Here's a `fish 2` shell version, that works on the Mac. You can add it as `~/.config/fish/functions/mark.fish`:

    set MARKPATH $HOME/.marks
    function jump
      cd $MARKPATH/$argv; or echo "No such mark: $argv"
    end
    function mark
      mkdir -p $MARKPATH; and ln -s (pwd) $MARKPATH/$argv
    end
    function unmark
      rm -i $MARKPATH/$argv
    end
    function marks
      ls -l $MARKPATH | cut -d' ' -f12-; and echo
    end