[gus_massa]

Racket has a strong respect for back compatibility, so it should work in the current version. There has been a lot of improvements in the compiler (in particular changing the "back end" from C to Chez Scheme) but it's stil back compatible and sligthly faster.

[bmitc]

It is my recollection that the official Arc uses incompatible features. I can't remember where I read this, so it's entirely possible I am incorrect.

[gus_massa]

Arc mutates inmutable pairs (cons), it worked initially because at the C level mutable and immutable pairs were the same struct. Now there is a primitive unsafe-set-immutable-car! [1] for cases like this. It's a weird primitive and should be used with a lot of care as explained in the docs.

[1] https://docs.racket-lang.org/reference/unsafe.html#%28def._%...