This module provides the input layer for Vty, including methods for initializing an Input structure and reading Events from the terminal.

Note that due to the evolution of terminal emulators, some keys and combinations will not reliably map to the expected events by any terminal program. 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.

VTY's Implementation

There are two input modes:

1. 7-bit
2. 8-bit

The 7-bit input mode is the default and the expected mode 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 7-bit input. Historically 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. For example,

• Control-I is i, `01101001`, and 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 is `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 which is useless for terminal emulators. Some terminal emulators support an 8-bit input encoding. While this provides some advantages, the actual usage is low. Most systems use 7-bit mode but recognize 8-bit 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 event from a control key, the timing of the input is used.

1. ESC individually: ESC byte; no bytes following for a period of VMIN milliseconds.
2. Control keys that contain ESC in their encoding: The @ESC byte is followed by more bytes read within VMIN milliseconds. 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.

Unicode Input and Escaped Control Key Sequences

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

• 7-bit mode: UTF-8 can be input unambiguously. 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.

Terminfo

The terminfo system is used to determine how some keys are encoded. Terminfo is incomplete and 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 fundamental 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. This would be a reasonable option for Vty if everybody used the linux kernel console but for obvious reasons this is not possible.

The Graphics.Vty.Config module 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.

See also

• http://www.leonerd.org.uk/hacks/fixterms/