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doc-page	Explicit Nulls

The "explicit nulls" feature (enabled via a flag) changes the Scala type hierarchy so that reference types (e.g. String) are non-nullable. We can still express nullability with union types: e.g. val x: String|Null = null.

The implementation of the feature in dotty can be conceptually divided in several parts:

1. changes to the type hierarchy so that Null is only a subtype of Any
2. a "translation layer" for Java interop that exposes the nullability in Java APIs
3. a "magic" JavaNull type (an alias for Null) that is recognized by the compiler and allows unsound member selections (trading soundness for usability)
4. a module for "flow typing", so we can work more naturally with nullable values

Feature Flag

Explicit nulls are disabled by default. They can be enabled via -Yexplicit-nulls defined in ScalaSettings.scala. All of the explicit-nulls-related changes should be gated behind the flag.

Type Hierarchy

We change the type hierarchy so that Null is only a subtype of Any by:

• modifying the notion of what is a nullable class (isNullableClass) in SymDenotations to include only Null and Any
• changing the parent of Null in Definitions to point to Any and not AnyRef
• changing isBottomType and isBottomClass in Definitions

Java Interop

The problem we're trying to solve here is: if we see a Java method String foo(String), what should that method look like to Scala?

• since we should be able to pass null into Java methods, the argument type should be String|JavaNull
• since Java methods might return null, the return type should be String|JavaNull

JavaNull here is a type alias for Null with "magic" properties (see below).

At a high-level:

• we track the loading of Java fields and methods as they're loaded by the compiler
• we do this in two places: Namer (for Java sources) and ClassFileParser (for bytecode)
• whenever we load a Java member, we "nullify" its argument and return types

The nullification logic lives in JavaNullInterop.scala, a new file.

The entry point is the function def nullifyMember(sym: Symbol, tp: Type)(implicit ctx: Context): Type which, given a symbol and its "regular" type, produces what the type of the symbol should be in the explicit nulls world.

In order to nullify a member, we first pass it through a "whitelist" of symbols that need special handling (e.g. constructors, which never return null). If none of the "policies" in the whitelist apply, we then process the symbol with a TypeMap that implements the following nullification function n:

1. n(T) = T|JavaNull if T is a reference type
2. n(T) = T if T is a value type
3. n(C[T]) = C[T]|JavaNull if C is Java-defined
4. n(C[T]) = C[n(T)]|JavaNull if C is Scala-defined
5. n(A|B) = n(A)|n(B)|JavaNull
6. n(A&B) = n(A) & n(B)
7. n((A1, ..., Am)R) = (n(A1), ..., n(Am))n(R) for a method with arguments (A1, ..., Am) and return type R
8. n(T) = T otherwise

JavaNull

JavaNull is just an alias for Null, but with magic power. JavaNull's magic (anti-)power is that it's unsound.

val s: String|JavaNull = "hello"
s.length // allowed, but might throw NPE

JavaNull is defined as JavaNullAlias in Definitions. The logic to allow member selections is defined in findMember in Types.scala:

• if we're finding a member in a type union
• and the union contains JavaNull on the r.h.s. after normalization (see below)
• then we can continue with findMember on the l.h.s of the union (as opposed to failing)

Working with Nullable Unions

Within Types.scala, we defined a few utility methods to work with nullable unions. All of these are methods of the Type class, so call them with this as a receiver:

• isNullableUnion determines whether this is a nullable union. Here, what constitutes a nullable union is determined purely syntactically:
  1. first we "normalize" this (see below)
  2. if the result is of the form T | Null, then the type is considered a nullable union. Otherwise, it isn't.
• isJavaNullableUnion determines whether this is syntactically a union of the form T|JavaNull
• normNullableUnion normalizes this as follows:
  1. if this is not a nullable union, it's returned unchanged.
  2. if this is a union, then it's re-arranged so that all the Nulls are to the right of all the non-Nulls.
• stripNull syntactically strips nullability from this: e.g. String|Null => String. Notice this works only at the "top level": e.g. if we have an Array[String|Null]|Null and we call stripNull we'll get Array[String|Null] (only the outermost nullable union was removed).
• stripAllJavaNull is like stripNull but removes all nullable unions in the type (and only works for JavaNull). This is needed when we want to "revert" the Java nullification function.

Flow Typing

Flow typing is needed so we can work with nullable unions in a more natural way. The following is a common idiom that should work without additional casts:

val x: String|Null = ???
if (x != null && x.length < 10)

This is implemented as a "must be null in the current scope" analysis on stable paths:

• we add additional state to the Context in Contexts.scala. Specifically, we add a set of FlowFacts (right now just a set of TermRefs), which are the paths known to be non-nullable in the current scope.

• the bulk of the flow typing logic lives in a new FlowTyper.scala file.

  There are four entry points to FlowTyper:

  1. inferFromCond(cond: Tree): Inferred: given a tree representing a condition such as x != null && x.length < 10, return the Inferred facts.

  In turn, Inferred is defined as case class Inferred(ifTrue: FlowFacts, ifFalse: FlowFacts). That is, Inferred contains the paths that must be non-null if the condition is true and, separately, the paths that must be non-null if the condition is false.

    e.g. for `x != null` we'd get `Inferred({x}, {})`, but only if `x` is stable.
    However, if we had `x == null` we'd get `Inferred({}, {x})`.
  

  2. inferWithinCond(cond: Tree): FlowFacts: given a condition of the form lhs && rhs or lhs || rhs, calculate the paths that must be non-null for the rhs to execute (given that these operations) are short-circuiting.

  3. inferWithinBlock(stat: Tree): FlowFacts: if stat is a statement with a block, calculate which paths must be non-null when the statement that follows stat in the block executes. This is so we can handle things like

     val x: String|Null = ???

  if (x == null) return val y = x.length ``` Here, inferWithinBlock(if (x == null) return) gives back `{x}`, because we can tell that the next statement will execute only if `x` is non-null.

  4. refineType(tpe: Type): Type: given a type, refine it if possible using flow-sensitive type information. This uses a NonNullTermRef (see below).

• Each of the public APIs in FlowTyper is used to do flow typing in a different scenario (but all the use sites of FlowTyper are in Typer.scala):

  • refineType is used in typedIdent and typedSelect
  • inferFromCond is used for typing if statements
  • inferWithinCond is used when typing "applications" (which is how "&&" and "||" are encoded)
  • inferWithinBlock is used when typing blocks

  For example, to do FlowTyping on if expressions:

  • we type the condition
  • we give the typed condition to the FlowTyper and obtain a pair of sets of paths (ifTrue, ifFalse). We type the then branch with the ifTrue facts, and the else branch with the ifFalse facts.
  • profit

Flow typing also introduces two new abstractions: NonNullTermRef and ValDefInBlockCompleter.

NonNullTermRef

This is a new type of TermRef (path-dependent type) that, whenever its denotation is updated, makes sure that the underlying widened type is non-null. It's defined in Types.scala. A NonNullTermRef is identified by computeDenot whenever the denotation is updated, and then we call stripNull on the widened type.

To use the flow-typing information, whenever we see a path that we know must be non-null (in typedIdent or typedSelect), we replace its TermRef by a NonNullTermRef.

ValDefInBlockCompleter

This a new type of completer defined in Namer.scala that completes itself using the completion context, asopposed to the creation context.

The problem we're trying to solve here is the following:

val x: String|Null = ???
if (x == null) return
val y = x.length

The block is usually typed as follows:

1. first, we scan the block to create symbols for the new definitions (val x, val y)
2. then, we type statement by statement
3. the completers for the symbols created in 1. are all invoked in step 2. However, regular completers use the creation context, so that means that val y is completed with a context that doesn't contain the new flow fact "x != null".

To fix this, whenever we're inside a block and we create completers for vals, we use a ValDefInBlockCompleter instead of a regular completer. This new completer uses the completion context, which is aware of the new flow fact "x != null".