vty-5.2.3: A simple terminal UI library

Safe HaskellNone



The input layer for VTY. This provides methods for initializing an Input structure which can then be used to read Events from the terminal.

The history of terminals has resulted in a broken input process. Some keys and combinations will not reliably map to the expected events by any terminal program. Even those not using vty. There is no 1:1 mapping from key events to bytes read from the terminal input device. In very limited cases the terminal and vty's input process can be customized to resolve these issues.

See Graphics.Vty.Config for how to configure vty's input processing. Customizing terminfo and the terminal is beyond the scope of this documentation.

VTY's Implementation

One can get the brain rot trying to understand all this. So, as far as I can care...

There are two input modes:

  1. 7 bit
  2. 8 bit

7 bit input is the default and the expected in most use cases. This is what vty uses.

7 bit input encoding

Control key combinations are represented by masking the two high bits of the 7bit input. Back in the day the control key actually grounded the two high bit wires: 6 and 7. This is why control key combos map to single character events: The input bytes are identical. The input byte is the bit encoding of the character with bits 6 and 7 masked. Bit 6 is set by shift. Bit 6 and 7 are masked by control. EG:

  • Control-I is i, `01101001`, has bit 6 and 7 masked to become `00001001`. Which is the ASCII and UTF-8 encoding of the tab key.
  • Control+Shift-C is C, `01000011`, with bit 6 and 7 set to zero which makes `0000011` and is the "End of Text" code.
  • Hypothesis: This is why capital-A, A, has value 65 in ASCII: This is the value 1 with bit 7 set and 6 unset.
  • Hypothesis: Bit 6 is unset by upper case letters because, initially, there were only upper case letters used and a 5 bit encoding.

8 bit encoding

The 8th bit was originally used for parity checking. Useless for emulators. Some terminal emulators support a 8 bit input encoding. While this provides some advantages the actual usage is low. Most systems use 7bit mode but recognize 8bit control characters when escaped. This is what vty does.

Escaped Control Keys

Using 7 bit input encoding the ESC byte can signal the start of an encoded control key. To differentiate a single ESC eventfrom a control key the timing of the input is used.

  1. ESC individually: ESC byte; no bytes for singleEscPeriod.
  2. control keys that contain ESC in their encoding: The @ESC byte; followed by more bytes read within singleEscPeriod. All bytes up until the next valid input block are passed to the classifier.

If the current runtime is the threaded runtime then the terminal's VMIN and VTIME behavior reliably implement the above rules. If the current runtime does not support forkOS then there is currently no implementation.

Vty used to emulate VMIN and VTIME. This was a input loop which did tricky things with non-blocking reads and timers. The implementation was not reliable. A reliable implementation is possible, but there are no plans to implement this.

Unicode Input and Escaped Control Key Sequences

The input encoding determines how UTF-8 encoded characters are recognize.

  • 7 bit mode: UTF-8 can be input unambiguiously. UTF-8 input is a superset of ASCII. UTF-8 does not overlap escaped control key sequences. However, the escape key must be differentiated from escaped control key sequences by the timing of the input bytes.
  • 8 bit mode: UTF-8 cannot be input unambiguously. This does not require using the timing of input bytes to differentiate the escape key. Many terminals do not support 8 bit mode.


The terminfo system is used to determine how some keys are encoded. Terminfo is incomplete. In some cases terminfo is incorrect. Vty assumes terminfo is correct but provides a mechanism to override terminfo. See Graphics.Vty.Config specifically inputOverrides.

Terminal Input is Broken

Clearly terminal input has fundemental issues. There is no easy way to reliably resolve these issues.

One resolution would be to ditch standard terminal interfaces entirely and just go directly to scancodes. A reasonable option for vty if everybody used the linux kernel console. I hear GUIs are popular these days. Sadly, GUI terminal emulators don't provide access to scancodes AFAIK.

All is lost? Not really. Graphics.Vty.Config supports customizing the input byte to event mapping and xterm supports customizing the scancode to input byte mapping. With a lot of work a user's system can be set up to encode all the key combos in an almost-sane manner.

There are other tricky work arounds that can be done. I have no interest in implementing most of these. They are not really worth the time.

Terminal Output is Also Broken

This isn't the only odd aspect of terminals due to historical aspects that no longer apply. EG: Some terminfo capabilities specify millisecond delays. (Capabilities are how terminfo describes the control sequence to output red, for instance) This is to account for the slow speed of hardcopy teletype interfaces. Cause, uh, we totally still use those.

The output encoding of colors and attributes are also rife with issues.

See also

In my experience this cannot resolve the issues without changes to the terminal emulator and device.



data Key Source

Representations of non-modifier keys.

  • KFun is indexed from 0 to 63. Range of supported FKeys varies by terminal and keyboard.
  • KUpLeft, KUpRight, KDownLeft, KDownRight, KCenter support varies by terminal and keyboard.
  • Actually, support for most of these but KEsc, KChar, KBS, and KEnter vary by terminal and keyboard.


data Modifier Source

Modifier keys. Key codes are interpreted such that users are more likely to have Meta than Alt; for instance on the PC Linux console, MMeta will generally correspond to the physical Alt key.



data Button Source

Mouse buttons.

todo not supported.




data Event Source



EvKey Key [Modifier] 
EvMouse Int Int Button [Modifier]

todo mouse events are not supported

EvResize Int Int

if read from eventChannel this is the size at the time of the signal. If read from nextEvent this is the size at the time the event was processed by Vty. Typically these are the same, but if somebody is resizing the terminal quickly they can be different.


data Input Source




_eventChannel :: Chan Event

Channel of events direct from input processing. Unlike nextEvent this will not refresh the display if the next event is an EvResize.

shutdownInput :: IO ()

Shuts down the input processing. This should return the terminal input state to before the input initialized.

_configRef :: IORef Config

Changes to this value are reflected after the next event.

_inputDebug :: Maybe Handle

input debug log

inputForConfig :: Config -> IO Input Source

Set up the terminal with file descriptor inputFd for input. Returns a Input.

The table used to determine the Events to produce for the input bytes comes from classifyMapForTerm. Which is then overridden by the the applicable entries from inputMap.

The terminal device is configured with the attributes:

  • IXON disabled
  • disables software flow control on outgoing data. This stops the process from being suspended if the output terminal cannot keep up. I presume this has little effect these days. I hope this means that output will be buffered if the terminal cannot keep up. In the old days the output might of been dropped?

"raw" mode is used for input.

  • ISIG disabled
  • enables keyboard combinations that result in signals. TODO: should probably be a dynamic option.
  • ECHO disabled
  • input is not echod to the output. TODO: should be a dynamic option.
  • ICANON disabled
  • canonical mode (line mode) input is not used. TODO: should be a dynamic option.
  • IEXTEN disabled
  • extended functions are disabled. TODO: I don't know what those are.