Newsgroups: comp.lang.scheme
Path: utzoo!utgpu!news-server.csri.toronto.edu!torsqnt!geac!sq!dak
From: dak@sq.sq.com (David A Keldsen)
Subject: Re: Logic does not Apply
Message-ID: <1991May2.180853.11028@sq.sq.com>
Organization: SoftQuad Inc.
References: <9104300906.aa04392@mc.lcs.mit.edu>
Date: Thu, 2 May 91 18:08:53 GMT
Lines: 33

Gordon Oulsnam (stcs8004@iruccvax.ucc.hea.ie) writes:

[why aren't 'and' and 'or' strict in Scheme?]
>It  seems to  me that  the difference  lies in  the fact  that the  logical
>operators are  defined in the R3  and R3.99 reports (section  7.3) as being
>*derived* expression types, so presumably do not qualify as 'procedures' in
>the sense specified for the first argument to 'apply' (section 6.9).

Correct, although I'd put it more simply:  'and' and 'or' are syntax, not
procedures.  All Scheme procedures are strict.  'and' and 'or' are not
strict, which is how "short-circuiting" boolean evaluation is made available
to you in Scheme.

>That the logical operators are derived  expressions may well be the *legal*
>answer as to  why 'and' and 'or' cannot be  used with 'apply'---my question
>is though: is it a *reasonable* answer, with or without the RxRs Reports to
>hand?  Why shouldn't 'and' and 'or' be first class citizens of Scheme?

Because they are syntax, not procedures.  You can only apply procedures.

>It seems that logic does not apply! :-)

As you point out, if you want the strict behavior, you can get it by the
usual eta transformation; going the other way is much more difficult.
In this sense, scheme is providing you with the "more fundamental" tools.

Regards,
Dak
-- 
David A. 'Dak' Keldsen of SoftQuad, Inc. email: dak@sq.com  phone: 416-963-8337
"I daresay you haven't had much practice," said the Queen.  "When I was your 
age, I always did it for half-an-hour a day.  Why, sometimes I've believed as 
many as six impossible things before breakfast." _Through the Looking Glass_