CompositionPost1

Usecase.scala

sealed trait Result
class Success extends Result
class Failed extends Result

// For some types A, B, C, D, we have these async computations which may fail:
def f1(a: A): Future[Either[Failed, B]] = ???
def f2(b: B): Future[Either[Failed, C]] = ???
def f3(c: C): Future[Either[Failed, D]] = ???
def f4(d: D): Future[Either[Failed, Result]] = ???

// We want to apply each function to the result of the previous one, so that:
// - If one returns a Failed result, return that failure without applying the next function
// - If a Future fails, return that failed future without applying the next function

// This obviously won't compile - but that's what we want conceptually:
def processRequest(a: A): Future[Result] = f4(f3(f2(f1(a))))