syntax-rules problem

syntax-rules problem

[David Pirotte]

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

Re: syntax-rules problem

[Andreas Rottmann]

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/>

Re: syntax-rules problem

[David Pirotte]

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