scalaz

scalaz.Free.Trampoline is a monadic data type that performs tail call optimization. It can be built from a @reset code block within some !-notation, similar to the each method in ThoughtWorks Each.

import _root_.scalaz.Trampoline
import _root_.scalaz.Free.Trampoline
import com.thoughtworks.dsl.keywords.Monadic._
import com.thoughtworks.dsl.domains.scalaz._
import com.thoughtworks.dsl.Dsl.reset
val trampoline3 = Trampoline.done(3)
def dslSquare = Trampoline.delay {
  s"This string is produced by a trampoline: ${!trampoline3 * !trampoline3}"
}: @reset
dslSquare.run should be("This string is produced by a trampoline: 9")

!trampoline3 is a shortcut of !Monadic(trampoline3), which will be converted to flatMap calls by our DSL interpreter. Thus, the method dslSquare is equivalent to the following code in scalaz.syntax:

def scalazSyntaxSquare = trampoline3.flatMap { tmp1 =>
  trampoline3.flatMap { tmp2 =>
    Trampoline.delay {
      s"This string is produced by a trampoline: ${tmp1 * tmp2}"
    }
  }
}
scalazSyntaxSquare.run should be("This string is produced by a trampoline: 9")

import com.thoughtworks.tryt.invariant.TryT, TryT._
import scala.util.{Try, Success}
type TryTTransfomredTrampoline[A] = TryT[Trampoline, A]
val trampolineSuccess0: TryTTransfomredTrampoline[Int] = TryT(Trampoline.done(Try(0)))
def dslTryCatch: TryTTransfomredTrampoline[String] = TryT(Trampoline.delay(Try {
  try {
    s"Division result: ${!trampoline3 / !trampolineSuccess0}"
  } catch {
    case e: ArithmeticException =>
      s"Cannot divide ${!trampoline3} by ${!trampolineSuccess0}"
  }
})): @reset
inside(dslTryCatch) {
  case TryT(trampoline) =>
    trampoline.run should be(Success("Cannot divide 3 by 0"))
}

Note that !-notation can be used on both trampoline3 and trampolineSuccess0 even when they are different types, i.e. trampoline3 is a vanilla Trampoline, while trampolineSuccess0 is a TryT-transfomred Trampoline. It is possible because the interpreters of the keywords.Monadic invoke scalaz.MonadTrans.liftM automatically. The above dslTryCatch method is equivalent to the following code in scalaz.syntax:

def scalazSyntaxTryCatch: TryTTransfomredTrampoline[String] = {
  import _root_.scalaz.syntax.monadError._
  trampoline3.liftM[TryT].flatMap { tmp0 =>
    trampolineSuccess0.flatMap { tmp1 =>
       TryT(Trampoline.delay(Try(s"Division result: ${tmp0 / tmp1}")))
    }
  }.handleError {
    case e: ArithmeticException =>
      trampoline3.liftM[TryT].flatMap { tmp2 =>
        trampolineSuccess0.flatMap { tmp3 =>
           TryT(Trampoline.delay(Try(s"Cannot divide ${tmp2} by ${tmp3}")))
        }
      }
    case e =>
      e.raiseError[TryTTransfomredTrampoline, String]
  }
}
inside(scalazSyntaxTryCatch) {
  case TryT(trampoline) =>
    trampoline.run should be(Success("Cannot divide 3 by 0"))
}