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Re: new, simpler formal specification

This page is part of the web mail archives of SRFI 62 from before July 7th, 2015. The new archives for SRFI 62 contain all messages, not just those from before July 7th, 2015.

In hopes it may be helpful, the following is a somewhat simpler version of
an earlier one: http://srfi.schemers.org/srfi-62/mail-archive/msg00045.html
which only modifies R5RS's definition of <comment> (effectively treated as
separator/white-space); which includes <block-comment>, and removes legally
quoting and/or commenting, a comment or <empty>:

Only modifying one R5RS's exiting grammar specification:

   <comment> -> <line-comment> | <block-comment> | <datum-comment>

And augmented it with these additions:

  <line-comment> -> ; <all-chars-to-end-of-line>

  <block-comment> -> #| <all-char-except-#|-or-|#> |#

  <datum-comment> -> #; <datum>

(i.e. #;#; '#; (#;) (') etc. are illegal sequences, vs. earlier versions.)

> From: Taylor Campbell <campbell@xxxxxxxxxxxxxxxxxx>
> I recently wrote an alternative specification of Scheme's syntax using
> (roughly) BNF, but in such a way that is much more in the spirit of
> Lisp: the syntax of the external representation of S-expressions is
> separated from the syntax of Scheme, and S-expressions are specified in
> such a way as to resemble a recursive-descent parser as closely as
> possible.  That is not the point of this message, though.  In writing
> the alternative specification, I realized that S-expression comments
> can be specified much more simply.  Specifically, all it requires is
> that the <datum>, <list>, & <vector> non-terminals be extended, and a
> <commented datum> be introduced.  (<Token> must of course be extended
> with a "#;" option, too.)  <Datum> becomes:
>   <datum> ---> <simple datum> | <compound datum> | <commented datum>
>   <commented datum> ---> "#;" <datum> <datum>
> <List> & <vector> use a couple of auxiliary rules; they are all
> presented here:
>   <list> ---> "(" <datum>* <optional dot> <delimiter prefix> ")"
>   <optional dot> ---> <empty> | <datum> <delimiter prefix> "." <datum>
>   <vector> ---> "#(" <datum>* <delimiter prefix> ")"
>   <delimiter prefix> ---> <empty> | "#;" <datum> <delimiter prefix>
> The first datum within a commented datum is ignored, as is any datum
> immediately following the "#;" token in a delimiter prefix.
> This formal specification is much simpler than what Al* Petrofsky
> suggested earlier, and I think it should satisfy Paul Schlie's demands
> for a grammar more consistent with Scheme's existing one.  If no one
> sees any problems with it, I'll change the SRFI document to use it and
> finalize the SRFI, which is a bit overdue.
> Should Paul Schlie still prefer his own specification, which I still
> don't understand, he should provide an implementation of it in terms of
> Scheme48's reader (as I show in the document), and he should explain
> why he wants to so drastically change the various reader abbreviations,
> those for QUOTE, QUASIQUOTE, &c., to magically disappear due to their
> being in what was previously syntactically invalid locations.
> (My alternative specification of Scheme's syntax, by the way, is at
> <http://www.bloodandcoffee.net/campbell/code/scm-syntax.text>, for
> those curious.  Any comments, questions, or other feedback on it would
> be appreciated, though such feedback should be sent to me directly, not
> to this mailing list.)