Web Assembly, struct to JavaScript Object and back

Tue Jun 23 18:15:25 UTC 2020

I'm experimenting with generating wasm files with ldc2 but I'm 
having problems when trying to pass JavaScript Objects and 
receive them as structs and the other way around.

I found this super nice getting started guide that works fine for 
basic types like double and so on 
https://wiki.dlang.org/Generating_WebAssembly_with_LDC , but I'm 
puzzled on how to pass more advanced data structures.

I found this C example where it's mentioned that structs in that 
case will always be passed by pointer 
https://stackoverflow.com/questions/50615377/how-do-you-call-a-c-function-that-takes-or-returns-a-struct-by-value-from-js-v#answer-56405899 . Is something similar possible with D?

My setup so far:

test.d:

extern (C): // disable D mangling

void callback(double a, double b, double c);

double add(double a, double b) {
   const c = a + b;
   callback(a, b, c);
   return c;
}

struct Vector2 {
   double x;
   double y;
}

Vector2 addVectors(Vector2 a, Vector2 b) {
   return Vector2(a.x + b.x, a.y + b.y);
}

void _start() {
}

index.html:

<html>
   <head>
     <script>
       const callback = (a, b, c) => {
         console.log(`callback from D: ${a} + ${b} = ${c}`);
       };

       fetch('test.wasm').then((response) => 
response.arrayBuffer()).then((code) => {
         const importObject = {env: {callback}};
         WebAssembly.instantiate(code, importObject).then(result 
=> {
           const {exports} = result.instance;

           const r = exports.add(42, -2.5);
           console.log('r = ' + r);

           const r2 = exports.addVectors({x: 2, y: 4}, {x: 1, y: 
-1.5});
           console.log('r2 =', r2);
         });
       });
     </script>
   </head>
   <body>
     Test page, open console to see the WASM results
   </body>
</html>

Then I build test.d with:

$ ldc2 -mtriple=wasm32-unknown-unknown-wasm -L-allow-undefined 
-betterC test.d

When I serve and open the address in a browser and open the 
console I get:

r = 39.5
r2 = undefined

Anyone that has something similar working with struct objects?