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.