A type-erased AsyncSequence

AnyAsyncSequence.swift

struct AnyAsyncSequence<Element>: AsyncSequence {
    struct AnyAsyncIterator: AsyncIteratorProtocol {
        private let _next: () async throws -> Element?

        init<Base: AsyncSequence>(_ base: Base) where Base.Element == Element {
            var baseIterator = base.makeAsyncIterator()
            self._next = { try await baseIterator.next() }
        }

        func next() async throws -> Element? {
            try await _next()
        }
    }

    private let _makeAsyncIterator: () -> AnyAsyncIterator

    init<Base: AsyncSequence>(_ base: Base) where Base.Element == Element {
        self._makeAsyncIterator = { AnyAsyncIterator(base) }
    }

    func makeAsyncIterator() -> AnyAsyncIterator {
        _makeAsyncIterator()
    }
}

extension AsyncSequence {
    func eraseToAnyAsyncSequence() -> AnyAsyncSequence<Element> {
        AnyAsyncSequence(self)
    }
}