Sequencers. Markus Roggenbach. 18. März 2004

Transcription

1 Sequencers Markus Roggenbach 18. März 2004

2 Programming Languages today

3 Programming Languages today 2 M.Broy, J.Siedersleben: Objektorientierte Programmierung und Softwareentwicklung, Informatik Spektrum, Februar Thesen: OO is the most popular paradigm OO does not serve the actual needs

4 Programming Languages today 3 S.Jähnichen, S.Herrmann: Was, bitte, bedeutet Objektorientierung? Informatik Spektrum, Juli (?) These: OO-concepts need to be changed/completed Dimensions: Programming language methodology infrastructure Problems: identity of objects referencing, aliasing, encapsulation inheritance modularisation Result: OO is a good basis for further development

5 Programming Languages today 4 Topic of the lecture Commands for Sequencing: Changing the (normal) control flow

6 Topic of the lecture 5 Content Jumps Exits Exceptions

7 Jumps (Goto)

8 Jumps 7 What is a jump? Commands up to now... C ::= skip V := E new (V) p(e1,..., Ek) begin C1;... ; Cn end if E then C else C case E of v1 : C1... vn: Cn end while E do C repeat C until B for i in L do C end

9 What is a jump? 8 now also... : C ::=... M: C -- label goto M -- jump ( use of label )

10 What is a jump? 9 An old debate... E.W. Dijkstra: Goto statement considered harmful, CACM 11, , The goto statement as it stands is just too primitive; it is too much an invitation to make a mess of one s program. D.E. Knuth: Structured Programming with goto statements, ACM Computing Surveys 6, , There are the occasions when the efficiency of the goto outweighs its harm to readability.

11 Jumps 10 Why is goto so bad? problems are entry/exit of a command jump from one block into another block (block: piece of program code that defines the scope of bindings) jumps out of (recursive) procedures programs with jumps are hard to understand (?)

12 Why is goto so bad? 11 Example: Entry/Exit if E1 then C1 else begin C2; goto L end; C3; while E2 do begin C4; L:C5 end;

13 Why is goto so bad? 12 Example: recursive procedure procedure print (num, width: Natural); begin if width <= 0 then goto L; if num >= 10 then print(num div 10, width-1); write(chr(ord( 0 ) + (num mod 10)) end;... print (n,w);... L:

14 Jumps 13 Design decisions in Programming Languages no goto: Modula-2, Java with goto: Language Label Algol 60, C, Ada identifier Fortran, Pascal integer constant PL/I label variables

15 Jumps 14 domestication in Pascal labels: need to be declared scope: enclosing block can not be passed as parameter, stored, or changed.

16 Jumps 15 Allowed labels to jump to: constants (no expressions!) jumps only into active command groups command groups have only one entry. command group: compound command, subprogram active: currently in execution

17 domestication in Pascal 16 procedure sub1; label 100; procedure sub2; begin... goto 100; end; { of sub2 } begin :... end; { of sub1 } Legal example in Pascal

18 Exits (exit, return, continue,... )

19 Exits 18 The idea Quitting a piece of program (ahead of normal schedule), can be encoded as a jump to the end of this piece of program. Advantages no jump back wards no problems with termination no jump over procedure/block borders all commands have one entry, but possibly several exits

20 Exits 19 Forms of exits Leaving loops exit Id [ when E ] (Ada) Return out of procedures/functions return [ E ] (Ada) Continue with the next loop cycle continue (C, C++, Java) Stop the whole program halt (Ada)

21 Exits 20 Example: Leaving a loop search: for m in Month loop for d in Day loop if matches (diary(m,d),i) then keydate:= (m, d); exit search; end if; end loop; end loop;

22 Exits 21 Example: Return out of a function function gcd (in i,j: Positive) return Positive is r: Positive; begin loop r := i mod j; if r=0 then return j; end if; i := j; j := r; end loop; end;

23 Exceptions

24 Exceptions 23 exceptional situations hardware/run-time system detects a situation which might need a special treatment. arithmetic overflow division by zero memory error incomplete input/output operation incomplete function definition (SML) out of range error of indices failed memory allocation...

25 exceptional situations 24 Treatment of exceptional situations Panic : stop the program

26 exceptional situations 24 Treatment of exceptional situations Panic : stop the program Do it Yourself

27 exceptional situations 24 Treatment of exceptional situations Panic : stop the program Do it Yourself (programmer provides a treatment provided she/he expected this)

28 Exceptions 25 Exceptions in Programming Languages Form: throw A when A then C activate (raise) treat (handle, catch) main questions: Where is the exception described? How is an exception assigned to its treatment? How to continue after the treatment?

29 Exceptions in Programming Languages 26 Further questions: Built-in exceptions? Is it possible to throw built-in exceptions? Hardware error as exceptions? How to define new exceptions? Standard-treatment of non-handled exceptions? Can exceptions be turned-off?

30 Exceptions 27 Exceptions in Ada Where is the exception described? Block/Unit end. begin -- Block/Unit exception when exception_name_1 => -- first exception handli when exception_name_2 => -- snd exception handli end;

31 Exceptions in Ada 28 How is an exception assigned to its treatment? exception x happens in block/unit B 1. B has handling for x local treatment 2. B hat no handling for x : propagate x Propagate: procedure point of call in the calling unit Unit calling unit (up to main program) Block calling Block...

32 Exceptions in Ada 29 Exception in declarations procedure River is current_flow : FLOAT := GET_FLOW;... begin... end River

33 Exceptions in Ada 30 How to continue after the treatment? Block/Unit, which activated exception x, is halted together with all units, to which x was propagated that could not treat x

34 Exceptions in Ada 31 Example: Repeat statements after exceptions type AGE_TYPE is range ; type AGE_LIST_TYPE is array (1..4) of AGE_TYPE; package AGE_IO is new INTEGER_IO (AGE_TYPE); use AGE_IO; AGE_LIST: AGE_LIST_TYPE;... begin

35 Exceptions in Ada 32 for AGE_COUNT in 1..4 loop loop EXECEPT_BLK: begin PUT_LINE( Enter an integer in the range ); GET(AGE_LIST(AGE_COUNT)); exit; exception when DATA_ERROR =>... when CONSTRAINT_ERROR =>.. end EXECTPT_BLK; end loop; end loop;

36 Exceptions in Ada 33 Now to the further questions Built-in exceptions CONSTRAINT ERROR, NUMERIC ERROR, PROGRAM ERROR, STORAGE ERROR, TASKING ERROR Throw built-in exceptions: raise exception name Defining new exceptions: exception name list: exception

37 Exceptions in Ada 34 Standard-treatment for non-handled exceptions: default handling for built-in exceptions: terminate the program Turning off exceptions: compiler directives (in the program) pragma SUPPRESS (check name)