How to refactor a function that throws exceptions? - scala

Question

Suppose I am refactoring a function like this:
def check(ox: Option[Int]): Unit = ox match {
case None => throw new Exception("X is missing")
case Some(x) if x < 0 => throw new Exception("X is negative")
case _ => ()
}
I'd like to get rid of the exceptions but I need to keep the check signature and behavior as is, so I'm factoring out a pure implementation -- doCheck:
import scala.util.{Try, Success, Failure}
def doCheck(ox: Option[Int]): Try[Unit] = ???
def check(ox: Option[Int]): Unit = doCheck(ox).get
Now I am implementing doCheck as follows:
def doCheck(ox: Option[Int]): Try[Unit] = for {
x <- ox toTry MissingX()
_ <- (x > 0) toTry NegativeX(x)
} yield ()
Using the following implicits:
implicit class OptionTry[T](o: Option[T]) {
def toTry(e: Exception): Try[T] = o match {
case Some(t) => Success(t)
case None => Failure(e)
}
}
implicit class BoolTry(bool: Boolean) {
def toTry(e: Exception): Try[Unit] = if (bool) Success(Unit) else Failure(e)
}
The implicits certainly add more code. I hope to replace OptionTry with an implicit from scalaz/cats someday and maybe find an analog for BoolTry.

Answer 1

You could refactor with a loan pattern and Try.
def withChecked[T](i: Option[Int])(f: Int => T): Try[T] = i match {
case None => Failure(new java.util.NoSuchElementException())
case Some(p) if p >= 0 => Success(p).map(f)
case _ => Failure(
new IllegalArgumentException(s"negative integer: $i"))
}
Then it can be used as following.
val res1: Try[String] = withChecked(None)(_.toString)
// res1 == Failure(NoSuchElement)
val res2: Try[Int] = withChecked(Some(-1))(identity)
// res2 == Failure(IllegalArgumentException)
def foo(id: Int): MyType = ???
val res3: Try[MyType] = withChecked(Some(2)) { id => foo(id) }
// res3 == Success(MyType)