GHCi in colour

This page documents efforts to colourise GHCi output.

Prompt only

The prompt can be colored by altering GHCi's prompt setting including ANSI terminal color codes. For example

   :set prompt "\ESC[34m\STX%s > \ESC[m\STX"

will give the prompt blue color:

As usual, adding this to ghci.conf will auto-execute the command when running GHCi, making the prompt colored by default.

Remember to use \STX to end an escape sequence. Without this control character, the prompt length will be computer incorrectly and your prompt will look wrong when editing long commands.

Full highlighting

Example

Or a type error:

Output like this would be the result of running, for example:

   ghci --colour

and would appear in the console/xterm as ansi terminal coloured output.

Implementation

Using sed and ghci.conf

You can use sed to color the non-prompt output, and color the prompt with ghci.conf. This works better than piping into HsColour for me. See this blog post for details.

This allows you to colorize everything you can catch with a regexp. Note that when coloring GHCi prompt using ghci.conf one should, in ultimate escapity, escape the escape codes with \001..\002, otherwise readline will mess up when editing long lines. For a working example of how this can be done refer to this example. Output:

This approach doesn't work well if you try to output an infinite sequence and then hit CTRL+C, ideas on how to fix that are welcome.

Using HsColour

An existing tool, HsColour, could be modified to operate interactively. In fact, HsColour is already interactive, and with a small patch added on 2006-12-14 to control ouput buffering better, this works relatively nicely:

   ghci 2>&1 | HsColour -tty

HsColour in action

There are small delays however, when lexing certain tokens, and the interaction with readline isn't ideal.

See http://www.reddit.com/r/haskell/comments/144biy/pretty_output_in_ghci_howto_in_comments/

GuiHaskell

Neil Mitchell has a prototype GuiHaskell wrapper, based on GTK.

If you have an idea of how to do this nicely, add your proposal here.