Path: utzoo!mnetor!uunet!mcvax!ukc!eagle!icdoc!qmc-cs!keithc
From: keithc@cs.qmc.ac.uk (Keith Clarke)
Newsgroups: comp.lang.misc
Subject: Re: History of := (was: == vs =)
Message-ID: <367@sequent.cs.qmc.ac.uk>
Date: 27 Jan 88 14:27:44 GMT
References: <2337@haddock.ISC.COM> <51300003@uxe.cso.uiuc.edu> <715@PT.CS.CMU.EDU> <3921@aw.sei.cmu.edu>
Reply-To: keithc@qmc.ac.uk (Keith Clarke)
Organization: Computer Science Dept, Queen Mary College, University of London, UK.
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In article <3921@aw.sei.cmu.edu> firth@bd.sei.cmu.edu.UUCP (Robert Firth) writes:
>Algol-60 does not allow an assignment within an expression, and there
>would be no ambiguity using '=' for both assignment and equality.  This
>is because the designers of Algol-60 had the good sense to make it a
>statement language.  They then used for equality the only reasonable
>symbol, and for assigment they used the left-pointing arrow, to capture
>the notion that something flowed from the RHS to the LHS.

Algol 60 doesn't allow assignments within expressions, but
there would too be an ambiguity, because it allows multiple
assignment:

(1) p :=  q = r
compare q and r, put the answer in p.

(2) p := q := r
put the value of r into q and also into p.

Parsers normally don't use type information to sort out
syntactic ambiguities, so it doesn't really matter if = isn't the
operator for comparing booleans; it is certainly the operator
for something!
I suspect the designers of Algol 60 would have used different
symbols anyway, since they stand for quite different things.
Maybe the statement/expression distinction faded because of a
wish to generalise constructs like the multiple assignment
statement?
-- 
Keith Clarke

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