On 2013-03-17, at 7:25 PM, Jason Felice jason.m.felice@gmail.com wrote:
Is there a way to register or implement a printer for a specific foreign type?
-Jason
The Scheme printer (that is used for the implementation of write, display, pretty-print, and print) is bound to the global variable ##wr, which can be assigned. By redefining ##wr it is possible to intercept the printing of all objects, and to filter out the foreign objects.
Here's some sample code. This approach could be integrated with a macro to define C structs or classes so that a printer for that type is automatically created.
Marc
(include "~~lib/_gambit#.scm")
(define foreign-writers (make-table))
(define (write-foreign we obj) (case (macro-writeenv-style we) ((mark) (##wr-mark we obj)) (else (let ((tags (##foreign-tags obj))) (if (##pair? tags) ((table-ref foreign-writers (##car tags) ##wr-foreign) we obj) (##wr-foreign we obj))))))
(let ((old-wr ##wr)) (set! ##wr (lambda (we obj) (if (##foreign? obj) (write-foreign we obj) (old-wr we obj)))))
(c-declare #<<end-of-c-declare
#include <stdlib.h>
typedef struct { int x, y; } point;
point *make_point( int x, int y ) { point *p = ___CAST(point*, malloc(sizeof(point))); p->x = x; p->y = y; }
int point_x( point* p ) { return p->x; } int point_y( point* p ) { return p->y; }
end-of-c-declare )
(c-define-type point "point") (c-define-type point* (pointer point))
(define make-point (c-lambda (int int) point* "make_point")) (define point-x (c-lambda (point*) int "point_x")) (define point-y (c-lambda (point*) int "point_y"))
(define p (make-point 11 22))
(pp p) ;; prints: #<point* #2 0x1006064d0>
(table-set! foreign-writers 'point* (lambda (we obj) (##wr-str we "{") (##wr we (point-x obj)) (##wr-str we ",") (##wr we (point-y obj)) (##wr-str we "}")))
(pp p) ;; prints: {11,22}