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.
