cats

A @reset code block can contain try / catch / finally if the monadic data type supports cats.MonadError. For example, cats.effect.IO is a monadic data type that supports cats.MonadError, therefore try / catch / finally expressions can be used inside a @reset code block whose return type is cats.effect.IO.

import com.thoughtworks.dsl.keywords.Monadic._
import com.thoughtworks.dsl.domains.cats._
import _root_.cats.effect.IO
import com.thoughtworks.dsl.Dsl.reset
val io0 = IO(0)
def dslTryCatch: IO[String] = IO {
  try {
    s"Division result: ${!io0 / !io0}"
  } catch {
    case e: ArithmeticException =>
      s"Cannot divide ${!io0} by itself"
  }
}: @reset
dslTryCatch.unsafeRunSync() should be("Cannot divide 0 by itself")

The above dslTryCatch method is equivalent to the following code in cats.syntax:

def catsSyntaxTryCatch: IO[String] = {
  import _root_.cats.syntax.applicativeError._
  io0.flatMap { tmp0 =>
    io0.flatMap { tmp1 =>
       IO(s"Division result: ${tmp0 / tmp1}")
    }
  }.handleErrorWith {
    case e: ArithmeticException =>
      io0.flatMap { tmp2 =>
         IO(s"Cannot divide ${tmp2} by itself")
      }
    case e =>
      e.raiseError[IO, String]
  }
}
catsSyntaxTryCatch.unsafeRunSync() should be("Cannot divide 0 by itself")

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_.cats.free.Trampoline
import _root_.cats.instances.function._
import com.thoughtworks.dsl.keywords.Monadic._
import com.thoughtworks.dsl.domains.cats._
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 cats.syntax:

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