Arbitrary

The arbitrary Generator

There is a special generator, org.scalacheck.Arbitrary.arbitrary, which generates arbitrary values of any supported type.

val evenInteger = Arbitrary.arbitrary[Int] suchThat (_ % 2 == 0)
val squares = for {
  xs <- Arbitrary.arbitrary[List[Int]]
} yield xs.map(x => x * x)

The arbitrary generator is the generator used by ScalaCheck when it generates values for property parameters. Most of the times, you have to supply the type of the value to arbitrary, like above, since Scala often can't infer the type automatically. You can use arbitrary for any type that has an implicit Arbitrary instance. As mentioned earlier, ScalaCheck has default support for common types, but it is also possible to define your own implicit Arbitrary instances for unsupported types. See the following implicit Arbitrary definition for booleans, that comes from the ScalaCheck implementation.

implicit lazy val arbBool: Arbitrary[Boolean] = Arbitrary(oneOf(true, false))

Let's see an example where we're defining an implicit arbitrary instance for Char 

import org.scalacheck.Arbitrary
import org.scalacheck.Gen
import org.scalacheck.Prop.forAll

implicit lazy val myCharArbitrary = Arbitrary(Gen.oneOf('A', 'E', 'I', 'O', 'U'))

val validChars: Seq[Char] = res0

check(forAll { c: Char => validChars.contains(c) })

This becomes more useful when we're dealing with our own data types. We'll use the case class defined in the Generators Section:

case class Foo(intValue: Int, charValue: Char)

Having an implicit def or val of our data type in the scope allow us to use the forAll method without specifying the generator 

import org.scalacheck.Arbitrary
import org.scalacheck.Gen
import org.scalacheck.Prop.forAll

val fooGen = for {
  intValue <- Gen.posNum[Int]
  charValue <- Gen.alphaChar
} yield Foo(intValue, charValue)

implicit lazy val myFooArbitrary = Arbitrary(fooGen)

check(forAll { foo: Foo => (foo.intValue < 0) == res0 && !foo.charValue.isDigit })

The Arbitrary.arbitrary method also returns a Gen object. 

import org.scalacheck.Arbitrary
import org.scalacheck.Gen.listOfN
import org.scalacheck.Prop.forAll

val genEightBytes = listOfN(8, Arbitrary.arbitrary[Byte])

check(forAll(genEightBytes)(list => list.size == res0))