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![Copyright © 2018 Pearson. All rights reserved. 1-18 Dynamic Type Binding • Dynamic Type Binding (JavaScript, Python, Ruby, PHP, and C# (limited)) • 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](https://image.slidesharecdn.com/pl12ch5-250514074513-ec7d8572/75/Theory-of-programming-language-chapter-5-18-2048.jpg)
Theory of programming language chapter 5
- 1. ISBN 0-321-49362-1 Chapter 5 Names,Bindings, and Scopes
- 2. Copyright © 2018Pearson. All rights reserved. 1-2 Chapter 5 Topics • Introduction • Names • Variables • The Concept of Binding • Scope • Scope and Lifetime • Referencing Environments • Named Constants
- 3. Copyright © 2018Pearson. All rights reserved. 1-3 Introduction • Imperative languages are abstractions of von Neumann architecture – Memory – Processor • Variables are characterized by attributes – To design a type, must consider scope, lifetime, type checking, initialization, and type compatibility
- 4. Copyright © 2018Pearson. All rights reserved. 1-4 Names • Design issues for names: – Are names case sensitive? – Are special words reserved words or keywords?
- 5. Copyright © 2018Pearson. All rights reserved. 1-5 Names (continued) • Length – If too short, they cannot be connotative – Language examples: • C99: no limit but only the first 63 are significant; also, external names are limited to a maximum of 31 • C# and Java: no limit, and all are significant • C++: no limit, but implementers often impose one
- 6. Copyright © 2018Pearson. All rights reserved. 1-6 Names (continued) • Special characters – PHP: all variable names must begin with dollar signs – Perl: all variable names begin with special characters, which specify the variable’s type – Ruby: variable names that begin with @ are instance variables; those that begin with @@ are class variables
- 7. Names (continued) • Casesensitivity – Disadvantage: readability (names that look alike are different) • Names in the C-based languages are case sensitive • Names in others are not • Worse in C++, Java, and C# because predefined names are mixed case (e.g. IndexOutOfBoundsException) Copyright © 2018 Pearson. All rights reserved. 1-7
- 8. Copyright © 2018Pearson. All rights reserved. 1-8 Names (continued) • Special words – An aid to readability; used to delimit or separate statement clauses - A keyword is a word that is special only in certain contexts – A reserved word is a special word that cannot be used as a user-defined name – Potential problem with reserved words: If there are too many, many collisions occur (e.g., COBOL has 300 reserved words!)
- 9. Copyright © 2018Pearson. All rights reserved. 1-9 Variables • A variable is an abstraction of a memory cell • Variables can be characterized as a six tuples of attributes: – Name – Address – Value – Type – Lifetime – Scope
- 10. Copyright © 2018Pearson. All rights reserved. 1-10 Variables Attributes • Name - not all variables have them • Address - the memory address with which it is associated – A variable may have different addresses at different times during execution – A variable may have different addresses at different places in a program – If two variable names can be used to access the same memory location, they are called aliases – Aliases are created via pointers, reference variables, C and C++ unions – Aliases are harmful to readability (program readers must remember all of them)
- 11. Copyright © 2018Pearson. All rights reserved. 1-11 Variables Attributes (continued) • Type - determines the range of values of variables and the set of operations that are defined for values of that type; in the case of floating point, type also determines the precision • Value - the contents of the location with which the variable is associated - The l-value of a variable is its address - The r-value of a variable is its value • Abstract memory cell - the physical cell or collection of cells associated with a variable
- 12. Copyright © 2018Pearson. All rights reserved. 1-12 The Concept of Binding A binding is an association between an entity and an attribute, such as between a variable and its type or value, or between an operation and a symbol • Binding time is the time at which a binding takes place.
- 13. Copyright © 2018Pearson. All rights reserved. 1-13 Possible Binding Times • Language design time -- bind operator symbols to operations • Language implementation time-- bind floating point type to a representation • Compile time -- bind a variable to a type in C or Java • Load time -- bind a C or C++ static variable to a memory cell) • Runtime -- bind a nonstatic local variable to a memory cell
- 14. Copyright © 2018Pearson. All rights reserved. 1-14 Static and Dynamic Binding • A binding is static if it first occurs before run time and remains unchanged throughout program execution. • A binding is dynamic if it first occurs during execution or can change during execution of the program
- 15. Copyright © 2018Pearson. All rights reserved. 1-15 Type Binding • 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
- 16. Copyright © 2018Pearson. All rights reserved. 1-16 Explicit/Implicit Declaration • An explicit declaration is a program statement used for declaring the types of variables • An implicit declaration is a default mechanism for specifying types of variables through default conventions, rather than declaration statements • Basic, Perl, Ruby, JavaScript, and PHP provide implicit declarations – Advantage: writability (a minor convenience) – Disadvantage: reliability (less trouble with Perl)
- 17. Explicit/Implicit Declaration (continued) •Some languages use type inferencing to determine types of variables (context) – C# - a variable can be declared with var and an initial value. The initial value sets the type – Visual Basic 9.0+, ML, Haskell, and F# use type inferencing. The context of the appearance of a variable determines its type Copyright © 2018 Pearson. All rights reserved. 1-17
- 18. Copyright © 2018Pearson. All rights reserved. 1-18 Dynamic Type Binding • Dynamic Type Binding (JavaScript, Python, Ruby, PHP, and C# (limited)) • 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
- 19. Copyright © 2018Pearson. All rights reserved. 1-19 Variable Attributes (continued) • Storage Bindings & Lifetime – Allocation - getting a cell from some pool of available cells – Deallocation - putting a cell back into the pool • The lifetime of a variable is the time during which it is bound to a particular memory cell
- 20. Copyright © 2018Pearson. All rights reserved. 1-20 Categories of Variables by Lifetimes • Static--bound to memory cells before execution begins and remains bound to the same memory cell throughout execution, e.g., C and C++ static variables in functions – Advantages: efficiency (direct addressing), history-sensitive subprogram support – Disadvantage: lack of flexibility (no recursion)
- 21. Copyright © 2018Pearson. All rights reserved. 1-21 Categories of Variables by Lifetimes • Stack-dynamic--Storage bindings are created for variables when their declaration statements are elaborated. (A declaration is elaborated when the executable code associated with it is executed) • If scalar, all attributes except address are statically bound – local variables in C subprograms (not declared static) and Java methods • Advantage: allows recursion; conserves storage • Disadvantages: – Overhead of allocation and deallocation – Subprograms cannot be history sensitive – Inefficient references (indirect addressing)
- 22. Copyright © 2018Pearson. All rights reserved. 1-22 Categories of Variables by Lifetimes • 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
- 23. Copyright © 2018Pearson. All rights reserved. 1-23 Categories of Variables by Lifetimes • Implicit heap-dynamic--Allocation and deallocation caused by assignment statements – all variables in APL; all strings and arrays in Perl, JavaScript, and PHP • Advantage: flexibility (generic code) • Disadvantages: – Inefficient, because all attributes are dynamic – Loss of error detection
- 24. Copyright © 2018Pearson. All rights reserved. 1-24 Variable Attributes: Scope • The scope of a variable is the range of statements over which it is visible • The local variables of a program unit are those that are declared in that unit • The nonlocal variables of a program unit are those that are visible in the unit but not declared there • Global variables are a special category of nonlocal variables • The scope rules of a language determine how references to names are associated with variables
- 25. Copyright © 2018Pearson. All rights reserved. 1-25 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 • Enclosing static scopes (to a specific scope) are called its static ancestors; the nearest static ancestor is called a static parent • Some languages allow nested subprogram definitions, which create nested static scopes (e.g., Ada, JavaScript, Common Lisp, Scheme, Fortran 2003+, F#, and Python)
- 26. Copyright © 2018Pearson. All rights reserved. 1-26 Scope (continued) • Variables can be hidden from a unit by having a "closer" variable with the same name • Ada allows access to “hidden” vars
- 27. Copyright © 2018Pearson. All rights reserved. 1-27 Blocks – A method of creating static scopes inside program units-- from ALGOL 60 – Example in C: void sub() { int count; while (...) { int count; count++; ... } … } - Note: legal in C and C++, but not in Java and C# - too error-prone
- 28. Declaration Order • C99,C++, Java, and C# allow variable declarations to appear anywhere a statement can appear – In C99, C++, and Java, the scope of all local variables is from the declaration to the end of the block – In the official documentation of C#, the scope of any variable declared in a block is the whole block, regardless of the position of the declaration in the block • However, that is misleading, because a variable still must be declared before it can be used Copyright © 2018 Pearson. All rights reserved. 1-28
- 29. Declaration Order (continued) •In C++, Java, and C#, variables can be declared in for statements – The scope of such variables is restricted to the for construct Copyright © 2018 Pearson. All rights reserved. 1-29
- 30. Global Scope • C,C++, PHP, and Python support a program structure that consists of a sequence of function definitions in a file – These languages allow variable declarations to appear outside function definitions • C and C++have both declarations (just attributes) and definitions (attributes and storage) – A declaration outside a function definition specifies that it is defined in another file Copyright © 2018 Pearson. All rights reserved. 1-30
- 31. Global Scope (continued) •PHP – Programs are embedded in HTML markup documents, in any number of fragments, some statements and some function definitions – The scope of a variable (implicitly) declared in a function is local to the function – The scope of a variable implicitly declared outside functions is from the declaration to the end of the program, but skips over any intervening functions • Global variables can be accessed in a function through the $GLOBALS array or by declaring it global Copyright © 2018 Pearson. All rights reserved. 1-31
- 32. Global Scope (continued) •Python – A global variable can be referenced in functions, but can be assigned in a function only if it has been declared to be global in the function Copyright © 2018 Pearson. All rights reserved. 1-32
- 33. Copyright © 2018Pearson. All rights reserved. 1-33 Evaluation of Static Scoping • Works well in many situations • Problems: – In most cases, too much access is possible – As a program evolves, the initial structure is destroyed and local variables often become global; subprograms also gravitate toward become global, rather than nested
- 34. Copyright © 2018Pearson. All rights reserved. 1-34 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 back through the chain of subprogram calls that forced execution to this point
- 35. Copyright © 2018Pearson. All rights reserved. 1-35 Scope Example • Evaluation of Dynamic Scoping: – Advantage: convenience – Disadvantages: 1. While a subprogram is executing, its variables are visible to all subprograms it calls 2. Impossible to statically type check 3. Poor readability- it is not possible to statically determine the type of a variable
- 36. Copyright © 2018Pearson. All rights reserved. 1-36 Scope and Lifetime • Scope and lifetime are sometimes closely related, but are different concepts • Consider a static variable in a C or C++ function
- 37. Copyright © 2018Pearson. All rights reserved. 1-37 Referencing Environments • The referencing environment of a statement is the collection of all names that are visible in the statement • In a static-scoped language, it is the local variables plus all of the visible variables in all of the enclosing scopes • A subprogram is active if its execution has begun but has not yet terminated • In a dynamic-scoped language, the referencing environment is the local variables plus all visible variables in all active subprograms
- 38. Copyright © 2018Pearson. All rights reserved. 1-38 Named Constants • A named constant is a variable that is bound to a value only when it is bound to storage • Advantages: readability and modifiability • Used to parameterize programs • The binding of values to named constants can be either static (called manifest constants) or dynamic • Languages: – C++ and Java: expressions of any kind, dynamically bound – C# has two kinds, readonly and const - the values of const named constants are bound at compile time - The values of readonly named constants are dynamically bound
- 39. Copyright © 2018Pearson. All rights reserved. 1-39 Summary • Case sensitivity and the relationship of names to special words represent design issues of names • Variables are characterized by the sextuples: name, address, value, type, lifetime, scope • Binding is the association of attributes with program entities • Scalar variables are categorized as: static, stack dynamic, explicit heap dynamic, implicit heap dynamic • Strong typing means detecting all type errors