1. Names, Bindings, Type Checking, and Scopes
Chapter 5
Names, Bindings, Type Checking, and Scopes

2. Introduction
High-level languages provide an abstration level that separates language features from the hardware
Imperative languages are abstractions of von Neumann architecture
A machine consists of
Memory - stores both data and instructions
Processor - can modify the contents of memory
Mention abstract memory cells
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3. Names A name is a character string used to represent something else
need a way to refer to variables, functions, user- defined types, labeled statements, …
Design issues for names:
Maximum length?
What characters are allowed?
Are names case sensitive?
C, C++, and Java names are case sensitive
this is not true of other languages
Are special words reserved words or keywords?
case sensitivity => readability issues
functions provide another level of abstraction
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4. Length of Names 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
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5. Keywords vs Reserved Words
Words that have a special meaning in the language
A keyword is a word that is special only in certain contexts, e.g., in Fortran
Real VarName (Real is a data type followed with a name, therefore Real is a keyword)‏
Real = 3.4 (Real is a variable)
A reserved word is a special word that cannot be used as a user-defined name
most reserved words are also keywords
keywords separate syntactic entities; aid readability
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6. Variables A variable is an abstraction of a memory cell
For primitive types correspondence is direct
For structured types (objects, arrays) things are more complicated
Variables can be characterized as a sextuple of attributes:
Name
Address
Value
Type
Lifetime
Scope
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7. Variable Attributes Name - not all variables have them
Address - the memory address with which it is associated (l-value)‏
Type - allowed range of values of variables and the set of defined operations
Value - the contents of the location with which the variable is associated (r- value)
Stack variables have different addresses at different times
Two names used to access the same variable are aliases
- Created using pointers, reference variables, unions, EQUIVALENCE
- harmful to readability
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8. The Concept of Binding
A binding is an association between a name and the "object" it is bound to
e.g. between an attribute and an entity, or between an operation and a symbol
entity could be a variable or a function or even a class
Binding time is the time at which a binding takes place.
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9. Possible Binding Times
Language design time -- bind operator symbols to operations
Language implementation time-- bind floating point type to a representation, I/O coupling
Program-writing time --
Compile time -- bind a variable to a type in C or Java
Link time --
Load time -- bind a FORTRAN 77 variable to a memory cell (or a C static variable)‏
Runtime -- bind a nonstatic local variable to a memory cell
binding primitive type to representation happens at language design time in Java
scripting languages bind type at run time
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10. Static and Dynamic Binding
A binding is static if it first occurs before run time and remains unchanged throughout program execution.
early binding is more efficient
A binding is dynamic if it first occurs during execution or can change during execution of the program
late binding provides flexibility
Give some examples of static and dynamic bindings in Java
compiled languages usually have static type binding
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11. Type Binding How is a type specified?
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 (the first appearance of the variable in the program)‏
When does the binding take place?
If static, the type may be specified by either an explicit or an implicit declaration
Java uses explicit declarations
scripting languages use implicit declaration (Scheme, ruby)‏
mention FORTRAN - name implies type
Perl - % imply scalar, array, hash
Type inference: ML, Haskell, Miranda
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12. Type Checking
Generalize the concept of operands and operators to include subprograms and assignments
Type checking is the activity of ensuring that the operands of an operator are of compatible types
insures that a program obeys type-compatibility rules
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
A type error is the application of an operator to an operand of an inappropriate type
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13. When is type checking done?
If all type bindings are static, nearly all type checking can be static (done at compile time)‏
If type bindings are dynamic, type checking must be dynamic (done at run time)‏
A programming language is strongly typed if type errors are always detected
Advantage: allows the detection of misuse of variables that result in type errors
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14. How strongly typed? FORTRAN 77 is not: parameters, EQUIVALENCE
Pascal has variant records
C and C++
parameter type checking can be avoided
unions are not type checked
Ada is almost
UNCHECKED CONVERSION is loophole
Java is similar
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15. Coercion The automatic conversion between types is called coercion.
Coercion rules they can weaken typing considerably
C and C++ allow both widening and narrowing coercions
Java allows only widening coercions
Java's strong typing is still far less effective than that of Ada
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16. Dynamic Type Binding
Dynamic Type Binding (Perl, JavaScript and PHP, Scheme)‏
Specified through an assignment statement e.g., JavaScript
list = [2, 4.33, 6, 8];
list = 17.3;
This provides a lot of flexibility
But …
How do you do any type checking?
Most functional languages and scripting languages; interpreters bind everything dynamically
Advantage: flexibility (generic program units)‏
Disadvantages:
High cost (dynamic type checking and interpretation)‏
Type error detection by the compiler is difficult
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17. Storage Bindings & Lifetime
The lifetime of a variable is the time during which it is bound to a particular memory cell
Allocation - getting a cell from some pool of available cells
Deallocation - putting a cell back into the pool
The object a variable is bound to may have a different lifetime than the variable itself
Depending on the language, allocation can be either controlled by the programmer or done automatically
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18. Categories of Variables by Lifetimes
Static -- lifetime is same as that of the program
Stack -- lifetime is duration of subprogram
more efficient use of memory than static
Heap -- lifetime is less predictable
Explicit allocation and deallocation specified by the programmer
Implicit -- Allocation and deallocation managed by run-time system
1: Fortran, C static; efficient; no recursion
2: Local variables in C, Java; efficient memory usage, recursion possible; overhead for allocation, deallocation, indirect addressing
3: Pointer variables in C, objects in Java; allows programmer control; error prone
4: Strings and arrays in Perl, JavaScript; flexible; inefficient
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19. Variable Scope
The scope of a variable is the range of statements over which it is visible
The nonlocal variables of a program unit are those that are visible but not declared there
The scope rules of a language determine how references to names are associated with variables
Scope and lifetime are sometimes closely related, but are different concepts
Class and instance variables in Java
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20. Approaches to Scope Only allow one global scope (Basic)‏
Allow global and local Scope (Fortran)‏
Allow nested scopes (Ada, Pascal)‏
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21. The current referencing environment is the set of currently active bindings at a particular point in the program
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22. Static Scope
The scope of a variable can be determined from the 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
Block scope
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23. Scope and Shadowed Variables
Variables can be hidden from a unit by having a "closer" variable with the same name
Sometimes say there is a hole in the scope
scope resolution operator (in some languages)‏
C++, Java and Ada allow access to some of these "hidden" variables
In Ada: unit.name
In C++: class_name::name or ::name for globals
In Java: this.name for variables declared at class level
Namespaces
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24. Static Scoping and Nested Functions
Assume MAIN calls A and B
A calls C and D
B calls A and E
MAIN E A C D B
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25. Nested Functions and Maintainability
Suppose the spec is changed so that D must now access some data in B
Solutions:
Put D in B (but then C can no longer call it and D cannot access A's variables)‏
Move the data from B that D needs to MAIN (but then all procedures can access them)‏
Same problem for procedure access
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26. Comments
Using nested functions with static scoping often encourages many globals
not considered to be good programming practice
Current thinking is that we can accomplish the same thing better with modules (classes)‏
Static scoping is still preferred to the alternative
Most current languages use static scoping at the block level even if they don't allow nested functions
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27. Nested Functions in Practice
Scheme, JavaScript, Ada and PHP allow you to nest functions
Java allows you to nest classes
scoping rules are similar to those of nested functions
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28. 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
This is easy to implement but it makes programs hard to follow
Used in APL, SNOBOL, early versions of LISP
Perl allows you to use dynamic scope for selected variables
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29. Scope Example MAIN calls SUB1 SUB1 calls SUB2 SUB2 uses x 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
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30. Scope Example Static scoping Dynamic scoping
Reference to x is to MAIN's x
Dynamic scoping
Reference to x is to SUB1's x
Evaluation of Dynamic Scoping:
Advantage: convenience
Disadvantage: poor readability
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31. 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
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
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