Eduardo,
On Oct 16, 2011, at 01:02 , Eduardo Cavazos wrote:
> "boot-eval.c" has an implementation of set-car however "eval.l" doesn't. Have
> mutable pairs been deprecated? :-)
eval.l contains little more than the minimum needed to run boot.l and emit.l.
boot-eval.c is a superset of eval.l because a few things that were used for
other experiments, but not needed to run boot.l and emit.l, were never removed.
> I added this implementation of set-car to my "eval.l" and it seems to work:
>
> (define-function subr_set_car (args env)
> (arity2 "set-car" args)
> (let ((arg (get_head args)))
> (if (is <pair> arg)
> (set_head arg
> (get_head (get_tail args)))
> ())))
Here's what's in my longer version of boot.l. These are more permissive about
their arguments (in the spirit of '(cdr (assq ...))') but sightly slower
because of the additional overhead of the calls to k_c*r and the tests they
perform for <pair>ness.
(define-function subr_set_car (args ctx)
(let ((arg (k_car args)))
(and (is <pair> arg)
(put <pair> head arg (k_cadr args)))))
(define-function subr_set_cdr (args ctx)
(let ((arg (k_car args)))
(and (is <pair> arg)
(put <pair> tail arg (k_cadr args)))))
> When I used "(pair? arg)" instead of "(is <pair> arg)" the build would fail.
'pair?' is an evaluator-only function that works on objects, whereas '(is
<type> obj)' is a macro that expands to low-level code that takes apart a
pointer and/or the structure that it references to figure out if the
representation of an object makes it smell like a <type>. You can use the
former only in interpreted code and the latter only in compiled code.
Regards,
Ian
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