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Comment by gumby | Hacker News Reader

Are jumps unrestricted or block only? ISTR in the first draft of WASM it was impossible to write continuation-passing code, coroutines, or even tail call.

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There's a proposal for effect handlers that allows for continuatios, coroutines and others.

> effect handlers as a unifying mechanism to enable efficient compilation of control idioms, such as async/await, generators/iterators, first-class continuations, etc.

https://wasmfx.dev/

So a coroutine would be a “stack” one frame deep?

Here's a good explanation: https://github.com/WebAssembly/stack-switching/blob/main/pro...

Block only. There’s a tail call proposal[1] that’s in phase 4 (nearly standardized).

[1]: https://github.com/WebAssembly/proposals

Note that phase 4 is the final stage of the standard process. Tail calls are at phase 4, which means that is a finalized and complete standard.

All that is left after that is for VMs to implement the standard, which is tracked here:

https://webassembly.org/features/

Most major VMs do support tail calls today, but not all.

Also note that that webpage can be somewhat out of date; for instance, see here for some recent edits to it (e.g. features Node had implemented that were marked as unavailable): https://github.com/WebAssembly/website/commits/main/

Jumps can either target the top of a 'loop' block or the bottom of a regular block, and the jump has to be inside of the block it's targeting.

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