Re: syntax-rules problem

[David Pirotte <david@altosw.be>]

Hi Andreas,

Thank you very much for the explication, very helpful. At the time I wondered how it
did work [guile-1.6] since the definition was quite clear about set! , but it did
and then I used it :-)

	[i am not using guile-1.8 but thanks for the advice, which might help other
	guilers off course]

Thanks,
David

;; --

Le Sun, 03 Apr 2011 21:16:00 +0200,
Andreas Rottmann <a.rottmann@gmx.at> a écrit :

> David Pirotte <david@altosw.be> writes:
> 
> > Hello,
> >
> > 	guile version:	2.0.0.160-39be
> >
> > this used to work:
> >
> > (define-syntax push*
> >   (syntax-rules ()
> >     ((push* . ?args)
> >      (set! (car (last-pair ?args))
> > 	   (cons* ?args)))))
> >
> Well, that's not well-formed code; there two problems here:
> 
> (1) The first operand to `set!' has to be an identifier; in the
>     expansion of `push*', it is an expression.  Actually, that's the
>     rule in plain R5RS and R6RS, but Guile contains hooks for
>     implementing SRFI-17, which allows for expressions in `set!'s first
>     operand; thus you get, with the above definition:
> 
> scheme@(guile-user)> (use-modules (language tree-il))
> scheme@(guile-user)> (tree-il->scheme (macroexpand '(push* 1 2 lst)))
> $4 = (((@@ (guile) setter) car) (last-pair (1 2 lst)) (cons* (1 2 lst)))
> 
>     And that, when called, yields the error you got (when SRFI-17 is not
>     loaded, as the default binding for `car' doesn't have a
>     setter). After importing SRFI-17, it still won't do what you
>     intended:
> 
> scheme@(guile-user)> (use-modules (srfi srfi-17))
> scheme@(guile-user)> (push* 1 2 lst)
> <unnamed port>:67:0: In procedure #<procedure 3dcdf00 at <current input>:68:0 ()>:
> <unnamed port>:67:0: Wrong type to apply: 1
> 
>     The reason is the second issue:
> 
> (2) `?args' is a pattern variable holding a list, and; so having
>     `(last-pair ?args)' is not OK: for `(push* 1 2 lst)', it expands to
>     `(last-pair? (1 2 lst))'. So you might quote `?args', but that
>     doesn't help to do what you want, because of the first issue.
> 
> That the above code worked in Guile 1.8 can be considered an accident
> (or even a bug, IMHO).
> 
> A correct version would be:
> 
> (define-syntax push*
>   (syntax-rules ()
>     ((push* elements ... identifier)
>      (set! identifier (cons* elements ... identifier)))))
> 
> Note that the above relies on R6RS-specified extensions to
> `syntax-rules' patterns that are not yet available in Guile 1.8.
> 
> HTH, Rotty