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When you declare a function as a

suspend

function, the compiler actually emits a function that always returns

Object

, and takes an extra

Continuation

parameter. The

Continuation

is basically just a callback that the function can use to return a result asynchronously.

What is the

delay(0)

for?

How do you know suspend function is not suuported? @Kroppeb

Presumably, if they were supported, you wouldn't get that exception. Retrofit is a library with a similar API as this looks to be (define an interface, annotate the methods), and It had to explicitly add support for coroutines by looking for functions that had that last coroutine parameter, constructing a continuation appropriately, and handling the special return value.