AsyncStream { (cont) in var a: A! var b: B! async { for await x in a_s { a = x cont.yield(a, b) } } async { for await x in b_s { b = x cont.yield(a, b) } } } (edited)AsyncStream { (cont) in var a: A! var b: B! async { for await x in a_s { a = x cont.yield(a, b) } } async { for await x in b_s { b = x cont.yield(a, b) } } } (edited)