operator and_then (operand1, operand2: Boolean) = Result: Boolean;
The and_then short-circuit logical operator performs the same operation as the logical operator and. But while the ISO standard does not specify anything about the evaluation of the operands of and – they may be evaluated in any order, or not at all – and_then has a well-defined behaviour: It evaluates the first operand. If the result is False, and_then returns False without evaluating the second operand. If it is True, the second operand is evaluated and returned.
Since the behaviour described above is the most efficient way to implement and, GPC by default treats and and and_then exactly the same. If you want, for some reason, to have both operands of and evaluated completely, you must assign both to temporary variables and then use and – or and_then, it does not matter.
and_then is an ISO 10206 Extended Pascal extension.
Some people think that the ISO standard requires both operands of and to be evaluated. This is false. What the ISO standard does say is that you cannot rely on a certain order of evaluation of the operands of and; in particular things like the following program can crash according to ISO Pascal, although they cannot crash when compiled with GNU Pascal running in default mode.
program AndBug;
var
p: ^Integer;
begin
New (p);
ReadLn (p^);
if (p <> nil) and (p^ < 42) then { This is NOT safe! }
WriteLn ('You''re lucky. But the test could have crashed ...')
end.
program And_ThenDemo;
var
p: ^Integer;
begin
New (p);
ReadLn (p^);
if (p <> nil) and_then (p^ < 42) then { This is safe. }
WriteLn (p^, ' is less than 42')
end.