Arc uses mutable cons, but Racket has immutable cons. So it's a problem.
In Racket BC mutable and immutable cons use the same struct at the C level, so both are quite fast and almost interchangeable, if you cross your fingers that the optimization passes don't notice the mess and get annoyed (somewhat like UB in C).
In Racket CS immutable cons are implemented as cons in Chez Scheme, but mutable cons are implemented as records in Chez Scheme, so they are not interchangeable at all.
Arc used a magic unsafe trick to mutate immutable(at the Racket level) cons that are actually mutable(at the Chez Scheme level) cons. The trick is slow because the Racket to Chez Scheme "transpiler" doesn't understand it and does not generate nice fast code.
One solution is to rewrite Arc to use mutable cons in Racket, but they are slow too because they are records in Chez Scheme that have less magic than cons in Chez Scheme. So my guess it that it will be a lot of work and little speed gain.
[Also, ¿kogir? asked a long time ago in the email list about how to use more memory in Racket BC, or how to use it better or something like that. I think he made a small patch for HN because it has some unusual requirements. Anyway, I'm not sure if it was still in use.]
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The takeaway is that mutable cons are slow in Racket.
Arc uses mutable cons, but Racket has immutable cons. So it's a problem.
In Racket BC mutable and immutable cons use the same struct at the C level, so both are quite fast and almost interchangeable, if you cross your fingers that the optimization passes don't notice the mess and get annoyed (somewhat like UB in C).
In Racket CS immutable cons are implemented as cons in Chez Scheme, but mutable cons are implemented as records in Chez Scheme, so they are not interchangeable at all.
Arc used a magic unsafe trick to mutate immutable(at the Racket level) cons that are actually mutable(at the Chez Scheme level) cons. The trick is slow because the Racket to Chez Scheme "transpiler" doesn't understand it and does not generate nice fast code.
One solution is to rewrite Arc to use mutable cons in Racket, but they are slow too because they are records in Chez Scheme that have less magic than cons in Chez Scheme. So my guess it that it will be a lot of work and little speed gain.
[Also, ¿kogir? asked a long time ago in the email list about how to use more memory in Racket BC, or how to use it better or something like that. I think he made a small patch for HN because it has some unusual requirements. Anyway, I'm not sure if it was still in use.]
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The takeaway is that mutable cons are slow in Racket.