rallyhealth / scalacheck-ops   2.12.0

Apache License 2.0 GitHub

ScalaCheck extension methods and JDK8 / Joda time typeclass instances

Scala versions: 3.x 2.13 2.12 2.11 2.10
Scala.js versions: 1.x
Scala Native versions: 0.4

Build Status CodeCov

scalacheck-ops_1-12 scalacheck-ops_1-13 scalacheck-ops_1-14 scalacheck-ops_1-15 scalacheck-ops_1
Download for ScalaCheck 1.12 Download for ScalaCheck 1.13 Download for ScalaCheck 1.14 Download for ScalaCheck 1.15 Download for ScalaCheck >= 1.16


A library that provides ScalaCheck implicits and helper methods made available via:

import org.scalacheck.ops._

See the use cases section for use cases and operations that are enabled with scalacheck-ops.



NOTE Version 1.x lives under the "me.jeffmay" organization. Version 2.x lives under "com.rallyhealth"

NOTE The artifact name in version 2.x and above changes from scalacheck-ops to include the version of scalacheck in the suffix of the artifact name. (i.e. scalacheck-ops_$major-$minor. Prior to this change, scalacheck-ops with no version suffix would pull in ScalaCheck version 1.12.6.

NOTE Version 2.x and above requires JDK >=8 and Scala >=2.11 as this library expects the java.time standard library module.

NOTE Version 2.11.x and above requires Scala >=2.12 and is only tested against JDK 11 and 17.

Artifact Name Version Limit ScalaCheck Supported JDK Supported Scala Supported Scala.js Supported Native
scalacheck-ops_1 x >= 2.11 1.17.0 11, 17 2.12, 2.13, 3.2 1 0.4
scalacheck-ops_1-15 x >= 2.5.2 1.15.4 11, 17 2.12, 2.13, 3.2 1 0.4
scalacheck-ops_1-14 x >= 2.0 1.14.3 11, 17 2.12, 2.13 N/A N/A
scalacheck-ops_1-13 x >= 2.0 1.13.5 11, 17 2.11, 2.12 N/A N/A
scalacheck-ops_1-13 1.5 <= x < 2.0 1.13.4 6 - 8 2.10 - 2.11 N/A N/A
scalacheck-ops_1-12 2.0 <= x < 2.11 1.12.6 8 2.11 N/A N/A
scalacheck-ops x < 2.0 1.12.6 6 - 8 2.10 - 2.11 N/A N/A

The same source code is compiled against specific versions of Scala and ScalaCheck. We use separate artifacts to avoid causing issues with transitive dependencies on ScalaCheck being evicted with binary incompatible versions.

ScalaTest Compatibility

Specifically, when using this library with ScalaTest you might notice the following exception:

java.lang.IncompatibleClassChangeError: Found class org.scalacheck.Gen, but interface was expected

This is because you need scalacheck-ops_1-13 for ScalaTest 3.0.x

ScalaTest Version ScalaCheck Version
2.2.x 1.12.6
3.0.x 1.13.4

Use Cases

Converting Gen to Iterator

Probably one of the most used features of this library is when you want to use generators for testing properties as well as testing single case unit tests. In plain ScalaCheck, you are able to call the Gen.sample method to get an Option[T] from a Gen[T]. However, for single unit tests, you need a T, so you could call .get on the option, but this option could be empty if the sample is filtered. ScalaCheck handles this internally by limiting the number of attempts, but it does not expose this logic for the developer. This library provides a safe way (and some unsafe ways) to get values out of a Gen:

import org.scalacheck.Gen
import org.scalacheck.ops._

val genEvens = Gen.choose(1, 10).suchThat(_ % 2 == 0)
val exampleEven = genEvens.head

By default, .head will make 100 attempts to get a value out of the generator before giving up. The generated result will always be the same value, based on Seed in the GenConfig in scope. You can customize the number of attempts, the seed, and the Gen.Parameters by defining your own implicit GenConfig.

Alternatively, if you don't want a pure result, but rather a random one every time you run the code (at the risk of making your tests more flaky) you can use the .nextRandom() method. Generally, it is better to run multiple test iterations with the same initial seed to create a test that will reliably fail. However, if running dozens of iterations of a given test is too expensive and you would rather catch bugs at the risk of having a flaky test, then you can generate a single random sample each time.

val exampleEven = genEvens.nextRandom()

In addition to getting a single value, you can convert a Gen into an Iterator:

val evens = genEvens.iterator

By default, this iterable attempts a max of 100 times for each sample before giving up and throwing an exception. This is to avoid an infinite loop.

Generating Sets

One thing that is missing from ScalaCheck is a performant way to generate Sets of an arbitrary or specific size. The naive implementation is insufficient:

val genSetOf3 = Gen.listOfN(3, Gen.oneOf(0 to 10)).map(_.toSet[Int])

This may work in some of the early and simple cases, but when you run it long enough, you will find sets of size 1 and 2. This is because Gen.oneOf uses a psuedo-random number generator, which will on occasion have duplicates. When you call .toSet it will filter out the duplicates and leave you with a smaller Set than you wanted.

In a similar manner to generating iterators mentioned above, this library gives you a method to generate a specific sized Set (or throw an exception after a predefined number of tries).

import org.scalacheck.Arbitrary.arbitrary
import org.scalacheck.Gen
import org.scalacheck.ops._
val genSetOf3 = Gen.setOfN(3, Gen.oneOf(0 to 10))
// or give up on building the Set after 10 duplicates
val genSetOf3 = Gen.setOfN(3, 10, Gen.oneOf(0 to 10))

Generating Strings a specific size

Let's say you have a password validator that requires strings with a length of 20 characters. If you want to generate only valid passwords, then you could generate strings of length 20.

// vanilla scalacheck
import org.scalacheck.Gen
import org.scalacheck.Arbitrary.arbitrary
val genValidPasswords = for {
  cs <- Gen.collectionOfN[Array](20, arbitrary[Char])
} yield new String(cs, "utf-8")

// scalacheck-ops
import org.scalacheck.Gen
import org.scalacheck.ops._
val genValidPasswords = Gen.stringOfN(20)

You could even generate strings within a certain range of characters:

// vanilla scalacheck
import org.scalacheck.Gen
import org.scalacheck.Arbitrary.arbitrary
val genValidPasswords = for {
  n <- Gen.choose(20, 40)
  cs <- Gen.collectionOfN[Array](n, arbitrary[Char])
} yield new String(cs, "utf-8")

// scalacheck-ops
import org.scalacheck.Gen
import org.scalacheck.ops._
val genValidPasswords = Gen.stringOfNWithin(20 to 40)

Generating Enums, BitSets, and Binary

Some useful generators that were missing from the vanilla ScalaCheck:

import org.scalacheck.Gen
import org.scalacheck.ops._

object Colors extends scala.Enumeration {
  val Red, Blue, Green = Value

Gen.boolean // Gen[Boolean]
Gen.bits // Gen[BitSet]
Gen.enumValue(Colors) // Gen[Colors.Value]