Unexpected TypeError when experimenting with arrays

[Karol Walasek]

Bug Report

Overview

An unexpected error occurs when running a particular piece of code. No proper error description is given so there is no way of telling what is wrong.

Expected

A proper error message should be shown.

Actual

S:\GIT\walt-tests\node_modules\walt-compiler\dist\walt.js:5213
    body.push(index_9, kind.code, `${kind.text}  ${debug ? debug : ''}`);
                            ^

TypeError: Cannot read property 'code' of undefined
    at code.forEach (S:\GIT\walt-tests\node_modules\walt-compiler\dist\walt.js:5213:29)
    at Array.forEach (<anonymous>)
    at emitFunctionBody (S:\GIT\walt-tests\node_modules\walt-compiler\dist\walt.js:5211:8)
    at functions.forEach.func (S:\GIT\walt-tests\node_modules\walt-compiler\dist\walt.js:5273:29)
    at Array.forEach (<anonymous>)
    at emit$7 (S:\GIT\walt-tests\node_modules\walt-compiler\dist\walt.js:5273:13)
    at Object.ast [as code] (S:\GIT\walt-tests\node_modules\walt-compiler\dist\walt.js:5442:19)
    at emit (S:\GIT\walt-tests\node_modules\walt-compiler\dist\walt.js:5483:350)
    at compile (S:\GIT\walt-tests\node_modules\walt-compiler\dist\walt.js:5824:16)
    at Object.<anonymous> (S:\GIT\walt-tests\mat.js:3:16)

Example

This snippet fails (required npm install walt-compiler)

const { compile } = require('walt-compiler');

const buffer = compile(`
const memory: Memory<{initial: 1}>;

type ResultFnType = (i32) => void;

export function func(a: i32, l: i32, f: ResultFnType): i32 {
	let v: i32 = 0;
	let i: i32 = 0;
	for(i; i < l; i+=1)
		v += a[i];
	f(v);
}
`).buffer();

WebAssembly.instantiate(buffer).then(result => {
	let sum;
	result.instance.exports.func([1, 2, 1], 3, (r) => sum = r);
	console.log(sum);
});

Offtop

It would be nice if there was an example on how to work with array arguments/array results in Walt (or if it's even possible at the moment).

[Arthur Buldauskas]

Thank you for reporting this. There is a fix in a PR for improved messaging in flight #165

To help debug any other potential issues, could you provide the result of require('walt-compiler').VERSION.

A couple of notes on your code above though.

• passing arrays does not work like that from JS. You have to write the values into the memory instance. You would either export the memory from the module or import into the module from JS.
• Similar story for passing functions to other methods. You would need to use the web assembly table or import the function explicitly. (Tables can only hold other wasm functions)

[Arthur Buldauskas]

Had a chance to validate your code snippet. Turns out that you had a subscript on a non array type, which when generated created an undefined opcode. This caused a hard fatal as you already brought up.

You want to change this line

export function func(a: i32, l: i32, f: ResultFnType): i32 {

to

export function func(a: i32[], l: i32, f: ResultFnType): i32 {

then you should be able to access a like an array via a[i]

Thanks for reporting this, the code snippet was very helpful!