Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!umich!samsung!zaphod.mps.ohio-state.edu!swrinde!ucsd!hub!eiffel!bertrand
From: bertrand@eiffel.UUCP (Bertrand Meyer)
Newsgroups: comp.lang.eiffel
Subject: Re: Eiffel cleanup #1: The creation mechanism
Summary: No exception need be made for null
Keywords: Separate operation on references from operations on objects
Message-ID: <243@eiffel.UUCP>
Date: 16 Feb 90 20:00:07 GMT
References: <226@eiffel.UUCP> <7862@cognos.UUCP> <235@eiffel.UUCP> <607@m1.cs.man.ac.uk>
Organization: Interactive Software Engineering, Santa Barbara CA
Lines: 31

From <607@m1.cs.man.ac.uk> by chl@cs.man.ac.uk (Charles Lindsey):
> 
> Although Tony Hoare's principle [i.e., the meaning of an expression
> should be fully deducible from the meanings of its constituents]
> seems a fundamental part of strong typing, there are precedents for
> breaking it. [..] Thus in PASCAL (also ALGOL 68) one may write
> 
>     somepointervariable := nil;
> 
> even though the type of nil can only be determined from its context [...]
> 
> In the case if Eiffel, I should have thought that the same exception could be
> made for x := !!. It is not so different from x := null which apparently
> Bertrand is prepared to condone.

Actually it is different because in Pascal the type status of
`nil' is murky: nil is a constant which must belong to every type, which
is contradictory with the rest of the type system. No such problem arises
in Eiffel thanks to the introduction of NONE: because NONE is a descendant
of every class, we can define the type of `null' as being NONE, so that
the assignment

    x := null

is always typewise valid regardless of the type of x.



-- 
-- Bertrand Meyer
bertrand@eiffel.com