Add Nullable type to cats data with instances#4823
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| import scala.language.strictEquality | ||
| import scala.quoted.* | ||
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| opaque type Nullable[+A] = A | Null |
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we could make Nullable[+A] >: A to allow passing an A where a Nullable is being accepted. That said, you would be adding a bunch of no-op if checks to your code by doing so, and I tend to think the cost of calling Nullable(a) for such cases isn't a bad idea.
Maximum convenience isn't the goal here. Maximum correctness with zero-cost is my goal.
| given [A](using A: Arbitrary[A]): Arbitrary[Nullable[A]] = | ||
| Arbitrary(Gen.oneOf(Gen.const(Nullable.empty[A]), A.arbitrary.map(Nullable(_)))) | ||
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| given [A](using A: Cogen[A]): Cogen[Nullable[A]] = | ||
| Cogen[Option[A]].contramap(_.toOption) |
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Ideally, these instances should be defined in cats-laws module, object cats.laws.discipline.arbitrary.
However, since it is for Scala 3 only, it may require adding some tweaks to the file structure in that module.
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| test("option-like Applicative is not lawful for nested Nullable") { |
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I hate to break it to you, but Functor[Nullable] seems to be unlawful for nested Nullable too:
// These cases all pass:
checkAll("Option[Option[*]]", FunctorTests[Option].functor[Option[Int], Option[Int], Option[Int]])
checkAll("Option[Nullable[*]]", FunctorTests[Option].functor[Nullable[Int], Nullable[Int], Nullable[Int]])
checkAll("Nullable[Option[*]]", FunctorTests[Nullable].functor[Option[Int], Option[Int], Option[Int]])
// But this one fails:
checkAll("Nullable[Nullable[*]]", FunctorTests[Nullable].functor[Nullable[Int], Nullable[Int], Nullable[Int]])The latter fails on "invariant composition" and "covariant composition".
2 participants
closes #4822
The idea is to be able to use nullable types from java (or even scala for unboxed optionality) but do so fully safely, so we can't confuse the nullable type with a non-nullable type.
Note: as suspected in #4822 both Monad and Applicative are not lawful. So instances were not added (someone could add them to alleycats, but I'm not motivated to add non-lawful instances).
For those cases, the user should be able to use
.foldwhich is as efficient as a hand written if/else with casting and can express all the operations you want to do.The functor methods are implemented which could be more efficiently done than with map and we test against the default implementations.