The prompt may optionally be generated from a function, instead of a string. Take note, however, that prompts generated from functions aren't as efficient, due to the need to perform a fork and capture the output of the fork to use as the prompt. To use a function for generating the prompt, simply create a function whose name is PROMPT, and the output of that command will be used as the prompt. Below is an example:

fn PROMPT
    echo -n "${PWD}# "
end