Railway Oriented Programming example in Swift https://medium.com/@pavelshilyagov/railway-oriented-programming-in-swift-d4131c05d627

swift_rop.swift

import Cocoa

precedencegroup PipeOperatorPrecedence {
  associativity: left
  higherThan: LogicalConjunctionPrecedence
}

infix operator |>: PipeOperatorPrecedence

public func |> <A, B>(arg: A,
                      f: @escaping (A) -> B) -> B {
  return f(arg)
}

precedencegroup KleisliOperatorPrecedence {
  associativity: left
  higherThan: PipeOperatorPrecedence
}

infix operator >=>: KleisliOperatorPrecedence

public enum Result<T, E> {
  case ok(T)
  case error(E)
}

extension Result {
  public func bind<B>(_ f: @escaping (T) -> Result<B, E>) -> Result<B, E> {
    switch self {
    case .ok(let x):
      return f(x)
    case .error(let e):
      return Result<B, E>.error(e)
    }
  }
}

public func >=> <A, B, C, E>(f: @escaping (A) -> Result<B, E>,
                             g: @escaping (B) -> Result<C, E>) -> (A)->Result<C, E> {
  return { a in
    f(a).bind(g)
  }
}


struct UserInput {
  let email: String
  let password: String
}

struct User {
  let id = UUID.init()
  let email: String
}

extension User: CustomDebugStringConvertible {
  var debugDescription: String {
    return "{ id: \(id), email: \(email) }"
  }
}

enum UserError: Error {
  case invalidEmail, invalidPassword, alreadyExist
  case unknown(cause: Error)
}

//key-value as DB store
var usersDb = [String: User]()

//simulate DB runtime error
enum DbError: Error {
  case duplicateKeyError
}

func saveToDb(_ input: UserInput) throws -> User {
  if usersDb[input.email] != nil {
    print("user \(input.email) already exists")
    throw DbError.duplicateKeyError
  }
  let newUser = User(email: input.email)
  usersDb[input.email] = newUser
  print("user saved: \(newUser)")
  return newUser
}

func validateEmail(input: UserInput) -> Result<UserInput, UserError> {
  //Simple regular expression for email validation
  let emailPredicate = NSPredicate(format: "SELF MATCHES %@", "[A-Z0-9a-z._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,4}")
  //Validate email
  return emailPredicate.evaluate(with: input.email) ? .ok(input) : .error(UserError.invalidEmail)
}

func validatePassword(input: UserInput) -> Result<UserInput, UserError> {
  return input.password.count > 6 ? .ok(input) : .error(UserError.invalidPassword)
}

func saveToDbWithResult(input: UserInput) -> Result<User, UserError> {
  do {
    let user = try saveToDb(input)
    return .ok(user)
  } catch DbError.duplicateKeyError {
    return .error(UserError.alreadyExist)
  } catch {
    return .error(UserError.unknown(cause: error))
  }
}

func register(input: UserInput) -> Result<User, UserError> {
  return validateEmail(input: input)
    .bind(validatePassword)
    .bind(saveToDbWithResult)
}

func registerOperators(input: UserInput) -> Result<User, UserError> {
  return input
    |> validateEmail
    >=> validatePassword
    >=> saveToDbWithResult
}

let test1 = UserInput(email: "123@mail.com", password: "password")
let registrationResult = register(input: test1)
switch registrationResult {
case .ok(let user):
  print("user \(user) successfully registered")
case .error(let error):
  print("failed to register new user. Error: \(error)")
}