Programming Languages Tucker and Noonan

1. Programming LanguagesTucker and Noonan Chapter 7: Semantics 7.1 Motivation 7.2 Expression Semantics 7.3 Program State 7.4 Assignment Semantics 7.5 Control Flow Semantics 7.6 Input/Output Semantics 7.7 Exception Handling Semantics CSC321: Programming Languages

2. Motivation • To provide an authoritative definition of the meaning of all language constructs for: • Programmers • Compiler writers • Standards developers • A programming language is complete only when its syntax, type system, and semantics are well-defined. CSC321: Programming Languages

3. Semantics • Semantics is a precise definition of the meaning of a syntactically and type-wise correct program. • Three approaches • Operational semantics • The meaning attached by compiling using compiler C and executing using machine M. Ex: Fortran on IBM 709. • Direct and straightforward program meaning • Axiomatic semantics • Axiomatize the meaning of statements -- Chapter 12 • Exploration of formal properties of programs • Denotational semantics • Statements as state transforming functions • High-level mathematical precision of program meaning • This chapter uses an informal, operational model. CSC321: Programming Languages

4. Expression Semantics • Notation – Expression tree • Meaning • Mathematics: (a + b) - (c * d) • Polish prefix notation: • - + a b * c d • Preorder traversal • Polish postfix notation • a b + c d * - • Postorder traversal • Cambridge Polish: • (- (+ a b) (* c d)) • Operator precedes operands • Parentheses CSC321: Programming Languages

5. Infix Notation • Mathematics meaning • (a + b) - (c * d) • Uses associativity and precedence to disambiguate • Associativity of Operators Language+ - * / Unary - ** == != < ... C-likeLeft Right Left AdaLeft non non non FortranLeft Right Right Left • Meaning of a < x < b in C-like • a < x && x < b ? • If (a < x) 1 < b else 0 < b CSC321: Programming Languages

6. Precedence of Operators Operators C-like Ada Fortran Unary -7 3 3 ** 5 5 * / 6 4 4 + - 5 3 3 == != 4 2 2 < <= ... 3 2 2 Unary not 7 2 2 Logical and 2 1 1 Logical or 1 1 1 CSC321: Programming Languages

7. Short Circuit Evaluation • a and b evaluated as: if a then b else false • a or b evaluated as: if a then true else b • Benefits • Efficiency • Shorter code • Clearer code • Example in C-like • x < Math.pow(y, 3) || b – bad? • x<1 && (y*y > 100 || y>x) – good? CSC321: Programming Languages

8. More Example • Node p = head; while (p != null && p.info != key) p = p.next; • while (p != null && ! found) { if (p.info == key) break; else p = p.next; } // using break • boolean found = false; while (p != null && ! found) { if (p.info == key) found = true; else p = p.next; } // avoiding break CSC321: Programming Languages

9. Expression Meaning • Number precision • data type size • (a+b)+c == a+(b+c) ? • Side effect • A change to any non-local variable or I/O. • What is the value of: • i = 2; b = 2; c = 5; • x = b * i++ + c * i; // 19 • y = c * i + b * i++; // 14 • Consider • y = x++; • y = x+1; CSC321: Programming Languages

10. Program State • The state of a program is the collection of all active objects and their current values. • Two maps: • Active objects to specific memory locations • Active memory locations to specific values. • State = memory  environment • The current statement (portion of an abstract syntax tree) to be executed in a program is interpreted relative to the current state. • The individual steps that occur during a program run can be viewed as a series of state transformations. • For simplicity, ignore the memory location CSC321: Programming Languages

11. 1 void main ( ) { 2 int n, i, f; 3 n = 3; 4 i = 1; 5 f = 1; 6 while (i < n) { 7 i = i + 1; 8 f = f * i; 9 } 10 } n i f undef undef undef 3 undef undef 3 1 undef 3, 3, 3 1, 2, 3 1, 2, 6 3, 3 1, 2 1, 2 3, 3 2, 3 1, 2 3, 3 3, 3 2, 6 3 3 6 Program State Transformation Program: compute the factorial of n State Transformation CSC321: Programming Languages

12. Assignment Semantics • Issues • Multiple assignment • Example: • a = b = c = 0; • Sets all 3 variables to zero. • Problems??? • Assignment statement vs. expression • Copy vs. reference semantics CSC321: Programming Languages

13. Assignment Statement vs. Expression • In most languages, assignment is a statement; cannot appear in an expression. • In C-like languages, assignment is an expression. • Example: if (a = 0) ... // an error • while (*p++ = *q++) ; // strcpy • while (ch = getc(fp)) ... // ??? • while (p = p->next) ... // ??? CSC321: Programming Languages

14. Copy vs. Reference Semantics • Copy: a = b; • a, b have same value. • Changes to either have no effect on other. • Used in imperative languages. • Reference • a, b point to the same object. • A change in object state affects both • Used by many object-oriented languages. CSC321: Programming Languages

15. Control Flow Semantics • Turing complete • a programming language is Turing Complete if its program are capable of computing any computable function • To be complete, an imperative language needs: • Statement sequencing • Conditional statement • Looping statement CSC321: Programming Languages

16. Sequence • Format: s1 s2 • Semantics: in the absence of a branch: • First execute s1 • Then execute s2 • Output state of s1 is the input state of s2 CSC321: Programming Languages

17. Conditional • Format IfStatementif(Expression)Statement [ elseStatement ] • Semantics If the value of Expression is true, the meaning is the state resulting from evaluating Statement1 in the current state. Otherwise the meaning is the state resulting from evaluating Statement2 in the current state • Example: if (a > b) z = a; else z = b; CSC321: Programming Languages

18. Various Conditional Statements • if <E> then <S> : if <E> is true then execute <S> • if <E> then <S> else <S2>: if <E> is true then execute <S>, otherwise execute <S2> • Dangling-else: Ambiguity. Ways to avoid ambiguity • Syntax: the else-clause associated with the closest then – e.g. C, Java if <E1> then if <E2> then <S1> else <S2> • Use compound brackets • A: if <E1> then { if <E2> then <S1> } else <S2> • B: if <E1> then { if <E2> then <S1> else <S2> } • Avoid use of nesting in then parts • A: if !<E1> then <S2> else if <E2> then <S1> • B: if <E1> && <E2> then <S1> else if <E1> then <S2> CSC321: Programming Languages

19. Multiple Branches • Multiple branches – case (selection): use the value of an expression to select one of several substatements for execution • Pascal, Modula-2: case x of <value1>: <statements1> <value2>: <statements2> … [else: <statements>] end • Ada: case x is when <value1> => <statements1>; when <value2> => <statements2>; … [others => <statements>]; • C/C++, Java : Notice: the use of break switch (x) { case <value1>: <statements1>; [break;] case <value2>: <statements2>; [break;] … [default: <statements>]; } CSC321: Programming Languages

20. Multiple Branches • Points: • Cases can appear in any order, but else/others/default must be the last one • The case value of x is discrete, such as 1, 10, 6, 9 • The case values must be distinct • Some languages allow ranges of case values, such as in Modula-2, ‘0’..’9’ represents the value either ‘0’, or ‘1’, …, or ‘9’ of x • Case can be replaced with if—then—else statements if x == <value1> then <statements1>; else if x == <value2> then <statements2>; … else: <statements>; CSC321: Programming Languages

21. Loops • Format WhileStatementwhile(Expression)Statement • Semantics • The expression is evaluated. • If it is true, first the statement is executed, and then the loop is executed again. • Otherwise the loop terminates. CSC321: Programming Languages

22. S1 C S2 General Loop Structure • General structure: Do S1 If C exit Otherwise Do S2 Repeat CSC321: Programming Languages

23. Ada: S1 while (!C) loop S1 end loop C/C++, Java: do S1 while (C) Pascal: repeat S1 until C Perl: S1 while (!C) { S1 } Ada: while (!C) loop S2 end loop C/C++, Java: while (!C) { S2 } Pascal: while (!C) do begin S2 end Perl: unless (C) { S2 } Ada: loop S1 exit when C S2 end loop C/C++, Java: while (t) { S1 if (C) break; S2 } Pascal: S1; while (C) do begin S2 S1 end Loop Implementation General loop While loop Repeat loop CSC321: Programming Languages

24. For Loop Implementation • For statement: definite iteration – for each element do • General form: • for (<var> = <initial>; [<step>]; <termination>) do statements • <var> varies from the initial value to the termination, each loop varies with a stepwise • Step can be omitted, default value is 1 • Stepwise can be going either up from small initial to large termination, or down from large initial to small termination • Pascal: • for x := low to high do S • for x:= high downto low do S • Fortran/Algol: • for x:= start step increment until end do S • increment may be negative to implement “going down” • Ada: • for x in low..high loop S end loop; • for x in reverse low..high loop S end loop • C/C++/Java: • for (e1; e2; e3) S CSC321: Programming Languages

25. For Loop Implementation • Some languages allow the stepwise to be real or other data types • Fortran/Algol: allow real steps such as 0.1, 0.2 • Pascal/Modula-2: allow step type of enumeration • for x in d do S; where d is a type of enumeration • Perl: allows enumeration in a list • foreach $x (@list) S; • Java: allows enumeration in a collection • Syntax foreach (Type variable : collection) statement • Example String names[]; …… foreach (String name : names) do something with name; CSC321: Programming Languages

26. Jump Statements • Goto: jump to a specific statement (labeled by a label). Recall jmp, jp in assembly languages • makes the program difficult to trace, hard to prove • violates the structured programming style • destroy the system if goto the kernel of the OS • Break (C/C++, Java): limited use of Goto as an exit of a loop or selection • Used inside a loop: exit the loop • Used inside a selection (case): exit the case • Exit in Ada and Modula-2 • Continue(C/C++, Java, Fortran): limited use of Goto • Begin the next loop, ignore the statements after it in the body CSC321: Programming Languages

27. 7.6 Input/Output Semantics • Binding: open, close • Access: sequential vs. random • Stream vs. fixed length records • Character vs. binary • Format • I/O Error handling CSC321: Programming Languages

28. Standard Files • Unix: stdin, stdout, stderr • C: stdin, stdout, stderr • C++: cin, cout, cerr • Java: System.in, System.out, System.err CSC321: Programming Languages

29. Input/Output Streams • Fortran integer :: i, a(8) write(8,*) “Enter 8 integers: “ read(*,*) a write(*,*) a • Java • file, pipe, memory, url • filter • reader, writer CSC321: Programming Languages

30. Formats • C • Codes: d, e, f, c, s (decimal. float, float, char, string) • Specifier: % opt-width code • Ex: %s %5d %20s %8.2f • Fortran • Codes: i, f, a (integer, float, string) • Specifier: op-repeat code width • Ex: 8i4, f8.2, a20 CSC321: Programming Languages

31. Exception Handling Semantics • Exception • an error condition occurring from an operation that cannot be resolved by the operation itself • Purposes • Simplify programming • Make applications more robust: continue to operate under all concevable error situations CSC321: Programming Languages

32. <exception event> Automaticallyjump <exception handler> Exception Handling Basic Structure • Exception events: abnormal • event that the system can not deal with so that leave to the programmer, such as • division by zero, • get value from a null pointer; • read a file that doesn’t exist • Exception handlers • Deal with the exception events, such as • output error • use default value • stop the execution CSC321: Programming Languages

33. Exception Handling ProgramStructure • C++/Java: • try { • …. • } catch (<exception1>) { • exception1-handler; • } catch (<exception2>) { • exception2-handler; • } • … • [finally { • …. • }] Ada: begin … exception when <exception1> => exception1-handler; when <exception2> => exception2-handler; …. end CSC321: Programming Languages

34. Raise/Throw/Catch Exceptions • Raise/throw exception • Implicitly throw: built-in method/function throw, e.g. division, file access • Explicitly throw: • throw <exception> --- C++/Java • raise <exception> --- Ada • Catch exception • If there exists a catch clause to catch a specific exception, the exception is handled by the corresponding handler. If no handler exists, the exception is raised/thrown to the caller. To implement this, the throws must be specified in the method prototype (signature). E.g. in Java public void method_sample(…) throws <exception> { … throw <exception>; … } CSC321: Programming Languages

35. Throwing an Exception Figure 7.13 CSC321: Programming Languages

36. Exception Post-process • Termination model • C++/Java, Ada: only enter the exception handler and execute the finally clause, if any. • Resumption model • Eiffel: can use the retry statement to re-execute the procedure/function in which the exception occurred CSC321: Programming Languages

37. Exception Handling Figure 7.9 CSC321: Programming Languages

38. Declare Exceptions • Predefined exceptions(built-in exception) • User-defined exception: e.g. in Java Public class MyException extends RuntimeException { …… } Creating a New Exception Class CSC321: Programming Languages

39. Java Exception Type Hierarchy Figure 7.10 CSC321: Programming Languages

40. Missing Argument Exception CSC321: Programming Languages

41. Invalid Input Exception Figure 7.12 CSC321: Programming Languages

42. AssertException Class CSC321: Programming Languages

43. Assert Class Figure 7.15 CSC321: Programming Languages

44. Using Asserts CSC321: Programming Languages