Canister events

[Hazel]

Thanks for the feedback! Keep it coming

@rossberg

A couple of drive-by comments:

• Candid is higher-order, so you can make SubscriptionRequest properly typed by passing real method handles of type (PublisherMessage) → () instead of a canister/method records.

type Publisher = service {
  icrc6_subscribe : (handler : (PublisherMessage) → ()) → (SubscriptionResponse);
  icrc6_unsubscribe : (handler : (PublisherMessage) → ()) → (SubscriptionResponse);
}

This was my first thought. But, last time I checked Motoko couldn’t extract the principal from the candid func type. This might not ever be an issue. Happy to use func but I wanted to keep things as future proof as possible. Thoughts?

@rossberg

Why have the indirection through the SubscriptionRequest type instead of two methods?

My intention here was to limit the amount of possible implementation errors by wrapping all actions inside the variant type; Rust forces developers to handle all cases explicitly, and I think Motoko does too? Similar thought process with the PublisherMessage.

@rossberg

• If you want the SubscriptionError type to be extensible in the future (without breaking clients), you need to wrap it in an option:

type SubscriptionResponse = variant {
  Ok;
  Err : opt SubscriptionError;
};

This wat, when you later add a new case to the error type, an old client will continue to work and just see null.

Glad you caught my intentions will update, thank you!