A nicer syntax for .XCompose files
There are two main ways in Linux to type special characters like λ and →. The first is to use the AltGr modifier key. For example, in my computer, typing AltGr+S results in §.
The second way is to use a Compose Key. The Compose Key allows you to type a sequence of keys to generate a character. For example, in my system you can also type § via Compose s o. Compose keys allow you to type many characters that cannot be typed with AltGr and often involve easier to remember mnemonics.
Note that when using the compose key you type the keys one after the other. When using AltGr you press the keys at the same time.
Keyboards don't typically come with a dedicated Compose key, so it is likely that you may need to configure your system to remap another key to the Compose. In my system I remapped the right Windows key to act as Compose.
The appropriate way to do this remap will depend on the desktop environment you are using. In XFCE this setting can be found under Settings > Keyboard > Layout
The default behavior of the Compose key (as well as other "dead-key" combinations such as ´+a=á) is locale-specific and is described in /usr/share/X11/locale/<mylocale>/Compose. Most of these Compose files use compose rules from /usr/share/X11/locale/en_US.UTF-8/Compose with small modifications.
It is also possible to define custom Compose rules. For example, in my thesis I use the "long right arrow" ⟶, which can't be easily typed using the default compose rules so I made a special compose sequence for it (Compose - - > space). These rules go in a .XCompose file in your home folder but they are cumbersome to write, which is where xcomposer comes in.
The ibus input system that is used by default in many Linux distributions does not respect the customizations present in the .XCompose file. You might to use a different input method or revert to the xim method. You can do this by setting the following environment variables in your X initialization (.xsessionrc)
export GTK_IM_MODULE="xim"
export QT_IM_MODULE="xim"
The easiest way to install xcomposer is via Luarocks
luarocks install xcomposer
xcomposer takes as input a rule file containing a description of your custom Compose rules and creates a suitable .XCompose file for you. You can invoke xcomposer as follows:
xcomposer myrules.xcompose
Passing - as the input file will read rules from the standard input
xcomposer - < myrules.xcompose
By default xcomposer writes its output to $HOME/.XCompose you can override this with the -o flag
xcomposer myrules.xcompose -o out.txt
You can also use -o - to output to standard output.
My personal xcomposer rule file can be found in the "examples" folder.