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.rottm...@gmx.at> a écrit :
> David Pirotte <da...@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
