BSD-3-Clause licensed by AUTHORS
Maintained by Jonathan Daugherty ([email protected])
This version can be pinned in stack with:vty-5.21@sha256:a5e61cd528f9573aa5748a3973dbf7542fceed813eb0b2700aba2e423507e960,20990

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. vt100, ansi, hurd, linux, screen, etc., or anything with a sufficient terminfo entry.

  • Automatic handling of window resizes.

  • Unicode output on terminals with UTF-8 support.

  • Handles multi-column glyphs. (Requires user to properly configure terminal.)

  • Efficient output. Output buffering and terminal state changes are minimized.

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

  • 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.

  • The interface is designed for easy extension.

  • 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

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

Uses terminfo to determine terminal protocol. With some special rules to handle some omissions from terminfo.


Unsupported (but contributions and testing here are very welcome!)


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.

Development Notes

Under NixOS

Using cabal

After installing ncurses to the user environment:

LIBRARY_PATH=$HOME/.nix-profile/lib/ cabal configure --enable-tests --extra-lib-dirs=$HOME/.nix-profile/lib
LIBRARY_PATH=$HOME/.nix-profile/lib/ cabal build
LIBRARY_PATH=$HOME/.nix-profile/lib/ cabal test

Code Coverage

As of last testing, profiling causes issues with coverage when enabled. To evaluate coverage, configure as follows:

rm -rf dist ; cabal configure --enable-tests --enable-library-coverage \
  --disable-library-profiling \


rm -rf dist ; cabal configure --enable-tests --disable-library-coverage \
  --enable-library-profiling \

Known Issues

  • Terminals have numerous quirks and bugs. Vty picks what works best for the author in ambiguous or buggy situations.

  • Signal handling of STOP, TERM and INT are nonexistent.

  • 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.

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


Good sources of documentation for terminal programming are:



  • 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

5.20 API changes:

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

5.19.1 API changes:

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

5.19 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.

5.18.1 Bug fixes:

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

5.18 API changes:


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

5.17 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)

5.16 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!)

5.15.1 Package changes:

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

Bug fixes:

  • translateX/Y: fix negative translations

5.15 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. 5.3.1
  • 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: