My fish shell lambda prompt functions (they go in ~/.config/fish/functions) it's extremely fast too! :D

fish_prompt.fish

# A lambda (λ) prompt.
# Green and red depending on exit status.
# Underlined if git status is dirty.
# Uppercase (Λ) if ahead of the remote.

function fish_prompt
  if is_status_okay
    set_color green
  else
    set_color red
  end

  if is_git_dirty
    set_color --underline
  end

  if is_git_ahead
    echo -n  'Λ'
  else
    echo -n  'λ'
  end

  set_color normal
  echo -n ' '
end

fish_right_prompt.fish

function fish_right_prompt

  if is_git
    set_color yellow
    echo -n (git_branch)
    set_color normal
    echo -n ' '
  end

  set_color blue
  echo -n (basename (pwd | sed "s#$HOME#\~#"))
  set_color normal
end

git_branch.fish

function git_branch
  if is_git
    echo (git rev-parse --abbrev-ref HEAD 2> /dev/null)
  end
end

is_git.fish

function is_git
  git symbolic-ref HEAD > /dev/null 2>&1
end

is_git_ahead.fish

function is_git_ahead
  set -l revs (git rev-list origin/(git_branch)..HEAD ^ /dev/null)
  [ "$revs" != "" ]
end

is_git_dirty.fish

function is_git_dirty
  is_git; and [ (git status | tail -n1) != "nothing to commit, working directory clean" ]
end

is_status_okay.fish

function is_status_okay
  [ $status = 0 ]
end

Wow, this is pretty much the perfect prompt! I love it, thank you!