Hi Matthew, Thanks for the explanations. But I'm still not convinced that the top-level-bind-scope is needed. This is my current understanding. The purpose of the top-level-bind-scope is to support recursion better at the top level. But for the case of `(define-values (x) ...)`, if `x` is not defined yet, then implicit #%top in `...` will let `...` refer to `x`. If `x` is defined, then the old definition will be used by `...`. Either way, the top-level-bind-scope is not needed. For the case of `(define-syntaxes (x) ...)`. As you explained, a macro can naturally recursively refer to itself, simply because of how macro expansion works, so the top-level-bind-scope is again not needed for recursion. Is my understanding correct?
On Monday, March 23, 2020 at 10:05:12 AM UTC-4, Matthew Flatt wrote: > > At Mon, 23 Mar 2020 01:45:40 -0700 (PDT), Yongming Shen wrote: > > I tried the example you gave for my first question and it resulted in an > > error. > > Oops --- you're right. I lost track of what we try to make work at the > top level. > > > I think this is because `(define-values (x) ...)` expands `...` without > the > > top-level-bind-scope, even when expand-context-to-parsed? is #t > (according > > to expander/expand/top.rkt). Is this a bug? > > Since the behavior goes far back, I think this is the behavior that we > decided to settle for. > > > Related to your answer to my second question, `define-syntaxes` > similarly > > does not add the top-level-bind-scope when expanding `...`. Does this > mean > > that even for `define-syntaxes`, `...` won't use the > top-level-bind-scope > > binding(s) after all? > > The way that evaluation, binding, and expansion are interleaved means > that a `define-syntaxes` macro can refer to itself in expansions. The > binding of an identifier in a macro template is resolved after the > macro is applied. > > The difference with `define` is that the right-hand side is > expanded/compiled before `define` binds. > > > A little bit off-topic, in the definition of define-values (in > > expander/expand/top.rkt), there is `(define-match m s ...)`, but for > > define-syntaxes it is `(define-match m disarmed-s ...)`. Is this > difference > > significant? Or does define-match not care whether `s` or `disarmed-s` > is > > used? > > Using `disarmed-s` in the definition of `define-values` is probably > a better idea, and I'll look into that more. > > It think it turns out not to matter, normally, because `define-values` > is transparent to syntax arming via `syntax-arm` with a #t second > argument (which is what the expander does). But it would be better to > not rely on that. > > -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/97759d2c-4d94-422e-b984-5ec9001b0fca%40googlegroups.com.
