Re: unquote in quasisyntax

[Jamison Hope <jrh at theptrgroup dot com>]

On Feb 6, 2012, at 4:20 AM, Helmut Eller wrote:

> * Per Bothner [2012-02-06 08:21] writes:
>
> > Perhaps the cleanest solution is warn about the old unquote syntax
> > *unless* srfi-72 is explicitly requested, with a require or import.
> > Or perhaps that is overkill?  How many would like Kawa to really
> > support SRFI-72?
>
> R6RS semantics should be used instead of SRFI-72.  Dybvig and the  
> Racket
> gang seem to write all the relevant documentation about this stuff, so
> using #, is less surprising.


Another incompatibility/shortcoming when compared to R6RS/Dybvig is that
Kawa can't handle a syntax-case pattern that isn't a list with the macro
name in the first car.

For example, TSPL4 section 8.4 [1] includes a "define-integrable" macro:

> (define-syntax define-integrable
>   (syntax-rules (lambda)
>     [(_ name (lambda formals form1 form2 ...))
>      (begin
>        (define xname
>          (let-syntax ([name (identifier-syntax xname)])
>            (lambda formals form1 form2 ...)))
>        (define-syntax name
>          (lambda (x)
>            (syntax-case x ()
>              [_ (identifier? x) #'xname]
>              [(_ arg (... ...))
>               #'((let-syntax ([name (identifier-syntax xname)])
>                    (lambda formals form1 form2 ...))
>                   arg (... ...))]))))]))


and R6RS Standard Libraries section 12.4 [2] has:

> (define p (cons 4 5))
> (define-syntax p.car
>   (make-variable-transformer
>     (lambda (x)
>       (syntax-case x (set!)
>         [(set! _ e) #â(set-car! p e)]
>         [(_ . rest) #â((car p) . rest)]
>         [_  #â(car p)]))))
> (set! p.car 15)
> p.car                   â 15
> p                       â (15 5)

A bit higher up in the section it says:

> A <pattern> is an identifier, constant, or one of the following.
>
> (<pattern> ...)
> (<pattern> <pattern> ... . <pattern>)
> (<pattern> ... <pattern> <ellipsis> <pattern> ...)
> (<pattern> ... <pattern> <ellipsis> <pattern> ... . <pattern>)
> #(<pattern> ...)
> #(<pattern> ... <pattern> <ellipsis> <pattern> ...)


Admittedly I've never encountered a situation where it really mattered,
but it might come up if someone is trying to port some fancy macros from
Chez or Racket to Kawa.


[1] http://www.scheme.com/tspl4/syntax.html#./syntax:h4
[2] http://www.r6rs.org/final/html/r6rs-lib/r6rs-lib-Z-H-13.html#node_sec_12.4

--
Jamison Hope
The PTR Group
www.theptrgroup.com