VHDL initialize signed of variable length to maximum value

Question

I've got the following function that returns the value variable when it can be represented with the length given as a parameter. If value is out of limits, it should return the maximum possible value or the maximum negative value depending on the sign of value.

function truncate (
  value   : in signed;
  length  : in integer)
  return signed is
constant max_positive_value  : signed(length-1 downto 0) := ((length-1) => '0', others => '1');
constant max_negative_value  : signed(length-1 downto 0) := ((length-1) => '1', others => '0');
variable return_value        : signed(length-1 downto 0) := (others => '0');
begin
  if (value >= max_positive_value) then
    return_value := max_positive_value;
  elsif (value <= max_negative_value) then
    return_value := max_negative_value;
  else
    return_value := resize(value, length);
  end if;
  return return_value;
end;

The problem is the initialization of the max_positive_value and max_negative_value. GHDL complains about not static choice exclude others choice. How can I initialize the limit values if the length is variable? I'm using VHDL 93.

Answer 1

I found another way to do it that Modelsim accepts without a warning but length needs to be smaller than 32:

    constant max_positive_value  : signed(length-1 downto 0) := to_signed(2**(length-1)-1, length);
    constant max_negative_value  : signed(length-1 downto 0) := to_signed(-2**(length-1), length);