Isocline: a portable readline alternative.

Isocline is a pure C library that can be used as an alternative to the GNU readline library (latest release v1.0.9, 2022-01-15).

• Small: less than 8k lines and can be compiled as a single C file without any dependencies or configuration (e.g. gcc -c src/isocline.c).

• Portable: works on Unix, Windows, and macOS, and uses a minimal subset of ANSI escape sequences.

• Features: extensive multi-line editing mode (shift-tab), (24-bit) color, history, completion, unicode, undo/redo, incremental history search, inline hints, syntax highlighting, brace matching, closing brace insertion, auto indentation, graceful fallback, support for custom allocators, etc.

• License: MIT.

• Comes with a Haskell binding (System.Console.Isocline.

Enjoy, Daan

Demo

Shows in order: unicode, syntax highlighting, brace matching, jump to matching brace, auto indent, multiline editing, 24-bit colors, inline hinting, filename completion, and incremental history search.
_{(screen capture was made with termtosvg by Nicolas Bedos)}

Usage

Include the isocline header in your C or C++ source:

#include <include/isocline.h>

and call ic_readline to get user input with rich editing abilities:

char* input;
while( (input = ic_readline("prompt")) != NULL ) { // ctrl+d/c or errors return NULL
  printf("you typed:\n%s\n", input); // use the input
  free(input);  
}

See the example for a full example with completion, syntax highligting, history, etc.

Run the Example

You can compile and run the example as:

$ gcc -o example -Iinclude test/example.c src/isocline.c
$ ./example

or, the Haskell example:

$ ghc -ihaskell test/Example.hs src/isocline.c
$ ./test/Example

Editing with Isocline

Isocline tries to be as compatible as possible with standard GNU Readline key bindings.

Overview:

       home/ctrl-a       cursor     end/ctrl-e
         ┌─────────────────┼───────────────┐    (navigate)
         │     ctrl-left   │  ctrl-right   │
         │         ┌───────┼──────┐        │    ctrl-r   : search history
         ▼         ▼       ▼      ▼        ▼    tab      : complete word
  prompt> it is the quintessential language     shift-tab: insert new line
         ▲         ▲              ▲        ▲    esc      : delete input, done
         │         └──────────────┘        │    ctrl-z   : undo
         │    alt-backsp        alt-d      │
         └─────────────────────────────────┘    (delete)
       ctrl-u                          ctrl-k

_{Note: on macOS, the meta (alt) key is not directly available in most terminals. Terminal/iTerm2 users can activate the meta key through Terminal → Preferences → Settings → Use option as meta key.}

Key Bindings

These are also shown when pressing F1 on a Isocline prompt. We use ^ as a shorthand for ctrl-:

Navigation
left,^b	go one character to the left
right,^f	go one character to the right
up	go one row up, or back in the history
down	go one row down, or forward in the history
^left	go to the start of the previous word
^right	go to the end the current word
home,^a	go to the start of the current line
end,^e	go to the end of the current line
pgup,^home	go to the start of the current input
pgdn,^end	go to the end of the current input
alt-m	jump to matching brace
^p	go back in the history
^n	go forward in the history
^r,^s	search the history starting with the current word

Deletion
del,^d	delete the current character
backsp,^h	delete the previous character
^w	delete to preceding white space
alt-backsp	delete to the start of the current word
alt-d	delete to the end of the current word
^u	delete to the start of the current line
^k	delete to the end of the current line
esc	delete the current input, or done with empty input

Editing
enter	accept current input
^enter,^j,shift-tab	create a new line for multi-line input
^l	clear screen
^t	swap with previous character (move character backward)
^z,^_	undo
^y	redo
tab	try to complete the current input

Completion menu
enter,left	use the currently selected completion
1 - 9	use completion N from the menu
tab, down	select the next completion
shift-tab, up	select the previous completion
esc	exit menu without completing
pgdn,^enter,^j	show all further possible completions

Incremental history search
enter	use the currently found history entry
backsp,^z	go back to the previous match (undo)
tab,^r,up	find the next match
shift-tab,^s,down	find an earlier match
esc	exit search

Build the Library

Build as a Single Source

Copy the sources (in include and src) into your project, or add the library as a submodule:

$ git submodule add https://github.com/daanx/isocline

and add isocline/src/isocline.c to your build rules -- no configuration is needed.

Build with CMake

Clone the repository and run cmake to build a static library (.a/.lib):

$ git clone https://github.com/daanx/isocline
$ cd isocline
$ mkdir -p build/release
$ cd build/release
$ cmake ../..
$ cmake --build .

This builds a static library libisocline.a (or isocline.lib on Windows) and the example program:

$ ./example

Build the Haskell Library

See the Haskell readme for instructions to build and use the Haskell library.

API Reference

• See the C API reference and the example for example usage of history, completion, etc.

• See the Haskell API reference on Hackage and the Haskell example.

Motivation

Isocline was created for use in the Koka interactive compiler. This required: pure C (no dependency on a C++ runtime or other libraries), portable (across Linux, macOS, and Windows), unicode support, a BSD-style license, and good functionality for completion and multi-line editing.

Some other excellent libraries that we considered: GNU readline, editline, linenoise, replxx, and Haskeline.

Formatted Output

Isocline also exposes functions for rich terminal output as ic_print (and ic_println and ic_printf). Inspired by the (Python) Rich library, this supports a form of bbcode's to format the output:

ic_println( "[b]bold [red]and red[/red][/b]" );

Each print automatically closes any open tags that were not yet closed. Also, you can use a general close tag as [/] to close the innermost tag, so the following print is equivalent to the earlier one:

ic_println( "[b]bold [red]and red[/]" );

There can be multiple styles in one tag (where the first name is used for the closing tag):

ic_println( "[u #FFD700]underlined gold[/]" );

Sometimes, you need to display arbitrary messages that may contain sequences that you would not like to be interpreted as bbcode tags. One way to do this is the [!tag] which ignores formatting up to a close tag of the form [/tag].

ic_printf( "[red]red? [!pre]%s[/pre].\n", "[blue]not blue!" );

Predefined styles include b (bold), u (underline), i (italic), and r (reverse video), but you can (re)define any style yourself as:

ic_style_def("warning", "crimson u");

and use them like any builtin style or property:

ic_println( "[warning]this is a warning![/]" );

which is great for adding themes to your application.

Each ic_print function always closes any unclosed tags automatically. To open a style persistently, use ic_style_open with a matching ic_style_close which scopes over any ic_print statements in between.

ic_style_open("warning");
ic_println("[b]crimson underlined and bold[/]");
ic_style_close();

Advanced

BBCode Format

An open tag can have multiple white space separated entries that are either a style name, or a primitive property[=value].

Styles

Isocline provides the following builtin styles as property shorthands: b (bold), u (underline), i (italic), r (reverse video), and some builtin styles for syntax highlighting: keyword, control (control-flow keywords), string, comment, number, type, constant.

Predefined styles used by Isocline itself are:

• ic-prompt: prompt style, e.g. ic_style_def("ic-prompt", "yellow on blue").
• ic-info: information (like the numbers in a completion menu).
• ic-diminish: dim text (used for example in history search).
• ic-emphasis: emphasized text (also used in history search).
• ic-hint: color of an inline hint.
• ic-error: error color (like an unmatched brace).
• ic-bracematch: color of matching parenthesis.

Properties

Boolean properties are by default on:

• bold [=(on|off)]
• italic [=(on|off)]
• underline [=(on|off)]
• reverse [=(on|off)]

Color properties can be assigned a color:

• color=color
• bgcolor=color
• color: equivalent to color=color.
• on color: equivalent to bgcolor=color.

A color value can be specified in many ways:

• any standard HTML color name.
• any of the 16 standard ANSI color names by prefixing ansi- (like ansi-black or ansi-maroon).
  The actual color value of these depend on the a terminal theme.
• #rrggbb or #rgb for a specific 24-bit color.
• ansi-color=idx or ansi-bgcolor=idx, where idx specifies an entry in the standard ANSI 256 color palette (between 0 and 255). Use idx 256 for the ANSI default color.

The width property makes the text at least width long:

• width=width [;align [;fill] ]

where width is the column with, align is left, center, or right, and fill the fill character (' ').

The maxwidth property makes text at most width long; when the content it is wider, the left- or right side (depending on the alignment) will have three dots (...) to visualize that content is cut off.

• maxwidth=width [;align]

Environment Variables

• NO_COLOR: if present no colors are displayed.
• CLICOLOR=1: if set, the LSCOLORS or LS_COLORS environment variables are used to colorize filename completions.
• COLORTERM=(truecolor|256color|16color|8color|monochrome): enable a certain color palette, see the next section.
• TERM: used on some systems to determine the color

Colors

Isocline supports 24-bit colors and any RGB colors are automatically mapped to a reduced palette on older terminals if these do not support true color. Detection of full color support is not always possible to do automatically and you can set the COLORTERM environment variable expicitly to force Isocline to use a specific palette:

• COLORTERM=truecolor: use 24-bit colors.
  
• COLORTERM=256color: use the ANSI 256 color palette.
  
• COLORTERM=16color : use the regular ANSI 16 color palette (8 normal and 8 bright colors).
  
• COLORTERM=8color: use bold for bright colors.
• COLORTERM=monochrome: use no color.

The above screenshots are made with the test_colors.c program. You can test your own terminal as:

$ gcc -o test_colors -Iinclude test/test_colors.c src/isocline.c
$ ./test_colors
$ COLORTERM=truecolor ./test_colors
$ COLORTERM=16color ./test_colors

ANSI Escape Sequences

Isocline uses just few ANSI escape sequences that are widely supported:

• ESC[nA, ESC[nB, ESC[nC, and ESC[nD, for moving the cursor n places up, down, right, and left.
• ESC[K to clear the line from the cursor.
• ESC[nm for colors, with n one of: 0 (reset), 1,22 (bold), 3,23 (italic), 4,24 (underline), 7,27 (reverse), 30-37,40-47,90-97,100-107 (color), and 39,49 (select default color).
• ESC[38;5;nm, ESC[48;5;nm, ESC[38;2;r;g;bm, ESC[48;2;r;g;bm: on terminals that support it, select entry n from the 256 color ANSI palette (used with XTERM=xterm-256color for example), or directly specify any 24-bit rgb color (used with COLORTERM=truecolor) for the foreground or background.

On Windows the above functionality is implemented using the Windows console API (except if running in the new Windows Terminal which supports these escape sequences natively).

Async and Threads

Isocline is not thread-safe and ic_readlinexxx and ic_printxxx should be used from one thread only.

The best way to use ic_readline asynchronously is to run it in a (blocking) dedicated thread and deliver results from there to the async event loop. Isocline has the

bool ic_async_stop(void)

function that is thread-safe and can deliver an asynchronous event to Isocline that unblocks a current ic_readline and makes it behave as if the user pressed ctrl-c (which returns NULL from the read line call).

Color Mapping

To map full RGB colors to an ANSI 256 or 16-color palette Isocline finds a palette color with the minimal "color distance" to the original color. There are various ways of calculating this: one way is to take the euclidean distance in the sRGB space (simple-rgb), a slightly better way is to take a weighted distance where the weight distribution is adjusted according to how big the red component is (redmean, denoted as delta-rgb in the figure), this is used by Isocline), and finally, we can first translate into a perceptually uniform color space (CIElab) and calculate the distance there using the CIEDE2000 algorithm (ciede2000). Here are these three methods compared on some colors:

Each top row is the true 24-bit RGB color. Surprisingly, the sophisticated CIEDE2000 distance seems less good here compared to the simpler methods (as in the upper left block for example) (perhaps because this algorithm was created to find close perceptual colors in images where lightness differences may be given less weight?). CIEDE2000 also leads to more "outliers", for example as seen in column 5. Given these results, Isocline uses redmean for color mapping. We also add a gray correction that makes it less likely to substitute a color for a gray value (and the other way around).

Possible Future Extensions

• Vi key bindings.
• kill buffer.
• make the ic_printxxx functions thread-safe.
• extended low-level terminal functions.
• status and progress bars.
• prompt variants: confirm, etc.
• ...

Contact me if you are interested in doing any of these :-)

Releases

• 2022-01-15: v1.0.9: fix missing ic_completion_arg (issue #6), fix null ptr check in ic_print (issue #7), fix crash when using /dev/null as both input and output.
• 2021-09-05: v1.0.5: use our own wcwidth for consistency; thanks to Hans-Georg Breunig for helping with testing on NetBSD.
• 2021-08-28: v1.0.4: fix color query on Ubuntu/Gnome
• 2021-08-27: v1.0.3: fix duplicates in completions
• 2021-08-23: v1.0.2: fix windows eol wrapping
• 2021-08-21: v1.0.1: fix line-buffering
• 2021-08-20: v1.0.0: initial release