Generators in Lone Lisp | Hacker News Reader

Generators in Lone Lisp

matheusmoreira.com

I like the overview given in this Stackoverflow answer [1] (based on an even earlier comment) which classifies different types of continuations:

- Asymmetric or symmetric

- Stackful or stackless

- Delimited or undelimited

- Multi-prompt or single prompt

- Reentrant or non-reentrant

- Clonable or not

Based on that these generators (or semi-coroutines as the article also calls them) seem to be asymmetric, stackful, delimited, single prompt(?), non-reentrant continuations.

[1] - https://stackoverflow.com/questions/62817878/what-are-the-sp...

That's a great overview. Yeah they are asymmetric, Wikipedia says symmetric and asymmetric correspond to coroutines and semicoroutines. They are also stackful and delimited. They are single shot by design, though I could easily make it possible to restart the generator from scratch if needed.

As for single prompt vs multiprompt... I'm not too sure about this one. I have a check to prevent recursion but nesting generators shouldn't be a problem since they keep track of their own callers.

I think lone's generators have composability issues due to the stack separation. For example, calling a generator g2 inside another generator g1 doesn't transparently yield values from g2 to g1's caller. I've been wondering about how to fix this without a Python-like yield from primitive.

Very cool stuff and great written article. Lone looks very interesting!

Are you planning to use your design to support things like exceptions as well? I think that's where that multi-prompt ability becomes important (yielding from a nested coroutine). Racket has prompts and aborts, which is essentially a 'yield to' primitive.

Thank you. Proper exception/condition handling is a high priority item on my list. It's something that is long overdue... Lone currently hard exits on literally any error.

My delimited continuations article also talks about exception handling:

https://www.matheusmoreira.com/articles/delimited-continuati...

Throw/catch are just delimited continuations which don't actually capture the continuation. The machinery is already in place. If I copied the control/transfer primitives and removed the continuation code, I'd have a functional exceptions handling mechanism.

In suppose I can use tagged prompts to catch some exceptions and not others. Place a symbol in the stack delimiter, then the transfer primitive can look for it and skip anything that doesn't match. Untagged prompts just use nil as the tag.

Fun read! If anyone is interested in more, I believe Andy Wingo has written about the implementation of delimited continuations in guile scheme over at https://wingolog.org

I love his blog. I think I've read almost the entirety of it. Here's his article about delimited continuations:

https://www.wingolog.org/archives/2010/02/26/guile-and-delim...

I even cited it in my own delimited continuations article:

https://www.matheusmoreira.com/articles/delimited-continuati...

After publishing that I actually emailed him about it. I wanted to show him how I solved the interleaving stacks problem he outlined in that article. No idea if he ever saw the email but his about page does mention the fact email has very variable latency.

This is awesome!

I haven't seen Lone Lisp before. Is it meant to be like a Symbolics Lisp Machine, where the entire userspace is lisp?

I really like using generators in typescript. They make a lot of problems much easier.

The idea is to create a language good enough to build a Lisp user space for Linux. I didn't dare to call it a lisp machine at first but other users suggested that when it was first submitted here on HN. Here's the discussion:

https://news.ycombinator.com/item?id=38126052

I'm doing some plumbing work on lone right now, then I'll start designing the iterator protocol. Gotta fix some usability details in the generators to make them more pleasant to use but I'm really proud of the fact that they even work at all. I'll read about typescript generators and try to understand what makes them great to use.

Thank you for great articles and what seems to be a very nice Lisp! I've written so many toy Lisps over the years that I've lost count, always fun to see when they are done more seriously.

A suggestion to the website index, could you add dates to the entries? I find the numbered list confusing as its not entirely clear in what order they are listed.

Thank you for your kind words and for the website feedback. I apologize for the confusion. I have edited the website index and replaced each article's list marker with its publish date.

In typescript/js, you can iterate over async generators with for-await-of. Very ergonomic.

This article sums it up pretty well: https://medium.com/@bhagyarana80/why-async-generators-were-t...

In nodejs, all streams are async iterables, so you can iterate over them. Really nice for handling stuff like connections to a server, or messages on a socket.

I’ve use generators to code ui “sagas”. You await async events like clicks etc, and yield the appropriate state updates. A sub-dialog can be implemented as a separate generator, and the main dialog generator can open it by with the yield* syntax to iterate over and pass on all the events until the generator returns. The return value would be the result of the dialog.

Super nifty.

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