1. Representation, Syntax, Paradigms, Types
Formal Syntax
Paradigms
Data Types
Type Inference
CSE S. Tanimoto Syntax and Types

2. CSE 341 -- S. Tanimoto Syntax and Types
General Issues
Representation (of data, of computation)
Form (Syntax)
Meaning (Semantics)
Paradigm (General way of thinking)
Naming (names, name spaces, bindings, locality)
Functionality (numeric, data manip, I/O, communication, synchronization, security)
Correctness (types, exception handling, error checking, bug avoidance)
CSE S. Tanimoto Syntax and Types

3. CSE 341 -- S. Tanimoto Syntax and Types
Syntax: The grammatical form of programs.
vs Semantics: The meaning of the program
Syntax (of textual languages) is typically specified by production rules for a context-free grammar using Backus-Naur Form (BNF) or Extended BNF (EBNF)
In visual languages, syntax is described by a set of restrictions on how diagrams may be constructed. (e.g., connection constraints)
CSE S. Tanimoto Syntax and Types

4. CSE 341 -- S. Tanimoto Syntax and Types
Syntactic Components
Identifiers and reserved words
Numeric constants
Parentheses, braces and brackets
Expressions
Statements
CSE S. Tanimoto Syntax and Types

5. BNF (Backus-Naur Form)
(2.0 * PI) / n
<expression> ::= <expression> + <term>
| <expression> - <term>
| <term>
<term> ::= <term> * <factor>
| <term> / <factor>
| <factor>
<factor> ::= number
| name
| ( <expression> )
CSE S. Tanimoto Syntax and Types

6. CSE 341 -- S. Tanimoto Syntax and Types
Extended BNF
Optional constructs written as [ x ]
Zero or more of x written as { x }
Choice (“or”) written using |
Grouping with parentheses ( x | y ) as in { (x | y ) z }
<expression> ::= <term> { (+ | -) <term> }
<term> ::= <factor> { (* | /) <factor> }
<factor> ::= ’(’ <expression> ’)’ | name | number
CSE S. Tanimoto Syntax and Types

7. CSE 341 -- S. Tanimoto Syntax and Types
Derivation
E ::= E + T | E - T | T
T ::= T * F | T / F | F
F ::= number | name | ( E ) E
E + T
T + T
T / F + T
F / F + T
F / F + F
25 / F + F
25 / F
25 / total
CSE S. Tanimoto Syntax and Types

8. Representation of Data
Constants, Variables
Types, classes
Compounds: arrays, structures.
Non-numeric objects: strings, images, audio.
Values vs references
Machine dependencies: word size, addressing resolution.
In C, characters and booleans are actually integers.
CSE S. Tanimoto Syntax and Types

9. Representation of Process
Arithmetic and logical expressions
Conditional expressions
Loops
Recursive and nonrecursive functions
Multiple threads of control, forking, joining, synchronizing
Single-threaded Parallel processing (in Single-instruction stream/multiple data stream processors)
Throwing and catching of exceptions
Declaration of constraints and rules
CSE S. Tanimoto Syntax and Types

10. CSE 341 -- S. Tanimoto Syntax and Types
Paradigm
General style of thinking that underlies a programming language
Webster’s New World Dictionary: “a pattern, example, or model”.
Imperative Rule-based
Functional Logic
Object-oriented Visual data-flow
CSE S. Tanimoto Syntax and Types

11. The Imperative Paradigm
An imperative program is a sequence of commands
Read a value from a file.
Evaluate an arithmetic expression.
Assign a value to a variable.
Test a condition and branch if it is true.
Iterate a loop body until a condition is false.
Print a value onto the screen.
CSE S. Tanimoto Syntax and Types

12. The Functional Paradigm
An functional program is a collection of function definitions and function applications.
Define SQR(x): {
Apply the * function to x and x}
Apply SQR to 7;
CSE S. Tanimoto Syntax and Types

13. The Object-Oriented Paradigm
An object-oriented program is a collection of object class definitions, in which both data members and methods are specified.
Class Student extends Person {
int student_number;
int get_student_number() { return student_number; }
int set_student_number (int num) {
student_number = num; }
CSE S. Tanimoto Syntax and Types

14. The Rule-Based Paradigm
A rule-based program is a collection of if-then rules.
if name = "" then input name;
if name starts with "A" then print "Early in the alphabet";
CSE S. Tanimoto Syntax and Types

15. The Logic-Programming Paradigm
A logic program is a collection of logical propositions and questions.
If x is a bird or an airplane, then x has wings.
Tweety is a bird.
Does Tweety have wings?
CSE S. Tanimoto Syntax and Types

16. The Visual Data-Flow Paradigm
A visual data-flow program is a diagram in which boxes represent operations and arrows indicate the flow of data from outputs of operations to inputs of other operations.
3x2 + 5x + 8 3 * *
input x + * + 5 8
CSE S. Tanimoto Syntax and Types

17. CSE 341 -- S. Tanimoto Syntax and Types
Category or class for a value (or object) that permits its bits to be interpreted.
Central Processing Unit instruction sets recognize certain types, such as integers and floats of different sizes.
A programming language is not limited to the types that are directly supported by the CPU.
CSE S. Tanimoto Syntax and Types

18. CSE 341 -- S. Tanimoto Syntax and Types
Strong vs Weak Typing
Strong typing: (static and usually monomorphic)
Every variable must have a type.
A variable may receive only values of its type.
Type-related bugs can be reduced.
Type identification tags are not needed a run time.
Weak typing: (dynamic and usually polymorphic)
Variables need not be declared with particular types.
The type of value held by a variable can change dynamically.
Values must be tagged during execution with their types.
CSE S. Tanimoto Syntax and Types

19. Coercion and Contagion
Coercion: Any automatic type conversion.
A value of type A is being assigned to a variable of type B.
The value is coerced into one of type B.
double x = 3 * 5;
Contagion:
Values of lower precision “catch” the precision of their higher precision co-arguments.
int y = 10 * ( / 10); /* result is 3, not 0. */
In Common Lisp, when a rational meets a float, the rule of floating-point contagion rules.
CSE S. Tanimoto Syntax and Types

20. CSE 341 -- S. Tanimoto Syntax and Types
Type Inference
With weak typing, type inference is needed. (But ML, which is strongly typed, also uses a kind of type inference.)
The resulting type can be determined from the types of the arguments, and the nature of the operations.
Contagion provides one method for type inference:
With strong typing, means for determining type equivalence are needed.
int x[10];
int y[10];
Here x and y have structurally equivalent types.
C and C++ recognize structurally equivalent types as equivalent.
In Modula, most structurally equivalent types are not automatically considered equivalent, but they can be declared equivalent.
CSE S. Tanimoto Syntax and Types