Path: utzoo!utgpu!attcan!uunet!lll-winken!ames!pasteur!ucbvax!decwrl!decvax!ima!haddock!karl
From: karl@haddock.ima.isc.com (Karl Heuer)
Newsgroups: comp.lang.misc
Subject: Re: Expression Based Language
Message-ID: <11398@haddock.ima.isc.com>
Date: 12 Jan 89 06:21:32 GMT
References: <3300001@uxg.cso.uiuc.edu> <3290002@hpctdls.HP.COM> <11359@haddock.ima.isc.com> <2659@ficc.uu.net>
Reply-To: karl@haddock.ima.isc.com (Karl Heuer)
Organization: Interactive Systems, Boston
Lines: 34

In article <2659@ficc.uu.net> you write:
>Three comments: First,
>
>	a = for(s1;e;s2)
>
>	Is this legal? If e fails, is the value s1?

I hadn't thought about that, but it's a reasonable definition -- provided the
type of s1 matches the type of the body of the for statement.  It does require
saving the value of s1 before executing e, which is somewhat couterintuitive.

>	For expressions like if-without-else, the value if no statement
>	was executed was zero, NULL, whatever. This was in analogy to
>	short-circuit AND (which wasn't in the language).

This would be self-consistent, but I'm not sure how useful it would be.  In a
while-statement, for example, if the value at the bottom of the loop is some
useful value, the default (zero-execution) value would probably be some sort
of error indication.  Zero is not always an out-of-band value.

On the other hand, a void value is even less useful, and the existence of else
(for if-statements), default (for switch-statements), and the above trick of
using `for' instead of `while' means that the user has complete control over
the default value in any case.  Given this, it seems acceptable to use zero as
the `default default', when no explicit default is specified.  It's also
within the spirit of C, I think.

>I think the value of a statement with varying types is soluble by looking
>at the coercion needed to use it in context.

Context outside of the expression itself?  I think that's dangerous ground.
Can you give an example of how one might use this?

Karl W. Z. Heuer (ima!haddock!karl or karl@haddock.isc.com), The Walking Lint