Thank you all so much! I'm glad it wasn't a huge mystery in the end. -------- Original Message -------- On Jun 17, 2020, 9:13 AM, Matthew Flatt wrote:
> Yes, clearly a BC compiler bug --- and probably almost as old as > Racket. The bug was specific to `set!` on a locally bound variable as > the first subexpression of `begin0`. > > I've pushed a repair. > > Thanks! > > At Wed, 17 Jun 2020 03:35:51 -0500, Alexis King wrote: >> This is quite curious. It appears to be a compiler bug. Here’s a very >> slightly >> smaller test case: >> >> #lang racket/base >> (define count! >> (let ([i 0]) >> (λ () (begin0 >> (set! i (add1 i)) >> (+ i))))) >> (count!) >> >> The fully-expanded program looks fine, so it isn’t the expander’s fault: >> >> (module counter racket/base >> (#%module-begin >> (module configure-runtime '#%kernel >> (#%module-begin (#%require racket/runtime-config) (#%app configure >> '#f))) >> (define-values >> (count!) >> (let-values (((i) '0)) >> (lambda () (begin0 (set! i (#%app add1 i)) (#%app + i))))) >> (#%app call-with-values (lambda () (#%app count!)) print-values))) >> >> But `raco decompile` reveals that begin0 has been mysteriously replaced with >> begin in the compiled program: >> >> (module counter .... >> (require (lib "racket/base.rkt")) >> (provide) >> (define-values >> (count!) >> (let ((local54 '0)) >> (begin >> (set! local54 (#%box local54)) >> (lambda () >> '#(count! #<path:/tmp/counter.rkt> 4 4 53 63 #f) >> '(flags: preserves-marks single-result) >> '(captures: (val/ref local54)) >> (begin >> (#%set-boxes! (local54) (add1 (#%unbox local54))) >> (+ (#%unbox local54))))))) >> (#%apply-values print-values (count!)) >> (void) >> (module (counter configure-runtime) .... >> (require '#%kernel (lib "racket/runtime-config.rkt")) >> (provide) >> (print-as-expression '#t) >> (void))) >> >> It seems perhaps an optimization has gone awry. The bug appears to be quite >> old: I can reproduce it as far back as 6.1.1. (I didn’t test any versions >> earlier than that.) Unsurprisingly, the issue does not occur on Racket CS, >> which is consistent with the hypothesis that this is a compiler bug. >> >> Alexis >> >> > On Jun 17, 2020, at 02:04, Sage Gerard <[email protected]> wrote: >> > >> > I attached a video demonstrating what I'm seeing. In case it does not load >> or is not available, I'll summarize here. Forgive any typos; it's been a late >> night of coding. >> > >> > Here's a module with an incorrect counter. It's incorrect because it uses >> begin0, and is therefore expected to return void instead of an incrementing >> integer. >> > >> > #lang racket >> > (provide count!) >> > (define count! >> > (let ([i 0]) >> > (λ () (begin0 >> > (set! i (add1 i)) >> > (~v i))))) >> > >> > Notice that I added the ~v to format the return value. If I launch Racket >> v7.7.0.5 using racket -it prog.rkt, (count!) returns the formatted value. But >> if I remove the ~v, it behaves as written (returning void). >> > >> > The video shows the behavior with and without ~v, both in DrRacket and >> racket. DrRacket is the only environment that consistently runs the code >> correctly. >> > >> > ~slg >> >> -- >> 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 [email protected]. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/racket-users/781BE0E5-E6E5-4F0B-8B46-5227FDFB >> 8835%40gmail.com. > > -- > 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 [email protected]. > To view this discussion on the web visit > https://groups.google.com/d/msgid/racket-users/20200617071306.bc%40sirmail.smtp.cs.utah.edu. -- 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 [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/IH8l5cBwN3MPSWGHBWcbvDTEMikj3_ZDa5UWVtZ36bEabxTUzD8qamaPRE42q8U6mYRKYGKWit9Zh6cGYc7J7ibz2DmCeJ3dt0_4zJU5Vyc%3D%40sagegerard.com.
