Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!ucsd!hub.ucsb.edu!eiffel!kimr
From: kimr@eiffel.UUCP (Kim Rochat)
Newsgroups: comp.object
Subject: Re: Do we really need types in OOPL's?
Summary: Why Eiffel is typed
Message-ID: <411@eiffel.UUCP>
Date: 21 Sep 90 18:04:24 GMT
References: <0yw10qr@Unify.Com>
Organization: Interactive Software Engineering, Santa Barbara CA
Lines: 34

In article <0yw10qr@Unify.Com>, dbrus.Unify.Com.brian@unify.uucp (Brian Meyerpeter ) writes:
> I have a question that has been bugging me for a long time.  
> 
> Why does Eiffel try to be a typed language? 
> 
> In an ideally object-oriented language shouldn't you be able to assign
> a class of any type to another class of any type as long as the 
> client that is assigning them knows their interface and does not 
> abuse it?

The last comment is the reason Eiffel is typed.  Because of typing, the
compiler can verify that class interfaces (i.e., types) are properly used, 
and a type error cannot occur at run time.  In contrast, dynamically 
typed languages get type errors at run time instead of compile time.

Typing inhibits dynamic binding only in the sense that you are only
allowed to send polymorphic messages to (families of) objects which are 
guaranteed to understand them, and you are not allowed to send messages 
to objects which aren't guaranteed to understand them. The combination
of multiple inheritance, static typing, and dynamic binding works quite 
well in practice.

Static typing also allows optimizations to be made which are difficult
or impossible in a dynamically typed language, such as the removal of
the code from an application for methods which the compiler can
guarantee are never executed by the application.

In Dr. Meyer's book, "Object-Oriented Software Construction", you 
will a find a complete explanation of "why Eiffel is a typed language".

Kim Rochat
Interactive Software Engineering

responses to eiffel@eiffel.com