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
--
Andreas Rottmann -- <http://rotty.yi.org/>
