UNIVERSITY AT BUFFALO, THE STATE UNIVERSITY OF NEW YORK
The Department of Computer Science & Engineering

STUART C. SHAPIRO: CSE 305

CSE 305
Programming Languages
Lecture Notes
Stuart C. Shapiro
Spring, 2005

The Program Level

It is interesting to compare programming languages on the characteristics of:

What is the top-level structure of a program?
What is done when the program/language/system is run with minimal command-line options?

For Fortran, C, C++, and Java, the top-level structure, aside from some compiler directives such as import and #define, is the definition of program/subprogram modules.

For Fortran, the modules are the PROGRAM, SUBROUTINEs, and FUNCTIONs.
For C, the modules are functions.
For C++, the modules are functions and classes.
For Java, the modules are classes.

C, C++, Fortran, and Java are all compiled languages. You must compile the program and executed the compiled version (even if, as for Java, it is not actually machine code). When you run the compiled program:

For Fortran, control is given to the PROGRAM module.
For C, C++, and Java, control is given to the function named main.

For Perl, the top-level structure of a program is a sequence of executable statements and subroutine definitions. When the program is run, the statements are executed. If you run the Perl interpreter by invoking perl with no command line options or arguments, the program is taken from standard input, and run when eof is reached.

For bash, the top-level structure of a program is a sequence of statements, called "commands", which include function definitions. When the program is run, the statements are executed. If you run the interpreter with no command line options or arguments, you enter an interactive read-eval-print loop, in which each statement you enter is immediately executed.

For Python, the top-level structure of a program is a sequence of executable expressions and statements, which include function and class definitions. When the program is run, the statements and expressions are evaluated in order. If you run the interpreter with no command line options or arguments, you enter an interactive read-eval-print loop, in which each statement you enter is immediately executed, and each expression you enter is immediately evaluated and its value is printed.

For Prolog, the top-level structure is the definition of subprogram modules, which Prolog calls "procedures", and are, in fact, relations. Each procedure/relation is defined by a sequence of clauses. Each clause must be one of the following:

rule: relation head plus body: <head> :- <body> .
fact: relation head without a body: <head> .
query: relation body without a head: :- <body> .

To run a Prolog program (with minimal command-line options), you run the Prolog interpreter, which puts you in an interactive read/eval/print loop, where the input must be a

query: call of one or more relation modules: <body> .
which could load a source-code file.
The only way to enter new facts and rules interactively is by loading ("consulting") the "file" called user.

The Prolog interpreter prints instances of the relation(s) you ask about.

For Common Lisp, the top-level structure of a program is a sequence of executable expressions, which include function and class definitions. When the program is run, the expressions are evaluated, and their values are printed. If you run the interpreter with no command line options or arguments, you enter an interactive read-eval-print loop, in which each expression you enter is immediately evaluated, and its value is printed.

This can all be summarized by the following table:

Language Top-Level Structure When Run Program When Run w/o Program

Fortran Module definitions Execute PROGRAM N/A

C Module definitions Execute main N/A

C++ Module definitions Execute main N/A

Java Module definitions Execute main N/A

Perl Module definitions and Statements Execute statements Enter program, then run it

bash Statements Execute statements Interactively execute statements

Python Statements and expressions Execute statements and evaluate expressions Interactively execute statements and evaluate expressions

Prolog Module definitions N/A Interactively evaluate queries

Common Lisp Expressions N/A Interactively evaluate expressions

Language	Top-Level Structure	When Run Program	When Run w/o Program
Fortran	Module definitions	Execute `PROGRAM`	N/A
C	Module definitions	Execute `main`	N/A
C++	Module definitions	Execute `main`	N/A
Java	Module definitions	Execute `main`	N/A
Perl	Module definitions and Statements	Execute statements	Enter program, then run it
bash	Statements	Execute statements	Interactively execute statements
Python	Statements and expressions	Execute statements and evaluate expressions	Interactively execute statements and evaluate expressions
Prolog	Module definitions	N/A	Interactively evaluate queries
Common Lisp	Expressions	N/A	Interactively evaluate expressions

CSE 305 Programming Languages Lecture Notes Stuart C. Shapiro Spring, 2005

The Program Level

Stuart C. Shapiro <shapiro@cse.buffalo.edu>

CSE 305
Programming Languages
Lecture Notes
Stuart C. Shapiro
Spring, 2005