From the beginning of Eiffel the language has been characterized as strongly typed. But since its beginning there has been a hole in the type system which has not yet been fixed up to now (the so called catcall problem).

What is the hole?

The existence of the hole can be demonstrated with very simple examples.

From the beginning of Eiffel the language has been characterized as strongly typed. But since its beginning there has been a hole in the type system which has not yet been fixed up to now (the so called catcall problem).

What is the hole?

The existence of the hole can be demonstrated with very simple examples.

The search for a type-safe version of Eiffel is a never-ending story in the development of the Eiffel language. Recently, Helmuth Brandl has created a series of blog entries with a nice description of is needed to make Eiffel type-safe. There have been others posting blog entries (including me) and it also had been a constant topic during the ECMA process of Eiffel standardization.

Interestingly, a question that was seldom raised was: why do we need type-safety? As absurd as it seems for all the mathematicians to raise this question, the Eiffel community has understood for a long time that mathematical soundness is not a reason in itself. Instead, it serves a purpose in the enterprise of language design: giving software developers means to express themselves in ways that reduce the number of defects in his/her programs and enable him/her to master complex problems he/she would not be able to master without.

From the beginning of Eiffel the language has been characterized as strongly typed. But since its beginning there has been a hole in the type system which has not yet been fixed up to now (the so called catcall problem).

What is the hole?

The existence of the hole can be demonstrated with very simple examples.

From the beginning of Eiffel the language has been characterized as strongly typed. But since its beginning there has been a hole in the type system which has not yet been fixed up to now (the so called catcall problem).

What is the hole?

The existence of the hole can be demonstrated with very simple examples.

From the beginning of Eiffel the language has been characterized as strongly typed. But since its beginning there has been a hole in the type system which has not yet been fixed up to now (the so called catcall problem).

What is the hole?

The existence of the hole can be demonstrated with very simple examples.

The following is a short description of a solution to the catcall problem (recently posted in comp.lang.eiffel as well):

In Eiffel it is easy to generate runtime type errors. Therefore Eiffel cannot (yet) be considered to be a typesafe language.

Main features of Eiffel

The main features of Eiffel are:

- clean object orientation with multiple inheritance

- design by contract

- elegant expressive syntax

- automatic memory management

- generics

- uniform type system with type safety (exept catcalls)

These features were unique at the time Eiffel was invented (some 25 years ago).

Eiffel in its current form is not completely type safe. Type errors (called catcalls in Eiffel speak) are possible. The compiler cannot detect these type errors. They usually trigger an exception at runtime.

These type errors are possible due to covariant redefinition of arguments and polymorphy. Both principles are very powerful in OO programming.