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From: wclodius@earthlink.net (William Clodius)
Newsgroups: comp.compilers
Subject: Re: Semantics, opt in Semantics
Date: Fri, 16 Jan 2015 12:22:21 -0700
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Seima Rao <seimarao@gmail.com> wrote:

>     The Backus Naur Form is a great mathematical model. It explains syntax
>     quite succintly.
>
>     In that form, the opt qualifier which stands for optional or
>     epsilon is utilized extensively for optional syntax.
>
>     Is there something similar for semantics i.e. is there something optional
>     in semantics.
>
>     Also, what is the equivalent in semantics of BNF ?
>
> Sincerely,
> Seima Rao.
> [Man, there's a can of worms. There's no semantic formalism that matches real
> semantics as well as BNF matches real syntax. -John]

This seems to combine three separate questions:

1. What notations are available for semantic descriptions?

2. What semantic concepts are considered optional?

3. How are those optional concepts expressed in the semantic notations?

As to the first question the complexity of semanticsx has resulted in a
large variety of notations: the van Wijngaarden two level grammar, the
Vienna Definition Language/Vienna Development Method, Denotational
semantics, attribute grammars,etc.

As to the second some obvious optional semantics include parallelism,
and side effects including exceptions and some forms of I/O.

I believe the latter reports on Algol68 included parallelism defined in
the van Wijngaarden two level grammar.