KETTERING@SPIKE.llnl.gov (Brett 'Volleyball Is My Game' Kettering) (04/11/90)
Erland Sommarskog - ENEA Data, Stockholm - sommar@enea.se writes: > Some years ago . . . rewrite the package so that PROCESS_ONE_ELEMENT should > take a second parameter, CONTINUE : out BOOLEAN. The problem with this is > that if the user in 90% of the time wants to iterate through the entire set, > this second parameter just clutters up his code . . . I don't think this clutters the code at all, it's a single assignment. > . . . he may even forget to initiate it with interesting. The compiler will require at least one assignment to a variable that is an 'out' parameter. This does not ensure that the code follows the thread of execution where this assignment occurs, so the code may not react as anticipated if there exists a thread of execution where an assignment is not made. That's why programmers are hired. > So I went for the other solution and simply wrote the invoking procedure as: > > DECLARE > done : EXCEPTION; > PROCEDURE Test_One_Element (E : IN My_Element_Subtype) is > BEGIN > -- some stuff > IF Found THEN > RAISE Done; > END IF; > END Test_One_Element; > > PROCEDURE Find_First_Element IS NEW > My_Set_Package.Process_Each_Element > (Process_One_Element => Test_One_Element); > BEGIN > Find_First_Element (S); > EXCEPTION > WHEN Done => NULL; > END; This, of course, assumes that MY_SET_PACKAGE.PROCESS_EACH_ELEMENT does not trap this exception and convert it to something else. Correctly the package MY_SET_PACKAGE should export an exception DONE, instructing the user to raise it in PROCESS_ONE_ELEMENT if he wishes to exit the iterate. This would then require the procedure TEST_ONE_ELEMENT to do a "raise MY_SET_PACKAGE.DONE". While this is possible, and I've even done it, I think it is not an elegant solution. I think exceptions are something that the package uses to notify the user of a condition, not vice-versa. Furthermore, I believe that a simple assignment is much easier code to generate and faster than the handling of an exception. Finally, the inclusion of the CONTINUE parameter makes it very obvious to the user how to stop the iteration and its use is virtually self-explanatory. Brett Kettering KETTERING%SPIKE@ICDC.llnl.gov