Either monad

gistfile1.scala

scala> def unsafe[A](f: => A): Either[Throwable, A] = try Right(f) catch { case x => Left(x) }
unsafe: [A](f: => A)Either[Throwable,A]

scala> def A(x: Int) = x + 10
A: (x: Int)Int

scala> def B(x: Int) = x.toString
B: (x: Int)java.lang.String

scala> def C(s: String) = s + "!"
C: (s: String)java.lang.String

scala> def D(s: String) = "Doh"
D: (s: String)java.lang.String

scala> def KaPow(s: String): String = sys.error("Ka Pow!")
KaPow: (s: String)String

scala> def result(i: Int) = for {
     |   a <- unsafe(A(i))
     |   b <- unsafe(B(a))
     |   k <- unsafe(KaPow(b))
     |   c <- unsafe(C(k))
     | } yield D(c)
result: (i: Int)Either[Throwable,java.lang.String]

scala> result(9)
res32: Either[Throwable,java.lang.String] = Left(java.lang.RuntimeException: Ka Pow!)

scala> def result(i: Int) = for {
     |   a <- unsafe(A(i))
     |   b <- unsafe(B(a))
     |   c <- unsafe(C(b))
     | } yield D(c)
result: (i: Int)Either[Throwable,java.lang.String]

scala> result(9)
res33: Either[Throwable,java.lang.String] = Right(Doh)