  • Cabal corrections.
    • Thanks Lennart Spitzner


  • Changed eventChannel of Graphics.Vty.Input from Chan to TChan. This enables clients to query if there are no pending events. The Graphics.Vty interface nextEvent is unchanged. Clients that use eventChannel directly will require updating.


  • Reverted cabal file to depend on Cabal >= 1.18 instead of 1.20 due to possibly breaking this on reasonable GHC versions


  • Upgraded QuickCheck dependency to 2.7
  • The standard IO Config (standardIOConfig) was overriding any provided application config. In addition, the inputFd and outputFd could not be changed if mkVty was used. Fixed.
  • Correct handling of display attributes at end of line. The output attributes are set to default at the end of content for the line and at the start of a new line. Previously the current attribute would extend to the next start of content. This was odd to reason about and was the cause of IIRC Yi requires the old behavior to display the selection region correctly.
  • shutdown of the input thread is now performed using killThread and synchronization on an MVar. For correct handling of the terminal read vmin and vtime the read must be a blocking read on an OS thread. This places a threadWaitRead, which will be interrupted by the killThread, prior to the uninterruptable read. An alternative would be to re-import the read foreign call as interruptable.


  • deepseq bounds increased for tests.
  • Clean up warnings when compiling on 7.10
    • Thanks Eric Mertens
  • Avoid discarding input bytes after multi-byte encoded codepoint
    • Thanks Eric Mertens


  • “str” now returns EmptyImage for empty strings to match behavior of other string-like Image constructors (fixes #74)
    • Thanks Jonathan Daugherty










  • Bump upper version bound for lens to 4.5. Thanks markus1189!


  • Config structure now specifies file descriptor to use. The default is stdInput and stdOutput file descriptors. Previously Vty used stdInput for input and the follow code for output:
    • hDuplicate stdout >>= handleToFd >>= (hSetBuffering NoBuffering)
    • the difference was required by Vty.Inline. Now, Vty.Inline uses the Config structure options to acheive the same effect.
  • removed: derivedVtime, derivedVmin, inputForCurrentTerminal, inputForNameAndIO, outputForCurrentTerminal, outputForNameAndIO
  • added: inputForConfig, outputForConfig
  • updates to vty-rogue from jtdaugherty. Thanks!
  • the oldest version of GHC tested to support vty is 7.6.2.
  • the oldest version of GHC that vty compiles under is 7.4.2




  • vmin and vtime can be specified however the application requires. See Graphics.Vty.Config.
  • fixed the processing of input when vmin is set > 1.


  • The naming convention now matches:
  • all projects using vty for input must be compiled with -threaded. Please notify vty author if this is not acceptable.
  • mkVtyEscDelay has been removed. Use “mkVty def”. Which initialized vty with the default configuration.
  • input handling changes
    • KASCII is now KChar
    • KPN5 is now KCenter
    • tests exist.
    • Applications can add to the input tables by setting inputMap of the Config. See Graphics.Vty.Config
    • Users can define input table extensions that will apply to all vty applications. See Graphics.Vty.Config
    • terminal timing is now handled by selecting an appropriate VTIME. Previously this was implemented within Vty itself. This reduced complexity in vty but provides a different meta key behavior and implies a requirement on -threaded.
    • The time vty will wait to verify an ESC byte means a single ESC key is the singleEscPeriod of the Input Config structure.
  • removed the typeclass based terminal and display context interface in favor of a data structure of properties interface.
  • renamed the Terminal interface to Output
  • The default picture for an image now uses the “clear” background. This background fills background spans with spaces or just ends the line.
    • Previously the background defaulted to the space character. This causes issues copying text from a text editor. The text would end up with extra spaces at the end of the line.
  • Layer support
    • Each layer is an image.
    • The layers for a picture are a list of images.
    • The first image is the top-most layer. The images are ordered from top to bottom.
    • The transparent areas for a layer are the backgroundFill areas. backgroundFill is added to pad images when images of different sizes are joined.
    • If the background is clear there is no background layer.
    • If there is a background character then the bottom layer is the background layer.
    • emptyPicture is a Picture with no layers and no cursor
    • addToTop and addToBottom add a layer to the top and bottom of the given Picture.
  • compatibility improvements:
    • terminfo based terminals with no cursor support are silently accepted. The cursor visibility changes in the Picture will have no effect.
    • alternate (setf/setb) color maps supported. Though colors beyond the first 8 are just a guess.
    • added “rgbColor” for easy support of RGB specified colors.
    • Both applications and users can add to the mapping used to translate from input bytes to events.
  • Additional information about input and output process can be appended to a debug log
    • Set environment variable VTY_DEBUG_LOG to path of debug log
    • Or use “debugLog ” config directive
    • Or set ‘debugLog’ property of the Config provided to mkVty.
  • examples moved to vty-examples package. See test directory for cabal file.
    • vty-interactive-terminal-test
      • interactive test. Useful for building a bug report for vty’s author.
      • test/interactive_terminal_test.hs
    • vty-event-echo
      • view a input event log for vty. Example of interacting with user.
      • test/EventEcho.hs
    • vty-rogue
      • The start of a rogue-like game. Example of layers and image build operations.
      • test/Rogue.hs
    • vty-benchmark
      • benchmarks vty. A series of tests that push random pictures to the terminal. The random pictures are generated using QuickCheck. The same generators used in the automated tests.
      • test/benchmark.hs

API changes:

  • Added Graphics.Vty.Image.crop: Ensure an image is no larger than the specified size.
  • Added Graphics.Vty.Image.pad: Ensure an image is no smaller than the specified size.
  • Added Graphics.Vty.Image.translate: Offset an image.
  • Thanks Ben Boeckel [email protected] for these features.

API changes:

  • Attr record accessor fore_color changed to attr_fore_color
  • Attr record accessor back_color changed to attr_back_color
  • Attr record accessor style changed to attr_style
  • Added an “inline” display attribute changing DSL:
    • put_attr_change applies a display attribute change immediately to a terminal
    • For instance, can be used to change the display attrbiutes of text output via putStrLn and putStr. EX: “put_attr_change $ back_color red” will set the background color to red.
    • Changes do not apply to a Picture output via output_picture.
    • See Graphics.Vty.Inline
  • Moved all IO actions into any monad an instance of MonadIO

  • binding for mk_wcswidth was incorrect. Most platforms just magically worked due to coincidence.


API changes:

  • “getSize” has been removed. Use “terminal vty >>= display_bounds” where “vty” is an instance of the Vty data structure.
  • added a “terminal” field to the Vty data structure. Accesses the TerminalHandle associated with the Vty instance.
  • Graphics.Vty.Types has undergone a number of changes. Summary:
    • Partitioned into Graphics.Vty.Attributes for display attributes. Graphics.Vty.Image for image combinators. Graphics.Vty.Picture for final picture construction.
  • Graphics.Vty.Attributes:
    • “setFG” and “setBG” are now “with_fore_color” and “with_back_color”
    • All other “set..” equations similarly replaced.
    • “attr” is now “def_attr”, short for “default display attributes” Also added a “current_attr” for “currently applied display attributes”
  • Graphics.Vty.Image:
    • “horzcat” is now “horiz_cat”
    • “vertcat” is now “vert_cat”
    • “renderBS” is now “utf8_bytestring”
    • “renderChar” is now “char”
    • “renderFill” is now “char_fill”
    • added a “utf8_string” and “string” (AKA “iso_10464_string”) for UTF-8 encoded Strings and ISO-10464 encoded Strings. String literals in GHC have an ISO-10464 runtime representation.
  • Graphics.Vty.Picture:
    • exports Graphics.Vty.Image
    • “pic” is now “pic_for_image”
    • added API for setting background fill pattern.
  • Completely rewritten output backend.
    • Efficient, scanline style output span generator. Has not been fully optimized, but good enough.
    • The details required to display the desired picture on a terminal are well encapsulated.
    • Terminfo based display terminal implementation. With specialized derivitives for xterm,, and
      • Attempts to robustly handle even terminals that don’t support all display attributes.
      • I’ve tested the following terminals with success:,, xterm, rxvt, mlterm, Eterm, gnome-terminal, konsole, screen, linux vty. Hopefully you will be as successfull.
    • Improved unicode support. Double wide characters will display as expected.
  • 256 color support. See Graphics.Vty.Attributes.Color240. The actual output color is adjusted according to the number of colors the terminal supports.
  • The Graphics.Vty.Image combinators no longer require matching dimensions to arguments. Unspecified areas are filled in with a user-customizable background pattern. See Graphics.Vty.Picture.
  • output images are always cropped to display size.
  • Significant code coverage by QuickCheck tests. An interactive test for those final properties that couldn’t be automatically verified.

Issues resolved: