How to convert std_logic_vector to hexadecimal

procedure hex2chr( variable hex  : in std_logic_vector( 3 downto 0);
                    variable char : out character) is
	 begin
		 case hex is
			 when "0000" =>    char:='0'; 
          when "0001" =>    char:='1'; 
          when "0010" =>    char:='2'; 
          when "0011" =>    char:='3'; 
          when "0100" =>    char:='4'; 
          when "0101" =>    char:='5'; 
          when "0110" =>    char:='6'; 
          when "0111" =>    char:='7'; 
          when "1000" =>    char:='8'; 
          when "1001" =>    char:='9'; 
          when "1010" =>    char:='a'; 
          when "1011" =>    char:='b'; 
          when "1100" =>    char:='c'; 
          when "1101" =>    char:='d'; 
          when "1110" =>    char:='e'; 
          when "1111" =>    char:='f';
          when others => 
           ASSERT (false) REPORT "no hex character read" SEVERITY failure;
         end case;
end hex2chr;

function to_hstring (value     : STD_ULOGIC_VECTOR) return STRING is
    constant ne     : INTEGER := (value'length+3)/4;
    variable pad    : STD_ULOGIC_VECTOR(0 to (ne*4 - value'length) - 1);
    variable ivalue : STD_ULOGIC_VECTOR(0 to ne*4 - 1);
    variable result : STRING(1 to ne);
    variable quad   : STD_ULOGIC_VECTOR(0 to 3);
  begin
    if value'length < 1 then
      return NUS;
    else
      if value (value'left) = 'Z' then
        pad := (others => 'Z');
      else
        pad := (others => '0');
      end if;
      ivalue := pad & value;
      for i in 0 to ne-1 loop
        quad := To_X01Z(ivalue(4*i to 4*i+3));
        case quad is
          when x"0"   => result(i+1) := '0';
          when x"1"   => result(i+1) := '1';
          when x"2"   => result(i+1) := '2';
          when x"3"   => result(i+1) := '3';
          when x"4"   => result(i+1) := '4';
          when x"5"   => result(i+1) := '5';
          when x"6"   => result(i+1) := '6';
          when x"7"   => result(i+1) := '7';
          when x"8"   => result(i+1) := '8';
          when x"9"   => result(i+1) := '9';
          when x"A"   => result(i+1) := 'A';
          when x"B"   => result(i+1) := 'B';
          when x"C"   => result(i+1) := 'C';
          when x"D"   => result(i+1) := 'D';
          when x"E"   => result(i+1) := 'E';
          when x"F"   => result(i+1) := 'F';
          when "ZZZZ" => result(i+1) := 'Z';
          when others => result(i+1) := 'X';
        end case;
      end loop;
      return result;
    end if;
  end function to_hstring;