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by mbrock 1474 days ago
I think a major limitation is that a WebAssembly module can’t run dynamically generated code, which is a huge part of typical Common Lisp implementations.
3 comments
jhgb 1473 days ago
I was thinking about this, and one solution that came to my mind was serializing the heap into a temporary image and restarting the environment from an updated Wasm module with new code. This would also collect all garbage at the point of transitioning to the new code. You might need to be very careful to do this reasonably quickly, but in principle I don't see why this wouldn't work. The idea came to me when I was thinking about how an interactive-but-native-speed Oberon environment could be implemented in Wasm.
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nickmain 1474 days ago
You could certainly generate a new Wasm module on the fly and then execute it from Javascript. Linking and sharing of memory should be possible.

Pyodide can dynamically load libraries that are separate Wasm modules so it is worth checking out how that works.

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aidenn0 1474 days ago
IIRC inter-module calls are more expensive than intra-module calls though.

WASM also doesn't allow for functions with variadic return counts.

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pmoriarty 1474 days ago
"a WebAssembly module can’t run dynamically generated code"

Why does it have that limitation, and is there any hope that it will someday be overcome?

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aidenn0 1474 days ago
On your computer, heap, stack, code, and global data all share the same address space. On the WASM virtual machine, only heap and global data share the same address space. Code and stack are abstracted away by the machine.

It was designed to support C-like languages, so you can do the equivalent of loading a DLL (or dynamic shared object). However in Lisp it is not unusual to compile just one function at a time. It's definitely possible to create a DLL for each function and load each DLL, but it requires a completely different architecture than most Lisp implementations use (in which, when you compile a function, it just writes the machine code to the heap).

There are similar impedance mismatches with so-called W^X systems (where you are disallowed from having a page be both (W)ritable and e(X)ecutable.

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