Department of Mathematics and Computer Science

Math 169 Notes
Introduction -- II

Contents

• A. Language
• B. Paradigms
• ---1. Functional
• ---2. Imperative
• ---3. Logic
• ---4. Object-Oriented
• ---5. Concurrent
• C. Comments on Paradigms
• D. Language Development

A. Language

Does a programming environment count as a language? Is MATLAB (``MATrix LABoratory'') a language or a computational environment? One can both write "routines/functions" in MATLAB and also do ad hoc computations. The same is true of LISP.

What about HTML? It is an abbreviation for "HyperText Markup Language" and is used to communicate with computers. So does that make it also a "computer language" in the common use of the term?

By a computer language, most people understand a system of commands and constructs that can be used to communicate with a computer, enabling a programmer to control and sequence the computer's computations and other activities. Usually, "computer language" is used synonymously with "programming language" with the result that a "markup" language or "query" language is seen to be in a different category.

The "ultimate" computer language for any machine is its own binary machine code, with assembly language being a close cousin. In reality, programming languages that are compiled are actually translated into machine code and it is the translation that a computer actually runs, rather than the source code.

B. Paradigms

Classification according to syntax/form
• Functional (Applicative): In these languages, everything is construed as a function, returning a value. The orientation of the language is toward values and the evaluation of expressions (usually functions). LISP and APL are classical examples. Languages derived from LISP include Scheme and ML. A functional language is sometimes termed an applicative language since a function definition is applied to a value (or set of values) to return another value. One characteristic of functional languages is that there are, in general, no side effects. Functions compute values and these are returned. Functions, in general, do not necessarily change values.

• Imperative (Procedural): These languages support operations that are performed not to obtain a value, but for some side effect. ("Imperative" is derived from the Latin word for "command.") Examples of such operations include assignment statements and I/O operations. The orientation of the language is toward variables and the execution of statements or operations. Because of "side-effects" as a result of executing statements, the order of evaluation is crucial, and thus sequencing is important. Languages which fall into this category include Fortran, Pascal, C/C++, Algol, Ada.

• Logic (Rule-Based): In such a language, programs consist of a set of rules for various goals. Program execution consists of a search for a combination of applications of rules that result in a valid logical expression. The prime example of such a language is Prolog.

  Classification according to Context/Purpose/Function

• Object-Oriented: In these languages, the focus is on objects that represent data and (usually) include actions (in the form of procedures or functions) that can be applied to data. Usually, "object-oriented" languages provide for inheritance, that is, the ability for one class to make use of properties, data structures, etc., from an "ancestor" class, and polymorphism, the ability for an operator or function to adjust its meaning depending on the types of the operands/arguments (sometimes called operator or function overloading). Functions that are parts of objects are seen as "messages" (Smalltalk) or "methods" (Java) rather than "functions." The major examples of such languages are Smalltalk, (C++,) and Java.

• Concurrent: Such languages (or language add-ons) are geared for working on parallel processors or multiple, networked machines. They include the ability to distribute sections of code to different processors and to synchronize the program flow after such distribution. One commonly-used set of routines to enable concurrent execution is MPI (Message-Passing Interface). (MPI can be used with Fortran or C code.) Fortran-95 has introduced certain explicit structures and attributes for possible concurrent execution. (HPF, or High Performance Fortran, has compiler commands that can specify how code can be executed on various processors.) There are some extensions of established languages created for testing parallel algorithms, e.g., Concurrent C, Path-Pascal. Java enables a program to have several activities running simultaneously.

C. Comments on Paradigms

C++ is a hybrid. It has been said that the better one knows C, the less one is able to write object-oriented code in C++ even though C++ has object-oriented features. The language itself is so close to its "parent" language C, that programmers can easily slip into a non-object-orientated programming style even though it is billed as a "object-oriented" language.

There are other terms in common use as well. For example, some authors speak about declarative languages which include logic, functional, and database languages, and use imperative to include object-oriented languages.

There are other languages which bridge various categories and are special purpose. SNOBOL and Icon are geared primarily for character manipulation (e.g., language translation or textual analysis). Both are based on pattern matching in character strings, although Icon looks like a typical imperative language. Database languages such as SQL presuppose a large database.

D. Language Development

Another way in which languages develop is by giving birth to new languages that are inspired by older languages and are similar in syntax but have distinctive differences that make code written in the "parent" language incompatible with code written in "child" language. Such is the case with the Pascal family of languages. Pascal was influenced by Algol-68 (itself derived from Algol-60) in its syntax, but was designed for ease of learning and included strict rules regarding data typing and the creation of new data structures. Modula-2 was built on Pascal, retained much of its syntax, but introduced new features now associated with objects and "data encapsulation." Oberon went further and was redesigned for single user workstations. All three languages bear a striking similarity to each other, but code written in one language cannot be compiled by another language's compiler.

Still another way that languages develop is by continuing the function of a language. SNOBOL4 (derived from SNOBOL) was designed for textual analysis and manipulation. SNOBOL's syntax is unlike any other language since the two basic "operations" are pattern matching (indicated by a blank) and text replacement (indicated by an =), both of which can occur in a single line. In addition, each line can include the equivalent of "GOTOs" that can be used to jump to other sections of code depending on whether the pattern matching was successful or not. SNOBOL's successor language, Icon, was developed based on the "best" features of Pascal and C, yet kept many of the intrinsic functions of SNOBOL geared toward textual analysis. Icon also includes other features, useful for numeric computation, not found in SNOBOL.

This page is maintained by Dennis C. Smolarski, S.J. dsmolarski@math.scu.edu
Last updated: 5 April 2002, minor correction 13 September 2005.