Re: New syntax: (: MODULE-NAME VARIABLE-NAME)

[Marius Vollmer <mvo@zagadka.de>]

Mikael Djurfeldt <mdj@mit.edu> writes:

> Marius Vollmer <mvo@zagadka.de> writes:
>
>> what about making the changes below?  The only unclean thing is that
>> we would need to change the interpretation of a single ':'.
>
> But isn't that pretty darn unclean?  It changes the lexical structure
> of the language.

Hmm, yes and no.  With the change, ':' remains a variable identifier
and the new syntax is actually a macro:

    (define-macro (: mod-name var-name)
      (let ((var (module-variable (resolve-interface mod-name) var-name)))
        (if (not var)
            (error "no such public variable" (list ': mod-name var-name)))
        var))

That part is clean, I think, and doesn't change the lexical structure.
It only depends on an implementational detail and does something
officially that you can't do in Standard Scheme.  However, Standard
Scheme doesn't have a module system either, and from that point of
view 'use-modules' etc already has changed the lexical structure
significantly.

> Note that we currently have an option which controls
> whether the reader conforms to R5RS ("Extended alphabetic characters
> may be used within identifiers as if they were letters.") or whether
> colon is marking a keyword, and that this option is defaulted to R5RS.

The reader is unchanged in the R5RS mode, only the non-R5RS mode now
treats an isolated colon differently:

    ;; R5RS
    guile> (symbol? ':)
    #t
    guile> (keyword? '#:)
    #t

    ;; non-R5RS with the change
    guile> (read-set! keywords 'prefix)
    guile> (symbol? ':)
    #t
    guile> (keyword? '#:)
    #t

    ;; non-R5RS without the change
    guile> (read-set! keywords 'prefix)
    guile> (symbol? ':)
    #f
    guile> (keyword? ':)
    #t

> I vote for using some syntax which is compatible with R5RS and would
> urge you strongly to at least keep the possibility to be compatible
> (=> new option controlling the interpretation of colon).

Given that the syntax is compatible, do you think we still need an
option for making (keyword ':) true again?


Here are the ChangeLog entries for the actual changes:

2003-10-24  Marius Vollmer  <mvo@zagadka.de>

	* read.c (scm_lreadr): Treat a single ':' always a symbol,
	never as an empty keyword.

2003-10-23  Marius Vollmer  <mvo@zagadka.de>

	* eval.c (scm_m_generalized_set_x): Macroexpand the target when it
	is a list.  This allows (: ...) to work with set!.

	* script.c (scm_compile_shell_switches): Use scm_c_read_string for
	the "-e" option instead of scm_str2symbol.  This allows things
	like (: ...) to be specified for the entry point.


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