* syntax-rules problem
@ 2011-04-03 4:34 David Pirotte
2011-04-03 19:16 ` Andreas Rottmann
0 siblings, 1 reply; 3+ messages in thread
From: David Pirotte @ 2011-04-03 4:34 UTC (permalink / raw)
To: guile-user
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)))
))
scheme@(guile-user)> (define ll '(1 2 3 4 5))
scheme@(guile-user)> (push* 'a 'b ll)
ERROR: In procedure setter:
ERROR: In procedure setter: Wrong type argument in position 1: #<procedure car (_)>
Entering a new prompt. Type `,bt' for a backtrace or `,q' to continue.
scheme@(guile-user) [1]> ,bt
In current input:
3:0 1 (#<procedure a164730 at <current input>:3:0 ()>)
In unknown file:
0 (setter #<procedure car (_)>)
scheme@(guile-user) [1]>
Cheers,
David
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: syntax-rules problem
2011-04-03 4:34 syntax-rules problem David Pirotte
@ 2011-04-03 19:16 ` Andreas Rottmann
2011-04-05 1:26 ` David Pirotte
0 siblings, 1 reply; 3+ messages in thread
From: Andreas Rottmann @ 2011-04-03 19:16 UTC (permalink / raw)
To: David Pirotte; +Cc: guile-user
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
--
Andreas Rottmann -- <http://rotty.yi.org/>
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: syntax-rules problem
2011-04-03 19:16 ` Andreas Rottmann
@ 2011-04-05 1:26 ` David Pirotte
0 siblings, 0 replies; 3+ messages in thread
From: David Pirotte @ 2011-04-05 1:26 UTC (permalink / raw)
To: Andreas Rottmann; +Cc: guile-user
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
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