Put contents of this file in your ~/.config/fish/functions/fish_prompt.fish in order to have a cool lambda prompt

lambda_prompt.fish

function fish_prompt --description 'Write out the prompt'
	set -l color_cwd
    set -l suffix
    switch "$USER"
        case root toor
            if set -q fish_color_cwd_root
                set color_cwd $fish_color_cwd_root
            else
                set color_cwd $fish_color_cwd
            end
            set suffix '#'
        case '*'
            set color_cwd $fish_color_cwd
            set suffix 'λ'
    end

    echo -n -s (set_color brgreen) "$USER" (set_color normal) @ (set_color blue) (prompt_hostname) ' ' (set_color $color_cwd) (prompt_pwd) (set_color brgreen) " $suffix "
    set_color normal
end