For Loop
Sequential Statement ----used in ----> Process
Function
Procedure
Syntax

optional_label: for parameter in range loop
	sequential statements
end loop label;

See LRM section 8.8
Rules and Examples
The for loop defines a loop parameter which takes on the type of the range specified. For example, the range 0 to 3 implies an integer: 
process (A)
begin
	Z <= "0000";
	for I in o to 3 loop
		if (A = I) then
			Z(I) <= '1';
		end if;
	end loop;
end process;

Attributes such as 'high, 'low and 'range may also be used to define the iterations of a for loop: 
process (A)#
	variable TMP : std_ulogic;
begin
	TMP := '0';
	for I in A'low to A'high loop
		TMP := TMP xor A(I);
	end loop;
	ODD <= TMP;
end process;

The range may be any discrete range, e.g. an enumerated type: 
type PRIMARY is (RED, GREEN, BLUE);
type COLOUR is ARRAY (PRIMARY) of integer range 0 to 255;
-- other statements
MUX: process
begin
	for SEL in PRIMARY loop
		V_BUS <= VIDEO(SEL);
		wait for 10 ns;
	end loop;
end process MUX;

The loop parameter does not need to be declared: it is implicitly declared within the loop. It may not be modified within the loop: 
for I in 1 to 10 loop
	if (REPEAT = '1') then
		I := I-1;	-- Illegal
	end if;
end loop;

Synthesis Issues
The for loop is supported for synthesis, providing:
  1. the loop range is static (i.e. implies a definite number of iterations), and
  2. the loop contains no wait statements.

Whats New in '93

The for ... loop statement has not changed in VHDL-93