Nested instruction representation in AST

[maurobringolf]

Our AST representation has different ways of representing nested instruction

• node.args: https://github.com/xtuc/webassemblyjs/blob/master/packages/wast-parser/test/fixtures/return/expected.json#L18

• node.instr: https://github.com/xtuc/webassemblyjs/blob/master/packages/wast-parser/test/fixtures/instruction/nested-block/expected.json#L20

• node.instrArgs: https://github.com/xtuc/webassemblyjs/blob/d73d782786a02d97b21dd9285e20ada9911c219c/packages/wast-parser/test/fixtures/wast-call-instruction/expected.json#L11

This makes code working with them (in my case the type checker) unnecessarily complicated. Can we unify the representation and always use the same field? Which one?

[xtuc]

Following our discussion on Slack. I would suggest adding a transformation that flatten the AST before the Module compilation (like the identifier ⟶ index transformation).

That would allow different representation for printing and interpreting, which means that we don't have the original source during interpretation and error message could become confusing. We can fix it later using sourcemaps or something.

[ColinEberhardt]

I'd vote for args

[maurobringolf]

args makes sense to me, although there is some overlap with arguments that are not instructions but literals (indices, numberliterals). These are currently also stored under args, in i32.const 0 for example. Maybe worth separating the two

[ColinEberhardt]

Good point - I like how it is documented here:

https://github.com/sunfishcode/wasm-reference-manual/blob/master/WebAssembly.md#constant

In this case, i32.const 0, the zero is an immediate, rather than an argument. It is not part of the signature of the const instruction.

[ColinEberhardt]

Basically, I think args should be used for anything that can be written in a nested form, i.e.

(f64.const 0)
(f64.abs)

can be nested as ...

(f64.abs (f64.const 0))

which reflects the fact that the signature for abs https://github.com/sunfishcode/wasm-reference-manual/blob/master/WebAssembly.md#floating-point-absolute-value is (f64) : (f64)