tabdulradi / nullable

Makes `A | Null` work with for-comprehensions

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" % "0.4.0"

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 a

Familiar 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.orNull

Prevents 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 instead

Working 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 res

The 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.