Nullable
Enriches union types A | Null with an interface similar scala.Option making it usable inside for comprehensions
Usage
Add the following to your build.sbt
scalacOptions += "-Yexplicit-nulls"
libraryDependencies += "com.abdulradi" %% "nullable-core" % "0.3.0"
0.3.0 is published for Scala 3 RC2 only ( If you are still in RC1, please use 0.2.0 instead)
Features
Plays nice with for comprehensions
import com.abdulradi.nullable.syntax.*
val maybeA: Int | Null = 42
val maybeB: String | Null = "foo"
// Compiles fine
for
a <- maybeA
yield 5
// Also compiles, no null pointer exceptions
for
a <- maybeA
b <- null
yield ()
// Compiles, and evaluates to null (because if condition doesn't match)
for
a <- maybeA
if (a < 0)
yield aFamiliar Option-like experience
maybeA.map(_ => 5)
maybeA.flatMap(_ => null)
maybeA.flatMap(_ => maybeB)
maybeA.fold(0)(_ + 1)
maybeA.getOrElse(0)
maybeA.contains(0)
maybeA.exists(_ == 0)
maybeA.toRight("Value was null")Convert from/to Option
val optA = Some(42)
maybeA.toOption == optA
maybeA == optA.orNullPrevents auto flattening
While Option[Option[A]] is a valid type, the equivalent A | Null | Null is indistinguishable from A | Null. So, the library ensures that map and flatMap are used correctly used at compile time.
The following examples don't compile
for a <- maybeA yield maybeB // Shows a compile time error suggesting to use flatMap instead
maybeA.flatMap(_ => 5) // Shows a compile time error suggesting message to use map instead
maybeA.map(_ => null) // Shows a compile time error suggesting to use flatMap instead
maybeA.map(_ => maybeB) // Shows a compile time error suggesting to use flatMap insteadWorking with Generics
The following doesn't compile, as we can't prove A won't be Null
def useFlatMapWithNullableInScope[A](f: Int => A): A | Null =
maybeA.flatMap(f)Solution: Propagate a Nullable instance and let the library check at usage site
def useFlatMapWithNullableInScope[A: Nullable](f: Int => A): A | Null =
maybeA.flatMap(f)
def useMapWithNotNullInScope[A: NotNull](f: Int => A): A | Null =
maybeA.map(f)Lightweight version
If you only care about for-comprehension features, but not the rest of Option-like methods, we also offer a lightweight syntax
import com.abdulradi.nullable.forSyntax.*
for
a <- maybeA
b <- maybeB
res = a + b
if (res % 2) == 0
yield resThe rest of the methods like fold, getOrElse, etc won't be in scope.
License & Acknowledgements
Since this library mimics the scala.Option behavior, it made sense to copy and adapt the documentation of it's methods and sometimes the implementation too. Those bits are copyrighted by EPFL and Lightbend under Apache License 2.0, which is the same license as this library.