scala with explicit nulls

Scala with Explicit Nulls

This is a version of the Dotty compiler where Scala reference types are non-nullable by default.

This means that val x: String = null no longer type-checks. Instead, you can use union types to get nullability back: val x: String|Null = null.

Design Doc

https://gist.github.com/abeln/9f79774bac111d99b3ae2cb9016a33e6

Features

✅ = done | 🔶 = in progress | 🔴 = planned

🔶 Initial version

• ✅ Null is not a subtype of reference types

  val x: String = null // no longer compiles
  

• ✅ Nullability is expressible via union types

  val x: String|Null = null // ok
  

• ✅ Scala "nullifies" types that come from Java (argument types and method argument and return types)

  val x = "abc".substring(2) // x has type String|JavaNull    
  
  nullify(T)    -> T|JavaNull             (if T is not generic)
  nullify(C[T]) -> C[nullify(T)]|JavaNull (if T is generic)
  

• ✅ The JavaNull type is "transparent" w.r.t field selections:

  val x: Person = ??? // Person is defined in Java
  val y = x.getName() // inferred type is String|JavaNull
  y.length() // type-checks, because JavaNull is "transparent"
             // can also throw NPE
  

• ✅ Basic flow-sensitive type inference (missing: TASTy support https://github.com/abeln/dotty/issues/7#issuecomment-445057335)

  val x: String|Null = ???
  if (x != null) {
    // x has type String here
  }
  

  Also propagated inside the condition

  if (x != null && x.length > 0) {} // x.length types, even though x declared as String|Null
  

  We also support ||s

  if (x == null || x.length == 0) {
  } else {
    // x has type String here
  } 
  

• 🔶 Positive, negative, and run tests pass
• ✅ Binary compatibility (details in https://github.com/abeln/dotty/issues/16)
• 🔴 Java @NotNull annotation recognized
• 🔴 Dotty bootstraps with explicit nulls
• 🔴 PR to Dotty team

🔴 Feature complete

• TBD