Re: Continuations

BGB <cr88192@gmail.com> wrote:

 
There is basically no real way to get dynamically allocated
stack-frames and argument copying to be performance competitive with
adjusting a stack pointer and passing arguments in registers.
Function calls are also, not exactly rare.

There is a way to combine a fast implementation and allow
continuations, and I think it's been done several times.

Proceed as usual, using a stack-based ABI, until a continuation is
captured. At that point, walk the stack and create the continuation by
copying stack frame contents into dynamically-allocated continuation
frames. Pass the continuation to whoever wants it. They may evaluate
it, inspect it, or whatever.

This does require precise information about the current stack frame to
be kept at all call sites, but that doesn't have to be much more than
the usual debuginfo.

I believe this technique was invented by Peter Deutsch for Smalltalk,
and it's used today for Kotlin.

Worse still, one is very likely going to need a garbage collector
(in the off chance anyone actually uses continuations, if the
continuation is captured via call/cc or similar, can no longer
automatically free it).

It doesn't really need GC, as long as it's the recipient's job to free
the continution. But I can't immediately think of any language with
continuations and without GC. They kinda go together.

Andrew.