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In TXR Lisp, I made delimited continuations by copying delimited sections of the native stack ("C stack"). Intrinsic functions in the Lisp runtime are not "for

In TXR Lisp, I made delimited continuations by copying delimited sections of the native stack ("C stack"). Intrinsic functions in the Lisp runtime are not "foreign"; capturing continuations across those is allowed. Just not across true foreign functions and certain library routines that interface with host functions, an example being ftw (file tree walk, wrapper for POSIX ftw). I have made it possible to throw exceptions out of the lambda callback invoked by ftw, but not to capture delimited continuations from there up across ftw itself. Speaking of exceptions (in the Blub language sense of "dynamic control transfers"), you would not want that to be based on delimited continuations, or certainly not ones based on this implementation technique. The pragmatic reason is that exceptions are bread-and-butter that you need in almost any everyday program, and that have to perform, whereas delimited continuations are boutique. Making exceptions work within delimited continuation contexts was also a little bit of an extra challenge/chore. When a saved continuation is resumed (blasted to the top of the current and reinstated) the exception-related wiring/plumbing inside that stack segment has to be surgically connected into that of the destination stack, for a correct graft. Basically the last/outermost exception handling frame of the grafted continuation segment has to fixed up to properly hook it into the exception frame linkage. In other words, there is a parallel exception stack consisting of linked frames, which runs through the native stack. A splice is needed there. To support the front-and-centre use case for delimited continuations, which is the ability to have yields with two-way communication (e.g. a recursive procedure can yield an item to a controlling procedure, which can then reply to it with a different item and resume it) I had to invent a new kind of dynamic control construct called "absconding". An abscond is exactly like a dynamic long return, except that it performs no unwinding! When a procedure yields, it is not unwinding. It needs to jump out to the context which is interested in the yielded item, but it must not dispose of its resources. There is the expectation that control will jump back in, and everything will need to be intact. Files that the procedure was working with must not be closed, etc. Absconding is an elegant solution which completely eliminates the need to even think about Scheme's dynamic-wind or similar. In situations in which absconding would lead to resource leakage, two mechanisms take care of it. Firstly, the program an explicitly "kill" a continuation by resuming it, and passing it a special "poison" value. When a continuation receives a poison value, it resumes as usual, and then immediately begins to unwind all the way to its prompt (top of its stack segment). After this unwinding is complete, the saved continuation object on the heap is marked as poisoned and cannot be resumed again. If that mechanism is not used, then we rely on the garbage collector to collect the resources in abandoned delimited continuations. The garbage collector does not automatically do the poisoning call on continuations. However, the application can use finalization for that; i.e. given a delimited continuation C, it could register a finalizer which calls (sys:cont-poison C). The obtain/yield implementation (stdlib/yield.tl) uses this trick.

matheusmoreira 1y ago
> TXR Lisp I remember. https://news.ycombinator.com/item?id=34740581 https://news.ycombinator.com/item?id=34740581 Nice to see you again! > you would not want that to be based on delimited continuations, or certainly not ones based on this implementation technique Yes. If I implemented exceptions on top of this, the continuation would be captured and then immediately thrown away. Needlessly wasteful. I don't really subscribe to Scheme's idea of implementing less powerful primitives in terms of more powerful ones. Conceptually it's amazing but I won't shy away from making new primitives in C if needed. I plan to add additional primitives for exceptions and generators at the very least. > To support the front-and-centre use case for delimited continuations, which is the ability to have yields with two-way communication (e.g. a recursive procedure can yield an item to a controlling procedure, which can then reply to it with a different item and resume it) I had to invent a new kind of dynamic control construct called "absconding". > An abscond is exactly like a dynamic long return, except that it performs no unwinding! When a procedure yields, it is not unwinding. It needs to jump out to the context which is interested in the yielded item, but it must not dispose of its resources. There is the expectation that control will jump back in, and everything will need to be intact. I think I get it... How did you implement this on your system? I implemented lone as a fully interruptible machine that can stop and yield control in between any computation step. I imagined that might come in handy for concurrency and parallelism someday. Wasn't sure how at first. The original idea was to run a separate machine per thread, or to multiplex numerous machines on a set of Linux threads. Now I'm playing with the idea of making the machine suspend execution by creating another machine with an independent stack, cycling it until it returns a value and then resuming. A machine subordinate to another machine. This concept would implement generators, coroutines... Does this make sense? Is this similar to what you implemented? I've read a lot about the complexity and difficulty of finalizers... I'm not sure how I'll implement them yet.
- kazinator 1y ago
  Abscond is really jut a crude hack: it's exactly like a dynamic block return in that it has a block symbol as its target. It finds the dynamic exist point by walking the dynamic frames looking for the block type with the right symbol. Only when it finds it, it simply jumps there without doing any unwinding. Jumping without unwinding is more primitive than with; e.g. C setjmp/lonjmp is more primitive than exceptions, right? It required a shift in attitude: in a Lisp with unwind-protect, we regard the unwinding as a Holy Cow. A dynamic control transfer guarantees that all unwinds are performed on penalty of the run-time implementor being struck by lightning. I had to convince myself that this heresy was beneficial. It's like threads. When the system switches from one thread to another, it doesn't unwind the resources of the thread left behind! With continuations, you can simulate coroutines, and as you go between them, you can't be unwinding and re-establishing resources. Not all resources can work that way. E.g. say there is a local variable holding a socket connection. You don't want to tear down and re-connect. Stuff has to stay intact. So how do you use the "abscond"? For instance to implement yield. yield works with obtain. obtain creates a dynamic block that is captured as a function. That function returns the yielded items. To continue the computation in the obtain block, you call the function again, to step it to the next yield. You can pass it an argument, which pops out of its yield call. This yield back and forth is facilitated by the "abscond" dynamic control transfers which don't unwind. The neat thing is that if, say, the yield process sis a recursive function with unwind protects, they all happen in its context undisturbed by the yield exchanges; it's like an exchange of virtual particles through a quantum tunnel.

matheusmoreira 1y ago

> TXR Lisp I remember. https://news.ycombinator.com/item?id=34740581 https://news.ycombinator.com/item?id=34740581 Nice to see you again! > you would not want that to be based on delimited continuations, or certainly not ones based on this implementation technique Yes. If I implemented exceptions on top of this, the continuation would be captured and then immediately thrown away. Needlessly wasteful. I don't really subscribe to Scheme's idea of implementing less powerful primitives in terms of more powerful ones. Conceptually it's amazing but I won't shy away from making new primitives in C if needed. I plan to add additional primitives for exceptions and generators at the very least. > To support the front-and-centre use case for delimited continuations, which is the ability to have yields with two-way communication (e.g. a recursive procedure can yield an item to a controlling procedure, which can then reply to it with a different item and resume it) I had to invent a new kind of dynamic control construct called "absconding". > An abscond is exactly like a dynamic long return, except that it performs no unwinding! When a procedure yields, it is not unwinding. It needs to jump out to the context which is interested in the yielded item, but it must not dispose of its resources. There is the expectation that control will jump back in, and everything will need to be intact. I think I get it... How did you implement this on your system? I implemented lone as a fully interruptible machine that can stop and yield control in between any computation step. I imagined that might come in handy for concurrency and parallelism someday. Wasn't sure how at first. The original idea was to run a separate machine per thread, or to multiplex numerous machines on a set of Linux threads. Now I'm playing with the idea of making the machine suspend execution by creating another machine with an independent stack, cycling it until it returns a value and then resuming. A machine subordinate to another machine. This concept would implement generators, coroutines... Does this make sense? Is this similar to what you implemented? I've read a lot about the complexity and difficulty of finalizers... I'm not sure how I'll implement them yet.

kazinator 1y ago
Abscond is really jut a crude hack: it's exactly like a dynamic block return in that it has a block symbol as its target. It finds the dynamic exist point by walking the dynamic frames looking for the block type with the right symbol. Only when it finds it, it simply jumps there without doing any unwinding. Jumping without unwinding is more primitive than with; e.g. C setjmp/lonjmp is more primitive than exceptions, right? It required a shift in attitude: in a Lisp with unwind-protect, we regard the unwinding as a Holy Cow. A dynamic control transfer guarantees that all unwinds are performed on penalty of the run-time implementor being struck by lightning. I had to convince myself that this heresy was beneficial. It's like threads. When the system switches from one thread to another, it doesn't unwind the resources of the thread left behind! With continuations, you can simulate coroutines, and as you go between them, you can't be unwinding and re-establishing resources. Not all resources can work that way. E.g. say there is a local variable holding a socket connection. You don't want to tear down and re-connect. Stuff has to stay intact. So how do you use the "abscond"? For instance to implement yield. yield works with obtain. obtain creates a dynamic block that is captured as a function. That function returns the yielded items. To continue the computation in the obtain block, you call the function again, to step it to the next yield. You can pass it an argument, which pops out of its yield call. This yield back and forth is facilitated by the "abscond" dynamic control transfers which don't unwind. The neat thing is that if, say, the yield process sis a recursive function with unwind protects, they all happen in its context undisturbed by the yield exchanges; it's like an exchange of virtual particles through a quantum tunnel.