extras.md

scalacheck-derived-extras

Since version 0.6.0, the additional module scalacheck-derived-extras provides support for literal and union types.

You can add it to your sbt project (as test-dependency) as follows:

libraryDependencies ++= Seq(
  "io.github.martinhh" %% "scalacheck-derived-extras" % "<version>" % "test"
)

Literal types

By importing io.github.martinhh.derived.extras.literal.scalacheck.given, you get Arbitrary- and Cogen-instances for any literal / singleton type (i.e. for any type for which an instance of ValueOf is available).

No Shrink instances are provided because there is no sense in shrinking singleton types.

N.B.: since there is only a single value for each literal singleton type, generating arbitrary values may not seem helpful at first - however: this feature can be very helpful when such types are combined via unions (like 1 | 2 | 3).

Unions

By importing io.github.martinhh.derived.extras.union.scalacheck.given, you get Arbitrary- and Cogen-instances for any union type A | B (if instances for A and B are in scope).

By importing io.github.martinhh.derived.extras.union.shrink.given, you can opt in to getting Shrink-instances for any union type A | B derived from the instances for A and B.

Caveat: Type Erasure for Cogen and Shrink

Derived Cogens and Shrinks for union types use scala.reflect.TypeTest for matching the type members of the union at runtime. TypeTest is subject to type erasure - you will see related warnings if, for example, you derive a Shrink[Option[Int] | String]. You may ignore these as long as the "outer" type does not occur twice in your union, i.e.: runtime errors should only occur if you derive something like a Shrink[Option[Int] | Option[String]].

Combined Example

The above enables you to things like this:

import io.github.martinhh.derived.extras.literal.scalacheck.given
import io.github.martinhh.derived.extras.union.scalacheck.given
import org.scalacheck.Prop

Prop.forAll { (oneTwoOrThree: 1 | 2 | 3) =>
  // your test using oneTwoOrThree here...
}