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Path: utzoo!mnetor!uunet!munnari!kre
From: kre@munnari.oz (Robert Elz)
Newsgroups: comp.mail.uucp
Subject: Re: Say "preference" rather than "precedence" in network addresses
Message-ID: <1770@munnari.oz>
Date: Wed, 5-Aug-87 12:15:44 EDT
Article-I.D.: munnari.1770
Posted: Wed Aug  5 12:15:44 1987
Date-Received: Sat, 8-Aug-87 02:25:43 EDT
References: <112@stargate.UUCP> <908@bsu-cs.UUCP>
Organization: Comp Sci, Melbourne Uni, Australia
Lines: 41
Summary: "precedence" is not misused in addressing discussions

In article <908@bsu-cs.UUCP>, dhesi@bsu-cs.UUCP (Rahul Dhesi) writes:
> The quoted use of the term "precedence" is frequently used to mean that
> when a mailer sees a construct of the form a!b@c it groups it as
> (a!b)@c rather than a!(b@c).
>	... 
> This, ... would imply that a+b*c would
> be grouped as (a+b)*c rather than as a+(b*c), which is wrong.

No, that's not quite right.  In both mathematics, and network
address parsing, "precedence" implies ordering of the operations,
highest precedence means "do it first".

In mathematics, parentheses can be used to force precedence to
a particular operator or expression, so its entirely sensible
to write "a+(b*c)" if that's what you prefer (or "(a+b)*c" if
that's what you mean).

In mail headers, parentheses mean comments, so its totally absurd
to write "h!u@d" as "h!(u@d)" or "(h!u)@d".   Nor is it sensible
to use those forms in any kind of analogy with mathematics.

Just think of precedence as "do it first" and apply that to the
address, in order to "do" a network address operator, you (notionally
at least) send the mail to the indicated host (rhs or lhs as
appropriate), and pass it the other operand to interpret locally.

With this definition, its entirely reasonable to claim that @
should have higher precedence than ! .. it just means that you
should "do" the '@' first.  This is totally consistent with
mathematics where '*' having higher precedence than '+' means
that you "do" the '*' first.

I suspect that where the confusion arises, is that the operands of
mathematical operators are numbers, and that's all (anything else
must be reducible to a number).  In net addresses the two operands
are strings, one a "host" and the other "local information", and
each of those contains all of the characters on the appropriate
side of the operator (not just those out as far as some other operator,
there isn't even a standard set of operators).

Robert Elz