Re: Smobs and setters?

[Daniel Skarda]

Hello,

  it is quite strange to reply to my own mail :) I was playing with guile
source, I extended smob with scm_set_smob_setter and scm_setter function with
ability to return smob's setter. Than I hit the wall :)

  Setters for smobs are different from procedure setters - they require one more
argument - smob itself (it is more like method than procedure). Current
generalized `set!' macro does:

   (set! var val)         
      -=>   (real-set! var val)

   (set! (proc arg1 ..) val)
      -=>   ((setter proc) arg1 ... val)

  I can think about two ways how to simply add smob setters to guile - rewrite
generalised set! macro to return

   (set! (dest arg1 ..  argn) val)

      -=>   (if (smob? val)
                ((setter dest) dest arg1 .. argn val)
                ((setter dest) arg1 ... argn val))

     or

      -=>   (setter-aply dest arg1 ... argn val)

   where setter-apply implements on C level what was done in previous example in
scheme level (if ...). I do not like the first way because it does not look
elegant (but `if' is imho necessary) and I do not know if setter-apply can by
created by scm_c_define_subr (I browsed through eval.c looking for 
implementation
of apply and I was quite mixed up - but maybe further study of guile sources
clears it :)

   As I am writing this mail I realized that I have not looked into GOOPS and
(as scm_setter function suggests) into structs/objects. How these areas of guile
would affect final solution?

Best regards,
Dan Skarda
 
> Hello,
> 
>   in my application I took an advantage of scm_set_smob_apply. Imagine we 
> create
> vec3d
> 
>                vec3d_tag = scm_make_smob_type ("vec3d", 3 * sizeof (double)); 
> 
>   I know that the usual Scheme way is to define two function - `vec-ref' and
> `vec-set!' for accessing values stored in vec3d. Using setter/getter we would
> like to write:
> 
>      (define v (make-vec 0 1 2))
> 
>      (v 1)
>         -=> 1
> 
>      (set! (v 2) 4)
>      v
>         -=>  #<vec3d 0 1 4>
> 
>   It is possible to implement getter (v 1) using scm_set_smob_apply. However 
> it
> is not possible to write setter function. It is possible to make procedure 
> with
> setter, but it is not possible to attach setter to smobs :-(
> 
>   I am sorry that my mail ends with "request for feature" instead of patch - 
> but
> my knowledge of guile internals is not that deep :(
> 
> Best regards,
> Dan Skarda
> 
> 
> 
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