How to define top-level thenable and generatorish variable with function attributes in Notebook?

[pearmini]

I implemented a proxy object (source) called shape to build SVG elements, such as:

const svg = shape.svg({width: 100, height: 100}, [
  shape.rect({x: 0, y: 0, height: 100, width: 100, fill: "black"}),
  shape.circle({cx: 50, cy: 50, r: 40, fill: "white"}),
]);

document.body.append(svg);

A simple implementation is as follows:

shape = new Proxy({}, {get: (_, name) => () => document.createElement("name")})

But it threw an error when I defined it at the top-level:

I found that it automatically called shape.then, shape.next and shape.return. After reading the source code, I discovered actual issue: the runtime can't handle thenable and generatorish variable as expected, especially when they have function attributes:

The generatorish issue is easy to solve relatively with a stricter check in generatorish.js:

export function generatorish(value) {
  if (!value) return false;
  const tag = value[Symbol.toStringTag];
  return tag === "Generator" || tag === "AsyncGenerator";
}

If "generatorish" is required, it is also possible to make it work with proxies like shape:

export function generatorish(value) {
  return value
      && 'next' in value && typeof value.next === "function"
      && 'return' in value && typeof value.return === "function";
}

However the thenable issue is not as easy to solve, because runtime use promises to wrap the value:

// https://github.com/observablehq/runtime/blob/622a1974087f03545b5e91c8625b46874e82e4df/src/runtime.js#L231
 variable._promise = variable._promise
    .then(init, init)
    // This will return a promise that never resolves, 
    // because the return value has a "then" attribute,
    // which is a function but does not call resolve.
    .then(define) 
    .then(generate);

function define(inputs) {
  // ....
  return definition.apply(value0, inputs); // Returns the value itself.
}

I made som attempts but failed. Before going further, I just want to confirm if I'm on the right track:

• Are these real issues? I didn't find any docs or tests that shows runtime supports thenable and generatorish variable (if I missed them, please let me know!). I think they should act like normal variables.
• If they are real issues, is fixing them necessary? This seems like a rare situation.

If they are real issues and should be fixed, I'm willing to open a PR to address them! If not, I'd love to hear suggestions to make shape compatible with Notebook. While I can modify the source code of shape to filter then, next, and return, I'd prefer not to do this solely for Notebook!

Anyway, thanks for the great work of Notebook!

• Reproduce Notebook