Effel

Eiffel is an interesting programming language developed by Bertrand Meyer and his company Eiffel Software in the 1980s. The language was named for Gustav Eiffel, the archi-tect who designed the famous tower in Paris. The language and accompanying methodology attracted considerable interest at software engineering conferences.

Eiffel fully supports (and in some ways pioneered) pro-gramming concepts found in more widely used languages today (see class and object-oriented programming). Syn-tactically, Eiffel emphasizes simple, reusable declarations that make the program easier to understand, and tries to avoid obscure or lower-level code such as compiler optimizations.

Program Structure

An Eiffel program is called a “system,” emphasizing its structure as a set of classes that represent the types of real-world data that need to be processed. A simple class might look like this:

class

COUNTER

feature—access counter value total: INTEGER

feature—manipulate counter value increment is—increase counter by one

do

total :- total + 1 end

decrement is—decrease counter by one do

total := total - 1 end

reset is—reset counter to zero do

total := 0 end

end

(In this listing language, keywords are in bold and user-defined objects are in italics. This formatting will be done automatically as the user enters the text.) Once the class is defined, making an instance of it is very simple:

my_counter COUNTER

create my_counter

The Eiffel compiler itself compiles to an intermediate “bytecode” that, in the final stage, is compiled into C, taking advantage of the ready availability of optimized C compilers.

A unique feature of Eiffel is the ability to set up “con-tracts” that specify in detail how classes will interact with one another. (This goes well beyond the usual declarations of parameters and enforcement of data types.) For example, with the COUNTER class an “invariant” can be declared such that total >= 0. This means that this condition must always remain true no matter what. A method can also require that the caller meet certain conditions. After pro-cessing and before returning to the caller, the method can ensure that a particular condition is true. The point of these specifications is that they make explicit what a given unit of code expects and what it promises to do in return. This can also improve program documentation.

Implementation and Uses

Eiffel’s proponents note that it is more than a language: It is designed to provide consistent ways to revise and reuse program components throughout the software development cycle. The current implementation of Eiffel is available for virtually all platforms and has interfaces to C, C++, and other languages. This allows Eiffel to be used to create a design framework for reusing existing software components in other languages. Eiffel’s consistent object-oriented design also makes it useful for documenting or modeling software projects (see modeling languages).

Eiffel was developed around the same time as C++. Eiffel is arguably cleaner and superior in design to the latter lan-guage. However, two factors led to the dominance of C++: the ready availability of inexpensive or free compilers and the existence of thousands of programmers who already knew C. Eiffel ended up being a niche language used for teaching software design and for a limited number of com-mercial applications using the EiffelStudio programming environment.

Eiffel has been recognized for its contributions to the development of object-oriented software design, most recently by the Association for Computing Machinery’s 2006 Software System Award for Impact on Software Quality.