How to turn your sync API into an async one

Let's suppose that you have a sync API send

(send params)

This API has at least 2 behaviors:

Minimal ex-handler

In our first atempt, we will add 1 extra parameter

(send-async params ex-handler)

This function now can:

Thow a sync exception (before go async)
Return a value, that is a promise
Call ex-handler with the async exception. The return of ex-handler will be delivered into the promise
Close the channel, even if ex-handler returns nil (that can't be inserted on a chan)

Is a core.async good pratice to allow the caller to pass it's return the channel

(send-async params ch ex-handler)

This function should still return ch

Sometimes the caller want full control over the channel, so you should allow the caller to control if you should close the channel or not.

(send-async params ch close? ex-handler)

Let's recap

send-async can throw before go async
ch is a optinal parameter, will always be returned by the function. On success, the result will be inserted and the channel closed.
close? will control if the channel will be closed or not
ex-handler is a function that receives an exception and returns a value. if it's value is not nil, it should be inserted on ch