scalaz

Attributes

Authors:

杨博 (Yang Bo)

Example:

scalaz.Free#Trampoline is a monadic data type that performs tail call optimization. It can be built from a @[[Dsl.reset 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.given
 import com.thoughtworks.dsl.macros.Reset.Default.reset
 import com.thoughtworks.dsl.keywords.Monadic.unary_!
 val trampoline3 = Trampoline.done(3)
 def dslSquare = reset(Trampoline.delay {
   s"This string is produced by a trampoline: ${!trampoline3 * !trampoline3}"
 })
 dslSquare.run should be("This string is produced by a trampoline: 9")

!trampoline3 is a shortcut of !Monadic(trampoline3), enabled by import com.thoughtworks.dsl.keywords.Monadic.given, 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")

A @[[Dsl.reset reset]] code block can contain try / catch / finally if the monadic data type supports scalaz.MonadError. tryt.scala is a monad transformer that provides scalaz.MonadError, therefore try / catch / finally expressions can be used inside a @[[Dsl.reset reset]] code block whose return type is TryT[Trampoline, ?].

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

 import _root_.scalaz.syntax.monad._
 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"))
 }

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Supertypes

class Object

trait Matchable

class Any

Self type

scalaz.type