CS 363 Comparative Programming Languages

1. CS 363 Comparative Programming Languages Names, Type Checking, and Scopes

2. Names • User-defined names include variables, functions, classes, types… • Design issues for names: • Maximum length? • Are connector characters (_,-,…) allowed? • Are names case sensitive? • Are special words reserved words or keywords? CS 363 Spring 2005 GMU

3. Names • Length • If too short, they cannot be connotative • Language examples: • FORTRAN I: maximum 6 • COBOL: maximum 30 • FORTRAN 90 and ANSI C: maximum 31 • Ada and Java: no limit, and all are significant • C++: no limit, but implementers often impose one CS 363 Spring 2005 GMU

4. Names • Case sensitivity • Disadvantage: readability (names that look alike are different) • In C++ /Java because predefined names are mixed case (e.g. IndexOutOfBoundsException) • C, C++, and Java names are case sensitive (b and B are different variables) • The names in some languages are not CS 363 Spring 2005 GMU

5. Names • Special words: keywords, reserved words • Ex: while, for, … • An aid to readability; used to delimit or separate statement clauses • Def: A keyword is a word that is special only in certain contexts • Disadvantage: poor readability, compiling • Def: A reserved word is a special word that cannot be used as a user-defined name CS 363 Spring 2005 GMU

6. Variables • A variable is an abstraction of a memory cell(s) • Variables can be characterized as a sextuple of attributes: (name, address, value, type, lifetime, and scope) • Not all variables have names (anonymous) CS 363 Spring 2005 GMU

7. Variables • Address - the memory address with which a variable is associated • A variable may have different addresses at different times during execution (variable local to a function) • A variable may have different addresses at different places in a program (variable name used in multiple scopes) • l-value of a variable (x := …) CS 363 Spring 2005 GMU

8. Variables • If two variable names can be used to access the same memory location, they are called aliases • Aliases are harmful to readability (program readers must remember all of them) • How aliases can be created: • Pointers, reference variables, C and C++ unions, (and through parameters - discussed in Chapter 9) • Some of the original justifications for aliases are no longer valid; e.g. memory reuse in FORTRAN • Replace them with dynamic allocation CS 363 Spring 2005 GMU

9. Variables • Type - determines the size of memory location, range of values of variables and the set of operations that are defined for values of that type, precision (floating point) • Value - the contents of the location with which the variable is associated • r-value of a variable (… := x …) CS 363 Spring 2005 GMU

10. Binding • A binding is an association, such as between an attribute and an entity, or between an operation and a symbol • Binding time is the time at which a binding takes place. CS 363 Spring 2005 GMU

11. Possible Binding Times • Language design time – e.g., operator symbols to operations • Language implementation time – e.g., bind floating point type to a representation • Compile time – e.g., bind a variable to a type • Load time – e.g., bind a FORTRAN 77 variable to a memory cell (or a C static variable) • Runtime – e.g., bind a local variable to a memory cell Different languages make different choices about binding times. CS 363 Spring 2005 GMU

12. The Concept of Binding • Def: A binding is static if it first occurs before run time and remains unchanged throughout program execution. • Def: A binding is dynamic if it first occurs during execution or can change during execution of the program. CS 363 Spring 2005 GMU

13. Overloading • More than one binding for a name in a given scope. • All languages offer limited overloading (+ for example) • Subroutine names (Ada, C++, Java) – differentiated by the arguments • Built-in Operators (Ada, C++, Fortran 90) CS 363 Spring 2005 GMU

14. Type Bindings • How is a type specified? • When does the binding take place? • If static, the type may be specified by either an explicit or an implicit declaration CS 363 Spring 2005 GMU

15. Types • Def: An explicit declaration is a program statement used for declaring the types of variables • Def: An implicit declaration is a default mechanism for specifying types of variables (the first appearance of the variable in the program) • FORTRAN, PL/I, BASIC, and Perl provide implicit declarations • Advantage: writability • Disadvantage: reliability (less trouble with Perl) CS 363 Spring 2005 GMU

16. Types • Dynamic Type Binding (JavaScript and PHP) • Specified through an assignment statement e.g., JavaScript list = [2, 4.33, 6, 8]; list = 17.3; • Advantage: flexibility (generic program units) • Disadvantages: • High cost (dynamic type checking and interpretation) • Type error detection by the compiler is difficult CS 363 Spring 2005 GMU

17. Types • Type Inferencing (ML, Miranda, and Haskell) • Rather than by assignment statement, types are determined from the context of the reference CS 363 Spring 2005 GMU

18. Type Checking • Generalize the concept of operands and operators to include subprograms and assignments • Def: Type checking is the activity of ensuring that the operands of an operator are of compatible types • Def: A compatible type is one that is either legal for the operator, or is allowed under language rules to be implicitly converted, by compiler- generated code, to a legal type. This automatic conversion is called a coercion. • Def: A type error is the application of an operator to an operand of an inappropriate type CS 363 Spring 2005 GMU

19. Type Checking • If all type bindings are static, nearly all type checking can be static • If type bindings are dynamic, type checking must be dynamic • Def: A programming language is strongly typed if type errors are always detected CS 363 Spring 2005 GMU

20. Strong Typing • Advantage of strong typing: allows the detection of the misuses of variables that result in type errors • What languages are strongly typed? • FORTRAN 77 is not: parameters, EQUIVALENCE • Pascal is not: variant records • C and C++ are not: parameter type checking can be avoided; unions are not type checked • Ada is, almost (UNCHECKED CONVERSION is explicit loophole) (Java is similar) CS 363 Spring 2005 GMU

21. Strong Typing • Coercion rules strongly affect strong typing--they can weaken it considerably (C++ versus Ada) • Although Java has just half the assignment coercions of C++, its strong typing is still far less effective than that of Ada CS 363 Spring 2005 GMU

22. Type Compatibility • Our concern is primarily for structured types • Def: Name type compatibility means the two variables have compatible types if they are in either the same declaration or in declarations that use the same type name • Easy to implement but highly restrictive: • Subranges of integer types are not compatible with integer types • Formal parameters must be the same type as their corresponding actual parameters (Pascal) CS 363 Spring 2005 GMU

23. Type Compatibility • Def: Structure type compatibilitymeans that two variables have compatible types if their types have identical structures • More flexible, but harder to implement CS 363 Spring 2005 GMU

24. Type Compatibility • Consider the problem of two structured types: • Are two record types compatible if they are structurally the same but use different field names? • Are two array types compatible if they are the same except that the subscripts are different? (e.g. [1..10] and [0..9]) • Are two enumeration types compatible if their components are spelled differently? • With structural type compatibility, you cannot differentiate between types of the same structure (e.g. different units of speed, both float) CS 363 Spring 2005 GMU

25. Type Compatibility • Language examples: • Pascal: usually structure, but in some cases name is used (formal parameters) • C: structure, except for records • Ada: restricted form of name • Derived types allow types with the same structure to be different • Anonymous types are all unique, even in: A, B : array (1..10) of INTEGER: CS 363 Spring 2005 GMU

26. Variable Lifetime • Storage Bindings & Lifetime • Allocation - getting a cell from some pool of available cells • Deallocation - putting a cell back into the pool • Def: The lifetime of a variable is the time during which it is bound to a particular memory cell • Lifetime dictated by the type of variable: static, stack, explicit heap, implicit heap. CS 363 Spring 2005 GMU

27. Lifetime Categories • Static--bound to memory cells before execution begins and remains bound to the same memory cell throughout execution. e.g. all FORTRAN 77 variables, C static variables • Advantages: efficiency (direct addressing), history-sensitive subprogram support • Disadvantage: lack of flexibility (no recursion) CS 363 Spring 2005 GMU

28. Lifetime Categories • Stack-dynamic--Storage bindings are created for variables when their declaration statements are elaborated. • If scalar, all attributes except address are statically bound e.g. local variables in C subprograms and Java methods • Advantage: allows recursion; conserves storage • Disadvantages: • Overhead of allocation and deallocation • Subprograms cannot be history sensitive • Inefficient references (indirect addressing) CS 363 Spring 2005 GMU

29. Lifetime Categories • Explicit heap-dynamic--Allocated and deallocated by explicit directives, specified by the programmer, which take effect during execution • Referenced only through pointers or references e.g. dynamic objects in C++ (via new and delete) all objects in Java • Advantage: provides for dynamic storage management • Disadvantage: inefficient and unreliable CS 363 Spring 2005 GMU

30. Lifetime Categories • Implicit heap-dynamic--Allocation and deallocation caused by assignment statements e.g. all variables in APL; all strings and arrays in Perl and JavaScript • Advantage: flexibility • Disadvantages: • Inefficient, because all attributes are dynamic • Loss of error detection CS 363 Spring 2005 GMU

31. Scope • Def: The scope of a variable declaration is the range of program statements over which it is visible • The scope rules of a language determine how references to names are associated with variables • The terms ‘scope’ and ‘name space’ are sometimes used interchangably. • Two approaches: static and dynamic CS 363 Spring 2005 GMU

32. Fortran 77 Name Space Global common block a Global scope holds procedure names and common block names. Procedures have local variables parameters, labels and can import common blocks common block b f1() variables parameters labels f2() variables parameters labels f3() variables parameters labels CS 363 Spring 2005 GMU

33. All objects (built-in and user-defined) reside in single global namespace ‘let’ expressions create nested lexical scopes Scheme Name Space Global map cons var 2 f1() f2() let let let CS 363 Spring 2005 GMU

34. Global scope holds variables and functions No function nesting Block level scope introduces variables and labels File level scope with static variables that are not visible outside the file (global otherwise) C Name Space Global a,b,c,d,. . . File scope static names w,x,y File scope static names x,y,z f2() f1() variables variables parameters labels f3() variables, param Block scope Block Scope variables labels Block scope CS 363 Spring 2005 GMU

35. Limited global name space with only public classes Fields and methods in a public class can be public  visible to classes in other packages Fields and methods in a class are visible to all classes in the same package unless declared private Class variables visible to all objects of the same class. Java Name Space Public Classes package p2 package p1 public class c1 fields: f1,f2 method: m1 locals method: m2 locals package p3 class c2 fields: f3 method: m3 CS 363 Spring 2005 GMU

36. Scope Understanding scope rules of a given language allows us to answer the following: • Where is a given variable visible? • What variables are visible at a given statement in the program? CS 363 Spring 2005 GMU

37. Static Scope • Based on program text • To connect a name reference to a variable, you (or the compiler) must find the declaration • Search process: search declarations, first locally, then in increasingly larger enclosing scopes, until one is found for the given name • A variable is local to a procedure if the declaration occurs in that procedure • A variable is nonlocal to a procedure if it is visible in the procedure but not declared there CS 363 Spring 2005 GMU

38. Scope • Variables can be hidden from a unit by having a "closer" variable with the same name • C++ and Ada allow access to these "hidden" variables • In Ada: unit.name • In C++: class_name::name CS 363 Spring 2005 GMU

39. Referencing Environments • Def: The referencing environment of a statement is the collection of all names that are visible to the statement • In a static-scoped language, it is the local variables plus all of the visible variables in all of the enclosing scopes CS 363 Spring 2005 GMU

40. Example: Pascal-like language Main Program main; a,b,c: real; procedure sub1(a: real); d: int; procedure sub2(c: int); d: real; body of sub2 procedure sub3(a:int) body of sub3 body of sub1 body of main sub1 sub2 sub3 CS 363 Spring 2005 GMU

41. Example Program main; a,b,c: real; procedure sub1(a: real); d: int; procedure sub2(c: int); d: real; body of sub2 procedure sub3(a:int) body of sub3 body of sub1 body of main Main has local variables a,b,c, and sub1 CS 363 Spring 2005 GMU

42. Example Program main; a,b,c: real; procedure sub1(a: real); d: int; procedure sub2(c: int); d: real; body of sub2 procedure sub3(a:int) body of sub3 body of sub1 body of main sub1 has local variables a,d, sub2 and sub3, as well as non-local variables b and c CS 363 Spring 2005 GMU

43. Example Program main; a,b,c: real; procedure sub1(a: real); d: int; procedure sub2(c: int); d: real; body of sub2 procedure sub3(a:int) body of sub3 body of sub1 body of main sub2 has local variables c,d and non-local variables a,b and sub1 (and potentially sub3 depending on the rules of the language) CS 363 Spring 2005 GMU

44. Example Program main; a,b,c: real; procedure sub1(a: real); d: int; procedure sub2(c: int); d: real; body of sub2 procedure sub3(a:int) body of sub3 body of sub1 body of main sub3 has local variable a and non-local variables b,c,d,sub2, and sub1 CS 363 Spring 2005 GMU

45. Static Scope • Advantages • Readability • Based on program text  can be evaluated by a compiler • Constant time implementation • Disadvantages: • Encourages global variables CS 363 Spring 2005 GMU

46. Dynamic Scope • Based on calling sequences of program units, not their textual layout (temporal versus spatial) • References to variables are connected to declarations by searching the chain of subprogram calls (runtime stack) that forced execution to this point CS 363 Spring 2005 GMU

47. Scope Example MAIN - declaration of x SUB1 - declaration of x - ... call SUB2 ... SUB2 ... - reference to x - ... ... call SUB1 … MAIN calls SUB1 SUB1 calls SUB2 SUB2 uses x Which x?? CS 363 Spring 2005 GMU

48. Scope Example MAIN - declaration of x SUB1 - declaration of x - ... call SUB2 ... SUB2 ... - reference to x - ... ... call SUB1 … MAIN calls SUB1 SUB1 calls SUB2 SUB2 uses x For static scoping, it is main’s x CS 363 Spring 2005 GMU

49. Scope Example • In a dynamic-scoped language, the referencing environment is the local variables plus all visible variables in all active subprograms. • A subprogram is active if its execution has begun but has not yet terminated. CS 363 Spring 2005 GMU

50. Scope Example MAIN - declaration of x SUB1 - declaration of x - ... call SUB2 ... SUB2 ... - reference to x - ... ... call SUB1 … MAIN (x) SUB1 (x) SUB2 MAIN calls SUB1 SUB1 calls SUB2 SUB2 uses x For dynamic scoping, it is sub1’s x CS 363 Spring 2005 GMU