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  1. object Main extends App {
  2.   val db = ""
  3.   def handleTransfer(req: TransferRequest): Either[Err, Result] = {
  4.     val resultNested = for {
  5.       u1 <- req.user1.lookup(db).nestError
  6.       u2 <- req.user2.lookup(db).earlyResult { e =>
  7.         Result(s"Cannot find ${req.user2}: $e")
  8.       }
  9.       depositor <- u2.checkAccess(u1).earlyResult { _ =>
  10.         Result(s"$u2 has not given you deposit access")
  11.       }
  12.       withdrawal <- u1.withdraw(req.amount).nestError
  13.       conf <- depositor.accept(withdrawal).nestError
  14.     } yield {
  15.       Result(s"You transferred $withdrawal to $u2, confirmation number $conf")
  16.     }
  17.     resultNested.joinLeft
  18.   }
  19.  
  20.   implicit class LeftNestingEither[A, B](either: Either[A, B]) {
  21.     def nestError: Either[Either[A, Nothing], B] =
  22.       either.left.map(Left(_))
  23.  
  24.     def earlyResult[C](errorToResult: A => C): Either[Either[Nothing, C], B] =
  25.       either.left.map(a => Right(errorToResult(a)))
  26.   }
  27. }
  28.  
  29. case class Err(msg: String)
  30. case class Result(msg: String)
  31. class User() {
  32.   def checkAccess(that: User): Either[Err, User] = ???
  33.   def withdraw(amount: Double): Either[Err, Double] = ???
  34.   def accept(amount: Double): Either[Err, Int] = ???
  35. }
  36. class UserName() {
  37.   def lookup(db: Any): Either[Err, User] = ???
  38. }
  39. case class TransferRequest(user1: UserName, user2: UserName, amount: Double)
  40.  
Success #stdin #stdout 0.28s 2181632KB
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https://ideone.com/yYGTu5
language:
Scala (scala 2.12.8)
created:
5 years ago
visibility:
public

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