Swift Async monad https://twitter.com/inamiy/status/1433228105728987140

async-monad.swift

/// `async` monad that wraps async function.
public struct Async<T>
{
    public let run: () async -> T
}

// MARK: - Functor

extension Async
{
    public func map<U>(_ f: @escaping (T) -> U) -> Async<U>
    {
        Async<U> {
            f(await run())
        }
    }
}

// MARK: - Applicative

extension Async
{
    public func zipWith<U>(_ u: Async<U>) -> Async<(T, U)>
    {
        Async<(T, U)> {
            await (self.run(), u.run())
        }
    }

    public func apply<U>(_ f: Async<(T) -> U>) -> Async<U>
    {
        zipWith(f).map { $1($0) }
    }
}

// MARK: - Monad

extension Async
{
    public func flatMap<U>(_ f: @escaping (T) -> Async<U>) -> Async<U>
    {
        Async<U> {
            await f(await run()).run()
        }
    }
}