When an actor is passed as a part of a struct, it isn't an `Actor` instance

[mathematician]

Describe the bug
When an actor is passed as a part of a struct, it isn't an Actor instance (but should be).

To Reproduce
Create a Motoko and a JavaScript file (this repo):

Motoko:

actor class canvalue_backend() = this {
  public type R = {
    value: canvalue_backend;
  };
  public query func test() : async R {
    return { value = this };
  };
};

TypeScript:

import { Actor } from "@dfinity/agent";
import { canvalue_backend } from "../../declarations/canvalue_backend";

const ac = await canvalue_backend.test();
Actor.canisterIdOf(ac.value)

Error:

Argument of type 'canvalue_backend' is not assignable to parameter of type 'Actor'.
  Property '[metadataSymbol]' is missing in type 'canvalue_backend' but required in type 'Actor'.

Generate:

dfx generate

Inspect the TypeScript code src/canvalue_frontend/src/index.ts in a text editor (such as MS Code) capable to spot TypeScript type errors.

Expected behavior
ac should be an instance of Actor.

Desktop (please complete the following information):

• OS: Ubuntu Linux
• Version 23.04

[Yan Chen]

This is working as expected. Here is the did file from the Motoko code:

type canvalue_backend =
  service {
    test: () -> (R) query;
  };
type R = record {value: canvalue_backend;};
service : {
  test: () -> (R) query;
}

test() returns a record, and ac.value is a service reference (see https://internetcomputer.org/docs/current/references/candid-ref#type-service-), which contains the canister id as a Principal. It's not an Actor instance.

[mathematician]

It's not an Actor instance.

But it should be an Actor instance. This is what I ask in my bug report.

[Yan Chen]

You can construct an Actor from the principal, but it's current not done automatically. It can be a feature request, but it's not a bug, the current format is specified in the doc.

[mathematician]

You can construct an Actor from the principal

No, ac in my original message isn't a Principal. It is some value that I have no idea how to use it.

Moreover,

  public query func getRootItem(): async ?(CanDBPartition.CanDBPartition, Nat) {
    rootItem;
  };

(here CanDBPartition is an actor class).

    main.getRootItem().then((data) => {
    })

crashed with the browser console message Uncaught (in promise) Error: Illegal service definition: services can only contain functions. So, it apparently crashes on any access to the value. It is a bug.

[mathematician]

The reverse case of passing an actor as an argument does work, but its value needs to be converted into a principal before passing. This seems to work:

actor class canvalue_backend() = this {
  public query func test(value: canvalue_backend) : async () {
  };
};

import { Actor } from "@dfinity/agent";
import { canvalue_backend } from "../../declarations/canvalue_backend";

await canvalue_backend.test(Actor.canisterIdOf(canvalue_backend));

A bad thing here is that if we allow (as it would be in an ideal world) passing directly an actor, this would create past compatibility problems (that however can be solved by expecting Actor | Principal.

[Yan Chen]

No, ac in my original message isn't a Principal. It is some value that I have no idea how to use it.

ac is a record. ac.value is a principal. If not, can you console.log(ac.value)?