Comment by samth
5 months ago
It's not true that you need to use CPS to implemented first-class continuations. There are plenty of slow ways to do it, and even if you want to be fast you can do multiple different things. Dybvig describes a number of options in his thesis: https://www.cs.unc.edu/xcms/wpfiles/dissertations/dybvig.pdf
Sam will definitely know more about this than I will, so if he contradicts me, listen to him.
If I am not mistaken, the racket language does not convert to CPS during compilation. Instead, when you want to get the continuation, I think you just get a pointer to the stack frame that you want. All I know for sure is that it uses something called a-normal form, which is kind of like SSA in some ways, and that the continuation is 2x 64/32-bit words depending on your architecture.
The main implementation of Racket today is built on top of Chez Scheme, which uses the techniques described by Dybvig that I linked to.
In the earlier implementation of Racket, indeed it doesn't convert to CPS but does use something like A-normal form. There, continuations are implemented by actually copying the C stack.