Here is the definition of + from lib/_num.scm:
(define-prim-nary (+ x y) 0 (if (##number? x) x '(1)) (##+ x y) macro-force-vars macro-no-check (##pair? ##fail-check-number))
The define-prim-nary macro will expand this to an n-ary procedure definition where the 0 argument case returns 0, the 1 argument case returns the argument if it is a number otherwise it raises a type error (by calling ##fail-check-number), and the general >= 2 argument case calls ##+ to fold the argument list. All arguments are passed to macro-force-vars to force the argument if it is a promise (and --enable-auto-forcing is used).
Using set! to “short-circuit” promises is not a good idea because it introduces a cell for the variable (if local and not previously assigned) and this slows things down. In early versions of Gambit (on Motorola 68K), the garbage collector did this short-circuiting (i.e. a reference to a promise was replaced with the value of the promise if it was previously forced). This isn’t done currently but probably easy to add.
Marc
On Jun 15, 2017, at 11:04 PM, Adam adam.mlmb@gmail.com wrote:
2017-06-13 5:30 GMT+07:00 Bradley Lucier lucier@math.purdue.edu:
On Jun 11, 2017, at 5:08 AM, Adam adam.mlmb@gmail.com wrote:
Possibly the "macro-force-vars", which is used all over the runtime and compiler, would have something to do with this, but I don't find its definition anywhere.
In _gambit#.scm:
(macro-define-syntax macro-force-vars (lambda (stx) (syntax-case stx () ((_ vars expr) (if (let* ((co (##global-var-ref (##make-global-var '##compilation-options))) (comp-opts (if (##unbound? co) '() co))) (assq 'force comp-opts))
(syntax-case (datum->syntax #'vars (map (lambda (x) `(,x (##force ,x))) (syntax->list #'vars))) () (bindings #'(let bindings expr))) #'expr)))))Ah right, https://github.com/gambit/gambit/blob/9c3dcbdc322a10673370c0880696ba13114425... , and used to be a define-macro, https://github.com/gambit/gambit/blob/29103e6a29b8fbbf7d6fc772a344b814be3f1c... , and all the rest of the code is meticulously padded with its use.
This also sheds a bit of light on why the slot containing the promise is not replaced with the forced value. Maybe that would be possible in some situations though.. When |x| is a symbol, it could be |set!| with the forced value?
That would cover standard variable slots and not typedef, vector, pair etc. slots though, I guess I'd need to dig in a bit more to understand how this one actually works out. If you have any spontaneous ideas, feel free to share.
Any idea where in the sources fundamental primitives like |+| , |if| , |or| autoforce?