1. Chapter 5 Variables: Names, Bindings, Type Checking and Scope

2. Introduction
This lecture introduces the fundamental semantic issues of variables
It covers the nature of names and special words in programming languages, attributes of variables, concepts of binding and binding times.
It investigates type checking, strong typing and type compatibility rules.
At the end it discusses named constraints and variable initialization techniques.

3. On Names Humans are the only species to have the concept of a name.
It’s given philosophers, writers and computer scientists lots to do for many years.
The logician Frege’s famous morning star and evening star example.
"What's in a name? That which we call a rose by any other word would smell as sweet." -- Romeo and Juliet, W. Shakespeare
The semantic web proposes using URLs as names for everything.
In programming languages, names are character strings used to refer to program entities – e.g., labels, procedures, parameters, storage locations, etc.

4. Names There are some basic design issues involving names:
Maximum length?
What characters are allowed?
Are names case sensitive?
Are special words reserved words or keywords?
Do names determine or suggest attributes

5. Names: length limitations?
Some examples:
FORTRAN I: maximum 6
COBOL: maximum 30
FORTRAN 90 and ANSI C: maximum 31
C++: no limit, but implementers often impose one
Java, Lisp, Prolog, Ada: no limit, and all are significant
The trend has been for programming languages to allow longer or unlimited length names
Is this always good?
What are some advantages and disadvantages of very long names?

6. Names: What characters are allowed?
Typical scheme:
Names must start with an alpha and contain a mixture of alpha, digits and connectors
Some languages (e.g., Lisp) are even more relaxed
Connectors:
Connectors might include underscore, hyphen, dot, …
Fortran 90 allowed spaces as connectors
Languages with infix operators typically only allow _, reserving ., -, + as operators
LISP: first-name
C: first_name
“Camel notation” is popular in C, Java, C#...
Using upper case characters to break up a long name, e.g. firstName

7. Names: case sensitivity
Foo = foo?
The first languages only had upper case (why?)
Case sensitivity was probably introduced by Unix and hence C (when?)
Disadvantage:
Poor readability, since names that look alike to a human are different; worse in Modula-2 because predefined names are mixed case (e.g. WriteCard)
Advantages:
Larger namespace, ability to use case to signify classes of variables (e.g., make constants be in uppercase)
C, C++, Java, and Modula-2 names are case sensitive but the names in many other languages are not

8. Special words
Def: A keyword is a word that is special only in certain contexts
Virtually all programming languages have keywords
Def: A reserved word is a special word that cannot be used as a user-defined name
Some PLs have reserved words and others do not
PLs try to minimize the number of reserved words
For languages which use keywords rather than reserved words
Disadvantage: poor readability thru possible confusion about the meaning of symbols
Advantage: flexibility – the programmer has fewer constraints on choice of names

9. Reserved words in C
Programming languages try to minimize the number of reserved words. Here are all of C’s reserved words.
auto
break
case
char
const
continue
default do
double
else
entry
signed
sizeof
static
struct
switch
typedef
union
unsigned
void
volatile
while
enum
extern
float
for
goto if
int
long
register
Return
short

10. Reserved words in Common Lisp
Programming languages try to minimize the number of reserved words. Here are all of Lisp’s reserved words.

11. Names: implied attributes
We often use conventions that associate attributes with name patterns.
Some of these are just style conventions while others are part of the language spec and are used by compilers

12. Examples of implied attributes
LISP: global variables begin and end with an asterisk, e.g.,
(if (> t *time-out-in-seconds*) (blue-screen))
JAVA: class names begin with an upper case character, field and method names with a lower case character
for(Student s : theClass) s.setGrade(“A”);
PERL: scalar variable names begin with a $, arrays and hashtables with %, subroutines begin with a &.
$d1 = “Monday”; $d2=“Wednesday”; $d3= “Friday”;
@days = ($d1, $d2, $d3);
FORTRAN: default variable type is float unless it begins with one of {I,J,K,L,M,N} in which case it’s integer
DO 100 I = 1, N
ISUM = ISUM + I

13. Variables A variable is an abstraction of a memory cell
Can represent complex data structures with lots of structure (e.g., records, arrays, objects, etc.)
current_student

14. Attributes of Variables
Variables can be characterized as a 6-tuple of attributes:
Name: identifier used to refer to the variable
Address: memory location(s) holding the variables value
Value: particular value at a moment
Type: range of possible values
Lifetime: when the variable can be accessed
Scope: where in the program it can be accessed

15. Variables: names and addresses
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 harmful to readability, but they are useful under certain circumstances

16. A variable which shall remain nameless
Many languages allow one to create new data structures with only a pointer to refer to them. This is like a variable that does not really have a name (“that thing over there”) , e.g.:
Int *intNode;
. . .
intNode = new int;
Delete intNode;
Some languages (e.g., shell scripts) assume you reference parameters not with names but by position (e.g., $1, $2)
Some languages don’t have named variables at all!
E.g., if every function takes exactly one argument, we can dispense with the variable name
Arguments that naturally take several arguments (e.g., +) can be reduced to functions that take only a single argument. More on this when we talk of lisp, maybe.

17. Aliases
When two names refer to the same memory location we can them aliases.
Aliases can be created in many ways, depend-ing on the language.
Pointers, reference variables, Pascal variant records, call by name, Prolog’s unification, C and C++ unions, and Fortran’s equivalence statemenr
Aliases can be trouble. (how?)
Some of the original justifications for aliases are no longer valid; e.g. memory reuse in FORTRAN
Aliases can be powerful.
Prolog’s unification offers new paradigms

18. Variables Type Typing is a rich subject in Computer Science
Most languages associate a variable with a single type
The type determines the range of values and the operations allowed for a variable
In the case of floating point, type usually also determines the precision (e.g., float vs. double)
In some languages (e.g., Lisp, Python, Prolog) a variable can take on values of any type.
OO languages (e.g. Java) have a few primitive types (int, float, char) and everything else is a pointer to an object, but the objects form a kind of user-defined type system

19. Functions have types too
Procedures or mehtods that return a value have a type, also: the type of the value returned
One of the most common way to describe a function is by its type signature
A type signature describes the types of each input and the type of the output
power: float × integer  float

20. Contrasts in Type Systems
Type systems are often described by their design decisions along several dimensions
Static vs. dynamic types
Strong vs. Weak typing
Explicit vs. implicit type conversion
Explicit vs. implicit type declarations
Although the dimensions appear to be binary choices, there are intermediate choices in many cases

21. Variable Value
The value is the contents of the memory location with which the variable is associated.
Think of an abstract memory location, rather than a physical one.
Abstract memory cell - the physical cell or collection of cells associated with a variable
A variable’s type will determine how the bits in the cell are interpreted to produce a value.
We sometimes talk about lvalues and rvalues.

22. lvalue and rvalue a := a + 1;
Are the two occurrences of “a” in this expression the same?
a := a + 1;
In a sense,
The one on the left of the assignment refers to the location of the variable whose name is a;
The one on the right of the assignment refers to the value of the variable whose name is a;
We sometimes speak of a variable’s lvalue and rvalue
The lvalue of a variable is its address
The rvalue of a variable is its value

23. Binding
Def: A binding is an association, such as between an attribute and an entity, or between an operation and a symbol
It’s like assignment, but more general
We often talk of binding
a variable to a value, as in
classSize is bound to the number of students
A symbol to an operator
+ is bound to the inner product operation
Def: Binding time is the time at which a binding takes place.

24. Possible binding times
Language design time, e.g., bind 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 in C or Java
Link time
Load time, e.g., bind a FORTRAN 77 variable to memory cell (or a C static variable)
Runtime, e.g., bind a nonstatic local variable to a memory cell

25. Type Bindings
Def: A binding is static if it occurs before run time and remains unchanged throughout program execution.
Def: A binding is dynamic if it occurs during execution or can change during execution of the program.
Type binding issues include:
How is a type specified?
When does the binding take place?
If static, type may be specified by either explicit or an implicit declarations

26. Declarations
Many languages require or allow the types of variables and functions to be declared
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)

27. Implicit Variable Declarations
Some examples of implicit type declarations
In C undeclared variables are assumed to be of type int
In Perl, variables of type scalar, array and hash begin with a or %, respectively.
Fortran variables beginning with I-N are assumed to be of type integer.
ML (and other languages) use sophisticated type inference mechanisms
Advantages and disadvantages
Advantages: writability, convenience
Disadvantages: reliability – requiring explicit type declarations catches bugs revealed by type mis-matches

28. Dynamic Type Binding
With dynamic binding, a variable’s type can change as the program runs and might be re-bound on every assignment.
Used in scripting languages (Javascript, PHP, Python) and some older languages (Lisp, Basic, Prolog, APL)
In this APL example LIST is first a vector of integers and then of floats:
LIST <
LIST <
Here’s a javascript example
list = [2, 4.33, 6, 8];
list = 17.3;

29. Dynamic Type Binding The advantages of dynamic typing include
Flexibility for the programmer
Obviates the need for “polymorphic” types
Development of generic functions (e.g. sort)
But there are disadvantages as well
Types have to be constantly checked at run time
A compiler can’t detect errors via type mis-matches
Mostly used by scripting languages today

30. Static Type Binding
In a static type system, types are fixed before the program is run (aka, compile time)
Compatibility checking can be done by a compiler and errors flagged
Some claim that most program errors are type errors
Another advantage is that the resulting code need not check for type mismatches at run time, which speeds up execution
It typically requires adding type declarations (a pain) but these can also be seen as a kind of documentation (a benefit)

31. Type Inferencing
Type inferencing is used in some programming languages, including ML, Miranda, and Haskell
Types are determined from the context of the reference, rather than just by assignment statement
The compiler can trace how values flow through variables and function arguments
The result is that the types of most variables can be deduced!
Any remaining ambiguity is treated as an error the programmer must fix by adding explicit declarations
Many feel it combines the advantages of dynamic typing and static typing

32. Type Inferencing in ML
fun circumf(r) = *r*r; // infer r is real
fun time10(x) = 10*x; // infer r is integer
fun square(x) = x*x; // can’t deduce types
// default type is int
We can explicitly type in several ways and enable the compiler to deduce that the function returns a real and takes a real argument
fun square(x):real = x*x;
fun square(x:real) = x*x;
fun square(x) = x:real*x;
fun square(x) = x*x:real;

33. Duck Typing A kind of dynamic typing typified by Python ad Ruby
An object's current set of methods and properties determines the valid semantics, rather than its inheritance from a particular class
If it walks like a duck and quacks like a duck, I would call it a duck.

34. Duck Typing example
def calculate(a, b, c): return (a+b)*c a = calculate(1, 2, 3) b = calculate([1, 2, 3], [4, 5, 6], 2) c = calculate('apples ’,'and oranges,', 3) print ‘a is’, a print ‘b is’, b print ‘c is’, c

35. Storage Bindings and 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
Categories of variables by lifetimes
Static
Stack dynamic
Explicit heap dynamic
Implicit heap dynamic

36. Static Variables
Static variables are bound to memory cells before execution begins and remains bound to the same memory cell throughout execution.
Examples:
All FORTRAN 77 variables
C static variables
Advantage: efficiency (direct addressing), history-sensitive subprogram support
Disadvantage: lack of flexibility, no recursion if this is the *only* kind of variable, as was the case in Fortran

37. Static Dynamic Variables
Stack-dynamic variables -- 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 Pascal and C subprograms
Advantages:
allows recursion
conserves storage
Disadvantages:
Overhead of allocation and deallocation
Subprograms cannot be history sensitive
Inefficient references (indirect addressing)

38. Explicit heap-dynamic
Explicit heap-dynamic variables are 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
Example:
int *intnode; intnode = new int; delete intnode;

39. Implicit heap-dynamic
Implicit heap-dynamic variables -- Allocation and deallocation caused by assignment statements and types not determined until assignment.
e.g. all variables in APL
Advantage:
flexibility
Disadvantages:
Inefficient, because all attributes are dynamic
Loss of error detection

40. 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
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.
A type error is the application of an operator to an operand of an inappropriate type
Note:
If all type bindings are static, nearly all checking can be static
If type bindings are dynamic, type checking must be dynamic

41. Strong vs. Weak Typing
We often categorize a programming languages into two classes:
Strongly typed
Weakly typed
based on their system of assigning types to variables and functions
The notions of strong and weak typing do not have consensus definitions, however

42. Strong Typing Features
A programming language is strongly typed if
type errors are always detected
There is strict enforcement of type rules with no exceptions.
All types are known at compile time, i.e. are statically bound.
With variables that can store values of more than one type, incorrect type usage can be detected at run-time.
Strong typing catches more errors at compile time than weak typing, resulting in fewer run-time exceptions.

43. Which languages have strong typing?
Fortran 77 isn’t because it doesn’t check parameters and because of variable equivalence statements.
The languages Ada, Java, and Haskell are strongly typed.
Pascal is (almost) strongly typed, but variant records screw it up
C and C++ are sometimes described as strongly typed, but are perhaps better described as weakly typed because parameter type checking can be avoided and unions are not type checked
Coercion rules strongly affect strong typing—they can weaken it considerably (C++ versus Ada)
See

44. Weak typing and coersion
Doing a lot of implicit (automatic) coersion weakens the type system
Most languages do it to some degree
X = 1
Y = 2.0
X + Y
But overuse can cause problems
Y = “2”

45. Weak typing and coercion
Doing a lot of implicit (automatic) coercion weakens the type system
Most languages do it to some degree
X = 1
Y = 2.0
X + Y
But overuse can cause problems
Y = “2”
Many languages will produce the float 3.0 for X+Y
X+Y is 3 in Visual Basic and “12” in javascript

46. What about Scheme and Python?
People argue about whether that are strongly or weakly typed
Partly it’s because the terms do not have a clear consensus definition and partly out of confusion and partly a result of conflating the issue with static vs. dynamic typing
Polymorphism and operator overloading obscure the judgment
I’m with the camp that describes both as strongly typed

47. Type Compatibility
Type compatibility by name means 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 aren’t compatible with integer types
Formal parameters must be the same type as their corresponding actual parameters (Pascal)
Type compatibility by structure means that two variables have compatible types if their types have identical structures
More flexible, but harder to implement

48. Subtypes and ranges
Some languages such as Ada make it easy to define subtypes, as in
subtype DAY_NUMBER_T is integer range 1..31;
subtype NATURAL is INTEGER range 0..INTEGER'LAST;
subtype POSITIVE is INTEGER range 1..INTEGER'LAST;
Pascal made good use of integer ranges
type a = ;
b = ;
c = ;

49. Consider the problem of two structured types
Type Compatibility
Consider the problem of two structured types
Suppose they are circularly defined
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 [-5..4])
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)

50. 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:

51. Type Safety
A programming language is type safe if the only operations that are performed on data in the language are those sanctioned by the type of the data
i.e., no type errors!
The checking can be done at compile time or run time
C is not type safe
Standard ML has been proven to be type safe
Haskell is thought to be type safe if you don’t use some features (type punning)

52. Variable Scope
The scope of a variable is the range of statements in a program over which it’s visible
Typical cases:
Explicitly declared => local variables
Explicitly passed to a subprogram => parameters
The nonlocal variables of a program unit are those that are visible but not declared.
Global variables => visible everywhere.
The scope rules of a language determine how references to names are associated with variables.
The two major schemes are static scoping and dynamic scoping

53. Static Scope Aka “lexical scope”
Based on program text and can be determined prior to execution (e.g., at compile time)
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

54. Blocks
A block is a section of code in which local variables are allocated/deallocated at the start/end of the block.
Provides a method of creating static scopes inside program units
Introduced by ALGOL 60 and found in most PLs.
Variables can be hidden from a unit by having a "closer" variable with same name
C++ and Ada allow access to these "hidden" variables

55. Examples of Blocks C and C++: Common Lisp: for (...) { int index;}
Ada:
declare LCL:FLOAT;
begin
end
Common Lisp:
(let ((a 1)
(b foo)
(c))
(setq a (* a a))
(bar a b c))

56. Static scoping example
MAIN A C D B E
MAIN
A B
C D E
MAIN calls A and B
A calls C and D
B calls A and E
MAIN MAIN
A B A B
C D E C D E

57. Evaluation of Static Scoping
Suppose the spec is changed so that D must now
access some data in B
Solutions:
1. Put D in B (but then C can no longer call it and D cannot access A's variables)
2. Move the data from B that D needs to MAIN (but then all procedures can access them)
Same problem for procedure access!
Overall: static scoping often encourages many globals

58. 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
Used in APL, Snobol and LISP
Note that these languages were all (initially) implemented as interpreters rather than compilers.
Consensus is that PLs with dynamic scoping leads to programs which are difficult to read and maintain.
Lisp switch to using static scoping as it’s default circa 1980, though dynamic scoping is still possible as an option.

59. Static vs. dynamic scope
MAIN calls SUB1 SUB1 calls SUB2 SUB2 uses x
Define MAIN
declare x
Define SUB1
declare x
call SUB2
...
Define SUB2
reference x
call SUB1
Static scoping - reference to x is to MAIN's x
Dynamic scoping - reference to x is to SUB1's x

60. Dynamic Scoping Evaluation of Dynamic Scoping: Advantage: convenience
Disadvantage: poor readability

61. Scope vs. Lifetime
While these two issues seem related, they can differ
In Pascal, the scope of a local variable and the lifetime of a local variable seem the same
In C/C++, a local variable in a function might be declared static but its lifetime extends over the entire execution of the program and therefore, even though it is inaccessible, it is still in memory

62. 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, that is the local variables plus all of the visible variables in all of the enclosing scopes. See book example (p. 184)
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.

63. Named Constants
A named constant is a variable that is bound to a value only when it is bound to storage.
The value of a named constant can’t be changed while the program is running.
The binding of values to named constants can be
either static (called manifest constants) or dynamic
Languages:
Pascal: literals only
Modula-2 and FORTRAN 90: constant-valued expressions
Ada, C++, and Java: expressions of any kind
Advantages: increased readability and modifiability without loss of efficiency

64. Example in Pascal Procedure example; type a1[1..100] of integer;
a2[1..100] of real;
...
begin
for I := 1 to 100 do
begin ... end;
for j := 1 to 100 do
avg = sum div 100;
Procedure example;
type const MAX 100;
a1[1..MAX] of integer;
a2[1..MAX] of real;
...
begin
for I := 1 to MAX do
begin ... end;
for j := 1 to MAX do
avg = sum div MAX;

65. Variable Initialization
For convenience, variable initialization can occur prior to execution
FORTRAN: Integer Sum Data Sum /0/
Ada: Sum : Integer := 0;
ALGOL 68: int first := 10;
Java: int num = 5;
LISP (Let (x y (z 10) (sum 0) ) ... )

66. Summary In this chapter, we see the following concepts being described
Variable Naming, Aliases
Binding and Lifetimes
Type variables
Scoping
Referencing environments
Named Constants
Type Compatibility Rules