Feasibility of native RTS support for continuations?

Alexis King lexi.lambda at gmail.com
Mon Feb 10 08:10:26 UTC 2020

> On Feb 6, 2020, at 02:28, Simon Marlow <marlowsd at gmail.com> wrote:
> The issue here is that raiseAsync is destructive - it *moves* the stack to the heap, rather than copying it. So if you want to continue execution where you left off, for shift#, you would have to copy it back onto the stack again. That's the point I was trying to highlight here.

Ah, yes, I see what you mean! It happens that for my use case I actually do want to unwind the stack when I capture a continuation, so that isn’t a problem for me.

> Yes, these are all the things that make raiseAsync tricky! You can either copy what raiseAsync does (but be warned, it has taken a lot of iteration to get right) or try to use raiseAsync and/or modify it to do what you want.

My point was more that I’m unsure that shift# should handle most of those cases. For raiseAsync, it makes sense, since asynchronous interrupts can, by their nature, occur at any time, even during pure code. But my shift# operation lives in IO, and the intent is to only capture up to a reset# in the same state thread.

My justification for this is that if you could use shift# in pure code, it would be ill-defined what you’d even be capturing. Suppose you return a thunk containing a call to shift#. When the thunk is evaluated, you capture up to the nearest reset#… but who knows what that is now? This opens you up to all sorts of general badness.

Therefore, I don’t think there should ever be an UPDATE_FRAME in the captured continuation—if there is, it’s probably a bug. So unless someone can think of any valid use cases, I’ll make that more explicit by modifying the continuation-capturing code to add some assertions that those frames never appear in the captured stack.
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