scala-steward / scalacheck-derived

Automatic derivation of scalacheck Arbitrary instances for Scala 3.

scalacheck-derived

Automatic derivation of scalacheck Arbitrary instances for Scala 3.

This enables automatic derivation for enums, case classes and sealed traits.

This library supports regular scala on jvm, scala-js and scala-native. See version matrix for details on versions.

Getting started

Add a (test-)dependency on this project to your project. For example, in a regular (jvm-targeting) sbt project:

libraryDependencies ++= Seq(
  "io.github.martinhh" %% "scalacheck-derived" % "0.2.0" % "test"
)

Usage

There are two ways of using this library. Let's look at them by examples using the following (example) data structure:

enum Color:
  case Red, Blue, Yellow

sealed trait LibItem
case class Magazine(title: String, issue: Int, color: Color) extends LibItem
case class Book(title: String, author: String, color: Color) extends LibItem

a) fully implicit

By importing io.github.martinhh.derived.scalacheck.given, you enable implicit resolution of derived instances so that you can do this:

import io.github.martinhh.derived.scalacheck.given
import org.scalacheck.Prop

Prop.forAll { (item: LibItem) =>
  // your test using item here...
}

b) derive explicitly

Alternatively, you can use io.github.martinhh.derived.scalacheck.deriveArbitrary to explicitly derive instances where you need them:

import io.github.martinhh.derived.scalacheck.deriveArbitrary
import org.scalacheck.Arbitrary

given arbLibItem: Arbitrary[LibItem] = deriveArbitrary

Limitations

There are a few limitations to the provided mechanism:

Recursive structures

Recursive structures are not supported. Trying derivation with the "fully implicit" approach will lead to a compiler error. Trying derivation with the "derive explicit" approach may even lead to an endless loop during generation at runtime(!).

Maximal number of successive inlines

The derivation mechanism uses inlining. Depending on the size of the data structure for which one wants to derive an Arbitrary, it is possible to hit the compiler's maximum limit for number of successive inlines.

In that case, compilation will fail with a message like this one:

[error]    |                     Maximal number of successive inlines (32) exceeded,
[error]    |                     Maybe this is caused by a recursive inline method?
[error]    |                     You can use -Xmax-inlines to change the limit.

Workarounds

There are two ways to handle this:

1. use -Xmax-inlines compile setting to increase the limit
2. instead of deriving an Arbitrary for the whole structure at once, derive "intermediate" instances for some of its members (and place them into implicit scope).

Version matrix

The following table documents which versions of scalacheck, scala, scala-js and scala-native were used for each release:

scalacheck-derived	scalacheck	scala	scala-js	scala-native
0.2.0	1.17.0	3.2.2	1.13.0	0.4.12
0.1.0	1.17.0	3.2.2	1.13.0	0.4.12