Using ScalaCheck forAll with a custom Generator that creates a tuple

forAll-customGen

// Instead of nesting forAll calls, we could have defined a custom generator in the following way:

  import org.scalacheck.Arbitrary.arbitrary
  import org.scalacheck.Gen.choose
  
  val genStringWithN = for {
    s <- arbitrary[String]
    n <- choose(0, s.length)
  } yield (s,n)

// We can now specify the property with only one forAll call:

  import org.scalacheck.Prop.forAll
  
  val propPrefix = forAll(genStringWithN) { case (s,n) =>
    val prefix = s.substring(0, n)
    s.startsWith(s)
  }
  
  // Notice that we have to use a case-expression since our property takes one tuple as 
  // its argument, not two separate arguments.