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From: news@stl.stc.co.uk (News admin c/o dww)
Newsgroups: comp.lang.misc
Subject: Re: bizarre instructions
Message-ID: <4100@stl.stc.co.uk>
Date: 1 Mar 91 11:26:18 GMT
References: <1991Feb25.194658.3712@linus.mitre.org>
Reply-To: tom@nw.stl.stc.co.uk
Organization: STC Technology Ltd., Harlow, Essex, GB
Lines: 31
From:tom@nw.stl.stc.co.uk

> I have never seen any language description in which the type of the
> result in a replacement statement affected what operation is
> performed.
 
There are quite a few such languages.  In the dialect of Algol we use here
for writing operating systems (and much else) it's easy to give an example.
Let's suppose that R1 and R2 are floating point variables, and I1 and I2 are
integer variables.  Now look at two statements
 
A:  R1:= I2+R2
B:  I1:= I2+R2
 
These are replacement statements. The type of the result in the first is
float, in the second it's integer.  In the first I2 is floated and a floating
point add performed.  In the second R2 is fixed and an integer addition is 
performed.  Given the ranges allowed for integer and float and the different
ways precision may be lost, it seems reasonable to consider the two operations
radically different (they even have different failures).
 
This sort of thing arises in every language I know which has implicit
coercions. Even in Fortran:-
  1  I1 = I2
  2  R1 = I2
the statements 1 and two perform different operations (copy and float 
respectively) just because the result type is different.
 
So what is Prof Rubin trying to say here?  Obviously not what he said
(he knows Fortran well enough to know that happens).
 
Tom Thomson   [tom@nw.stl.stc.co.uk
International Computers Ltd, Manchester M12 5DR,  UK