Path: utzoo!utgpu!water!watmath!clyde!rutgers!gatech!hubcap!ncrcae!ncr-sd!hp-sdd!hplabs!hpcea!hpfcdc!hpisla!wolpert
From: wolpert@hpisla.HP.COM (David Wolpert)
Newsgroups: comp.lang.misc
Subject: Re: History of := (was: == vs =)
Message-ID: <5410001@hpisla.HP.COM>
Date: 27 Jan 88 17:09:17 GMT
References: <2337@haddock.ISC.COM>
Organization: HP Instrument Systems Lab - Loveland, CO
Lines: 54

>>legal to have an assigment as a term in an expression, there had to
>>be a way to distinguish between equality and assignment. This is the

>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

Algol DOES allow assignment as a term in an expression.  At least,
Burroughs XALGOL did, and I can't imagine Algol without it.  Proof
by quoting excerpt from manual:

<expression> ::=
        <arithmetic_expression> | <Boolean_expression> |
        <designational_expression> | <concatenate_expression>

<arithmetic_expression> ::=
        <simple_arithmetic_expression> | <if_clause>
        <arithmetic_expression> ELSE <arithmetic_expression>

<simple_arithmetic_expression ::=
        <term> | <adding_operator> <term> |
        <simple_arithmetic_expression> <adding_operator> <term>

<term> ::=
        <factor> | <term> <multiplying_operator> <factor>

<factor> ::=
        <primary> | <factor>*<primary>

<primary> ::=
        <unsigned_number> | <variable> | <function_designator> |
        ( <arithmetic_expression> ) | <partial_word_designator> |
        <string> | <concatenate_expression> | <assignment_statement>
                                              -----here--it--is-----

I'm (almost) positive that this was a standard feature of Algol,
and not a Burroughs extention.  Wish I had a copy of the "plain"
language definition to quote from.

Actually, there were only two things I missed when I changed from
Algol to Pascal: assignment inside expressions, and OWN variables
(and, maybe a little, being able to hop into GTL/pseudo-Lisp).
                       _  _
                      / \/ \      David Wolpert   (303) 667-5000 x3533
  /_  __   HEWLETT    \    /             Measurement Systems Operation
 / / /_/   PACKARD     \  /          P O Box 301 - Loveland, CO  80539
    /                   \/                wolpert%hpisla@hplabs.HP.COM
======================================================================
"Be joyful always; pray continually; give thanks in all circumstances"

ps: I describe Algol as a "block" language; Pascal is closer to being
a "statement" language, since it doesn't allow the programmer to nest
blocks freely.