interface Stream extends RpcTarget {
      write(chunk): void;
      end(): void;
      [Symbol.dispose](): void;
    }

In any case, you implement this interface, and pass it over the RPC connection. The other side can now call it back to write chunks. Voila, streaming.

That said, getting flow control right is a little tricky here: if you await every `write()`, you won't fully utilize the connection, but if you don't await, you might buffer excessively. You end up wanting to count the number of bytes that aren't acknowledged yet and hold off on further writes if it goes over some threshold. Cap'n Proto actually has built-in features for this, but Cap'n Web does not (yet).

Workers RPC actually supports sending `ReadableStream` and `WritableStream` (JavaScript types) over RPC. I'd like to support that in Cap'n Web, too, but haven't gotten around to it yet. It'd basically work exactly like above, but you get to use the standard types.

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Exceptions work exactly like you'd expect. If the callee throws an exception, it is serialized, passed back to the caller, and used to reject the promise. The error also propagates to all pipelined calls that derive from the call that threw.

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The mapper function receives, as its parameter, an `RpcPromise`. So you cannot actually inspect the value, you can only pipeline on it. `friend.isBestFriend ?` won't work, because `friend.isBestFriend` will resolve as another RpcPromise (for the future property). I suppose that'll be considered truthy by JavaScript, so the branch will always evaluate true. But if you're using TypeScript, note that the type system is fully aware that `friend` is type `RpcPromise<Friend>`, so hopefully that helps steer you away from doing any computation on it.