Is this the right API? Should you be returning the ClientCall?

What do you suggest?

Maybe we make a new type which only allow cancel to be called.
But returning the LL.ClientCall is more general.
And the user only gets the ClientCall if they explicitly ask for it.

Returning the pair of ClientCall and the value would have been my approach, if it fits. If the only sensible thing you can do is cancel it then return a pair of IO () (the cancel function) and the value. FWIW at a previous company I defined newtype Cancel = Cancel (IO ()) and it was quite a successful abstraction.

We can't return a pair because the interface is set up in a with-resource style, the ClientCall value being obtained from withClientCall.

I was about to introduce the Cancel abstraction you suggested, when I noticed that ClientCall and clientCallCancel were already exposed in the HighLevel interface. (The only thing missing was how to obtain a ClientCall value.)

Since the type of ClientCall is abstract, the only thing to be done is call clientCallCancel on it. And so it seems to fulfill the role of the suggested newtype.

So I the end, didn't add it.

Fair enough on all points.