VHDL: setting a constant conditionally based on another constant's value - vhdl

Question

I need to set a constant's value using an "if-else" or "case", and select a different constant value based on another constant's value. Is this possible in VHDL? It would be a one time change of constant values at the beginning of simulation... Example:
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
entity bridge is
generic (
burst_mode :std_logic := '0'
);
end entity;
architecture rtl of bridge is
constant fifo_aw_wdata :natural := 2 when (burst_mode = '0') else 5;
begin
-- fifo: entity work myfifo
-- generic map(
-- aw => fifo_aw_wdata
-- )
-- port map(
-- ...
-- );
end architecture;
The above VHDL code gives the error message:
Error ';' is expected instead of 'when'
In Verilog, Its very easy to do this type of thing...so I assume VHDL has a away to do this as well? Verilog example:
module #(parameter burst_mode = 1'b0) bridge;
localparam fifo_aw_wdata = (!burst_mode) ? 2 : 5;
// fifo myfifo #(.aw(fifo_aw_wdata)) ( ...);
endmodule;

Answer 1

This solution is a little bit strange, but it works:
architecture rtl of bridge is
function setup1(s:std_logic; i0:integer; i1:integer)
return integer is
begin
if s = '1' then
return i1;
else
return i0;
end if;
end function;
constant fifo_aw_wdata :natural := setup1(burst_mode, 2, 5);
begin
-- fifo: entity work myfifo
-- generic map(
-- aw => fifo_aw_wdata
-- )
-- port map(
-- ...
-- );
end architecture;

Answer 2

Correct answers have been posted, but there are also one-line alternatives.
The simplest solution is to change the datatype of burst_mode to integer with range 0 to 1, and then use some math:
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
entity bridge is
generic (
burst_mode :integer range 0 to 1 := 0
);
end entity;
architecture rtl of bridge is
constant fifo_aw_wdata :natural := 2 + burst_mode * 3;
begin
If the datatype of burst_mode cannot be changed, then you can also do it with a type conversion:
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
entity bridge is
generic (
burst_mode :std_logic := '0'
);
end entity;
architecture rtl of bridge is
constant fifo_aw_wdata :natural := 2 + to_integer(unsigned('0' & burst_mode)) * 3;
begin

Answer 3

Though pico posted an answer already, it is a very relevant question worthy of elaboration.
The declaration of the function may be similar to "conditional expressions" or "ternary if" in other languages like Python with res_true if cond else res_false or C with cond ? res_true : res_false.
The condition is then a boolean, and the result for true comes before the result for false. The declaration could be like:
function tif(cond : boolean; ref_true, ref_false : integer) return integer is
begin
if cond then
return res_true;
else
return res_false;
end if;
end function;
And having multiple functions with different result types, a version for real could also be defined like:
function tif(cond : boolean; res_true, res_false : real) return real is
begin
if cond then
return res_true;
else
return res_false;
end if;
end function;