ISO Pascal reference manual

PROGRAM fail(output);
PROCEDURE try(FUNCTION x(n : integer):integer); BEGIN
writeln(x(123));
END;
BEGIN
try(PRED); (wrong}
try(SQR); (wrong}
END.
*ERROR 133 -- this function cannot execute an assignment
of the form: := result
All functions must contain at least one executable statement of the form:
functionid := expression;
in order that the result of the function shall be defined. The error message
indicates that either no such statement occurred at all, or that it is known to
the compiler that none can ever be executed.
PROGRAM fail;
CONST flag = 1;
VAR x : integer;
FUNCTION one:integer
BEGIN
IF flag = 0 THEN one := 1; [can never be executed}
END;
BEGIN
x := one;
END.
*ERROR 134 -- the assignment of a value to may occur only
within the definition of the function
The result of a function is assigned by a special form of the assignment
statement in which the name of the function appears on the left-hand side,
This form of assignment is only permitted within the definition of the
function, and will be faulted if it occurs in any other context.
92 Pascal Issue 1