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Sound non-nullable (by default) types with incremental migration


This document proposes a roadmap for implementing a sound null tracking type system in Dart similar to what has been explored previously. The purpose of this document is not to describe the technical details of a proposal, but instead to argue for a specific set of high-level goals and design choices that define the key properties of the final system, and of the migration path to get there. Specifically, we propose to aim for a system which allows for an incremental opt-in migration, and which is fully sound once all code in a program has opted in.

Motivation for non-nullability in Dart.

This has been fairly well explored previously, so just in brief. This is one of the most requested features in developer surveys, and one of the thing that developers most frequently mention missing from other languages. Kotlin, Swift, C#, Typescript, and numerous other languages now have, or are adding, support for non-nullability. Since Dart also has no primitive types or value types, it is possible that non-nullability could provide performance benefits as well.

Summary of proposed goals

We propose to set the following goals for nullability tracking in Dart.

  • Code can be migrated incrementally. A program can run and have well-defined semantics when some parts have been migrated to non-nullable types and others have not. We don't guarantee full safety in programs which mix migrated and unmigrated code. That is, when not all code has opted in, it is possible that a non-nullably typed value will receive null.

  • When a program has been fully migrated to non-nullable types, it uses only the subset of the type system that is sound. No null errors (returning null from an expression whose type is not nullable, calling methods on null that the Null class does not support) will occur.

  • During the migration, if a package opts in after all of the packages it depends on have already opted in (or if it correctly predicts and codes against the post-migration API of the packages it depends on) it can migrate completely in one step.

  • Migration should be minimally breaking for unmigrated packages. Migrating a package should never cause compilation to fail for unmigrated downstream dependencies, and as much as is possible should not introduce new runtime failures.

  • Packages can migrate before their upstream dependencies have migrated. In so far as a packages is able to correctly predict how its upstream dependencies will eventually null-annotate their code, it should not have to re-migrate after those packages opt in.

Proposed roadmap


Non-nullable types will be rolled out as an opt-in feature. At some point in the indefinite future, this may become the default in a future major release of Dart. Until then, packages can opt in whenever they want. Opting in will not break other downstream packages or apps that have not yet migrated.

In order to facilitate the migration, there will be two levels of null checking: weak null checking and strong null checking. Until all of the code in a program has opted in to strong null checking, only a subset of null errors will be caught, and it will still be possible to get unexpected null exceptions at runtime.

In addition to the technology for incremental migration, the Dart team will build a tool to help modify code to be null-safe. This tool will be run from the command line or the IDE, and will suggest fixes to a package to make it work with non-null types. We don't anticipate that the tool will be able to fix all issues for the programmer, but hope to be able to automate the bulk of the migration.

Weak null checking

Weak null checking will turn on static null checks, and also add some automatic runtime null assertions. It will not guarantee that users won't get unexpected null errors, since other libraries in the program may not have opted in yet. Any null safety violations are only warnings with weak null checking. That is, static errors that arise only because of nullability annotations will only appear as warnings at this level. Similarly, runtime cast failures which only fail because of nullability annotations may only be surfaced as warnings at this level as well.

Weak null checking will also cause the compiler to insert checked-mode style null checks on assignments to variables, parameters, and return values of non-nullable type. These null checks will not be warnings: they will behave as if the programmer had written the corresponding "assert" statement in the code. This is intended to allow programmers to remove their null assertions when they migrate their code without losing assertion checking in the period until all code in the program has migrated. Once all code in the program has migrated, these checks should be provably redundant and do not need to be inserted.

Concretely, opting in to weak null checking will proceed as follows:

  • The programmer opts into the weak null checks by marking their package as opted in.
  • This will cause warnings about null-safety violations within the package (only).
    • If the opted-in code uses other packages that have opted in, then the programmer will see null-safety warnings from any misuses of the opted in APIs from the other packages.
    • If the opted-in code uses other packages that have not opted in, the programmer will not see any null-safety warnings from uses of those APIs. They are free to treat those APIs as being null-accepting or non-null-accepting as appropriate.
  • If the programmer runs the migration tool, it will suggest fixes to get rid of null safety warnings as best it can.
  • Any remaining warnings should be dealt with by the programmer. However, the program can still be run with null-safety warnings.
  • The compiler will add null assertions to code in opted-in packages whenever a value is assigned to a variable or parameter of non-nullable type, or is returned from a function with non-nullable return type.
  • The compiler will warn at runtime if code casts between incompatible nullable types that are otherwise compatible.

Strong null checking

Strong null checking is turned on globally for a program on a per-compile basis. Turning on strong null checking turns all null-safety violations into errors. A programmer may turn on strong null checking before all code in their program has opted in and still run their program, provided that they have fixed all of the null-safety errors in the opted-in portion of their program. They may however see new runtime errors from non-opted in code if it misuses opted-in APIs. Turning on strong null-checking will provide stronger protection from null-safety violations, but still doesn't guarantee null-safety until all libraries in the program have been opted in. Consequently, the compiler will still insert null assertions in opted-in code to catch null-safety violations. With strong null checking enabled, the programmer may also encounter runtime cast failures if their code casts between incompatible nullable types (such as casting a List<int?> to a List<int>).

Once all of the code in a program has opted in to strong null checking, the compiler will stop adding null assertions, and has the option of generating better code using the static guarantees of the type system.


External migration will begin by releasing a stable release of Dart with non-nullability available ("the NNBD release"). This release will contain a migrated SDK along with full static and runtime support for non-nullable types. Once this is released, package migration will begin, both under the auspices of the Dart team, and independently by package authors. Packages required for Flutter will be migrated, and a Flutter SDK incorporating a Dart NNBD enabled SDK and a migrated Flutter framework will be released (subject to agreement and buy-in from the Flutter team).

The suggested process for migrating an external library published on Pub is to opt the package in to weak null checking, and then verify that all tests for the package run with strong null checking turned on. This makes it more likely that downstream packages will be able to run with strong null checking on as well without encountering runtime cast failures in their upstream dependencies. The package author may publish the opted-in package as a minor version release, since opting in will not break any downstream packages. The published package must specify an SDK lower bound greater than or equal to the NNBD release of Dart.

The process for migrating an app is similar: opt the app code in, fix all of the warnings, and get the app running with strong null checking on.

For both apps and packages, opting in after all upstream dependencies have opted in minimizes the chances of having to make subsequent fixes. When an upstream dependencies opts in, new warnings may appear in opted-in downstream code. However, to avoid packages and apps being blocked on slow to upgrade upstream dependencies, we plan to fully support opting in and running before upstream dependencies have done so.

Once all code in a program has opted in, the code will be fully null safe: the only null safety errors at runtime will be from dynamic invocations, and the compiler will be able to take advantage of non-nullability to produce smaller and faster code.

Opting in

Non-nullable types will be controlled by a language opt-in as described elsewhere. In short, a library may opt in via syntax in the code, or a package may opt in in entirety. Opting in applies only to the library or package so marked.

Within an opted-in library, unmarked types are interpreted as non-nullable, and only types suffixed with ? are considered nullable (with the possible exceptions of the top types, and of course Null itself). Additional syntax and semantics are enabled for null assertions, definite assignment analysis, type promotion. Additional static errors and warnings are enabled. If we choose to move forward as proposed in this roadmap, the details of this will be worked out by the language team over the next quarter, drawing heavily on previous proposals.


Nullable (and non-nullable) types are reified, and hence have a runtime effect. So for example, casts and instance checks can distinguish between List<int> and List<int?>. Without runtime reification, nullability is relegated to an odd and inconsistent state with respect to the rest of the Dart type system, which is uniformly reified. Null-soundness is also almost certainly impossible to achieve in Dart without reification, and without null-soundness we can neither derive performance benefits in our compilers, nor deliver complete protection from null pointer errors to developers.


When all libraries in a program have opted in to non-nullability, the type system is sound and both compilers and programmers can benefit from a robust guarantee that no non-nullably typed value may be observed to be null (and hence dynamic calls will be the only source of noSuchMethod errors on null receivers). We believe that a sound type system provides the most value to Dart programmers and brings us to parity with competing languages. Specifically, we make the following arguments.

First, Kotlin and Swift both provide the experience of a null-sound system. Kotlin makes a couple of small exceptions to soundness discussed in detail below. The key point is that these exceptions are not made in the interest of reducing implementation effort, nor because they feeel that sound null-checking is too painful for the programmer, but rather because of limitations of the JVM. The overall programmer experience is of a sound null checking system. As a result, from an implementation standpoint in Dart, the implementation effort required to achieve parity in the developer experience without soundness is not noticeably less than the implementation effort required for a fully sound system. Put another way, we can only save implementation effort by compromising on the developer experience relative to Kotlin and Swift.

Second, the languages that have chosen to take an unsound path (C# and Typescript) largely seem to have been forced to do so by legacy reasons that are less applicable to Dart.

Third, we believe that null-soundness can be leveraged by our compilers to generate better code. This is especially important for Dart. Languages like Java and C# have primitive types which are implicitly soundly non-nullable. You never pay a price for null when you use int in C#. In Dart, even "primitive" types are nullable.

Soundness is discussed in more detail below, including detailed comparisons to other languages.

Incremental unsoundness and dynamic checking

During the migration period, null-soundness is not guaranteed. Opted-in code that interacts with non-opted-in code may observe null values flowing from a non-nullably typed location. We propose to specify some set of migration period dynamic checks so that opted-in code can safely remove null assertions before the migration is complete. Depending on the measured performance impact of these checks, we may choose to make these dynamic checks something that are inserted in a debug mode only (either when assertions are turned on, or perhaps controlled by a separate flag). These dynamic checks are orthogonal to any dynamic checking specified as part of the core feature, and will not be required when all packages in a program have opted in. These checks may not be sufficient to recover full null-soundness during the migration period: that is, even with the debug mode null guards in place, it may still be possible to see a null pointer error in opted-in code because of interactions with non-opted-in code. Consider this code.

library opted_in;

void takesListNonNull(List<int> l) {


library opted_out;

import "opted_in.dart" as oi;


This proposal does not allow the call to takesListNonNulll to be statically rejected (since the caller has not opted in), and we believe that it is better to allow the call to be dynamically accepted (since most working code will not be passing null to unexpected locations). This implies that the read from l might return null (as it does in this case). We could require a null-assertion on every single non-nullable expression: this would "catch" this error exactly at the point of the method call (which is where the null pointer error would happen anyway). But it is likely that this would be prohibitively expensive to little benefit, and so we might choose to only insert debug mode null checks on variables, parameters, and return values (much as was done with checked mode in Dart 1).

Migration path

We don't necessarily need to require users to follow a "waterfall migration" in which a library cannot opt in until after the transitive closure of its imports have. Instead, we could support a model in which libraries can opt in whenever they want, independently of the state of libraries they import or that import them.

In the waterfall model, our proposal has the property that it is mostly non-breaking for a library to opt in. By this we mean that opting in will never cause static errors in any non-opted-in library, and that the behavior of a library should be the same whether it is used from an opted-in client or a non-opted-in client. Any new runtime errors that are caused by opting in will either reflect an actual violation of the nullability contract (that is, a failure of an automatically inserted dynamic null-check), or by a runtime interaction between two opted-in libraries mediated by a non-opted-in library (for example, reading a List<int?> from opted-in library A, treating it as a List<int> in a non-opted in library, and then passing it on to another opted-in library which tries to cast it to a List<int>).

In the package ecosystem, we likely will want to bump major version numbers of packages that opt in despite the "mostly non-breaking" property.

In the unconstrained model, packages may choose to opt in before their upstream dependencies. However, when an upstream dependency opts in, they may have to re-migrate to accommodate the breaking change. An advantage of the unconstrained model is that apps can migrate without having to wait for all of their transitive dependencies to migrate. This makes it easier for pieces of the ecosystem to migrate, and hopefully increases the velocity of adoption. While it may be desireable to follow a "mostly-waterfall" model, the ability to opt in arbitrary packages avoids the issue where a single unmaintained (or hard to migrate) package far upstream can block an arbitrary chunk of the ecosystem from migrating.

Language support for migration

Implicit in both of the above models (waterfall and unconstrained) is an assumption of language support. While in opted-in code we propose to treat unmarked types (e.g. int) as non-nullable, we propose that in non-opted-in code, unmarked types be treated as "assumed-non-nullable", both in the static and in the reified type system. Static nullability checking is essentially turned off for these "assumed-non-nullable" types, both at compile time and at runtime.

Why not just treat un-opted-in types as nullable?

Firstly, in the unconstrained case, this forces you to make all of the wrong assumptions about the APIs that you use. You must treat them as entirely nullable, even though it is highly likely that eventually most of the types in them will become non-nullable. Moreover, treating them as nullable, while safer, makes the unmigrated APIs much more painful to use.

// not_opted_in.dart
int getInt() => 3;

// opted_in.dart
import "no_opted_in.dart";

main() {
  getInt().abs(); // Error. Can't call abs() on `int?`.

Secondly, in both the waterfall and the unconstrained case, you must decide how to deal with un-opted-in downstream dependencies.

You could choose to treat them as having been opted in (with implicitly nullable types) by virtue of being in the same program as an opted-in package. Either this is massively breaking (since the package is almost certain to mis-use the opted-in APIs), or else you treat nullability violations in the un-opted-in packages as warnings and allow the compile to continue. However, you still run into the problem of runtime breakage.

library opted_in;
void test(Object f) {
    // Assert that f maps non-null ints to non-null ints
   assert(f is int Function(int)); 

library opted_out;
import "opted_in.dart" as oi;

// Interpreted as having type int? Function(int?)
int f(int x) => 3;

Before the migration of library opted_in, this code worked. After migration, it fails the assertion.

To avoid this, you could choose to turn off runtime reification of nullability if any package in the program is not opted-in (or equivalently, have a separate opt-in for the runtime component that requires all packages to have opted in). The implication of this is that when runtime reification is turned on, you can see new runtime failures, despite all packages having migrated already.

Why not just treat un-opted-in types as non-nullable.

This is the approach that C# takes (see more discussion of this below).

This allows code that migrates before its upstream dependencies have migrated to make less pessimistic (but still sometimes incorrect) assumptions about the API of its upstream dependencies. This means that in an unconstrained migration, multiple migrations may be necessary.

The same issues with runtime behavior changes apply in this option.

Migration tooling

It could be extremely valuable to provide a nullability inference tool which does best effort inference to add nullability annotations and required null-checks to make code pass the checker.

How do C# and Typescript deal with migration?

If nullability violations are only warnings, and there is no runtime component, then it is possible to compile against other libraries that have not migrated without any language support. You may get spurious warnings, and you may have to re-migrate your code once the libraries you depend on migrate, but you can continue to make progress. Moreover, it is possible for libraries to decorate their APIs without properly migrating their internals.

Typescript and C# more or less take this approach, supporting unconstrained library opt-in, while providing no language support for the migration. The warnings are all or nothing, but libraries can be provided with an interface file (in Typescript), or can simply be worked with as if they were de-facto non-nullable (C#). A key part of this is that nullability violations are warnings, and they can always be suppressed or ignored.

The C# commentary calls out all of the issues with this:

But of course you’ll be depending on libraries. Those libraries are unlikely to add nullable annotations at exactly the same time as you. If they do so before you turn the feature on, then great: once you turn it on you will start getting useful warnings from their annotations as well as from your own.

If they add anotations after you, however, then the situation is more annoying. Before they do, you will “wrongly” interpret some of their inputs and outputs as non-null. You’ll get warnings you didn’t “deserve”, and miss warnings you should have had. ...

After the library owners get around to adding ?s to their signatures, updating to their new version may “break” you in the sense that you now get new and different warnings from before ...

But ultimately they decided to simply go with this:

We spent a large amount of time thinking about mechanisms that could lessen the “blow” of this situation. But at the end of the day we think it’s probably not worth it. We base this in part on the experience from TypeScript, which added a similar feature recently.

This path is probably not feasible if there is a runtime component of types, since in that case the runtime behavior of libraries depends on whether or not you compile them with the flag on or off. This would be more feasible if we chose to go with a static only type system, with all of the attendant limitations.

Why (or why not) null-soundness? What do other languages do?

A sound type system is one in which the type of a location is guaranteed to be a conservative approximation of the set of values which may flow there. Concretely for nullability tracking, in a sound nullable type system a location with a non-nullable type can never be observed to be null. This is in contrast to an unsound system in which some level of effort is made to stop null values from reaching non-nullable locations, but no guarantees are provided.

void noNull(String x) { // String is non-nullable
    // In a sound system, this property access is guaranteed not to
    // cause a null pointer exception, but in an unsound system it
    // might

Note that in most cases, interop can fairly abitrarily violate the static and dynamic guarantees of the system, so we leave the question of interop on the side.

Why soundness?

Benefits of soundness fall into two categories: tooling benefits and programmability benefits.

Compilers can leverage sound non-nullability to generate better code. Null checks can be eliminated, and better representations can potentially be chosen for values in locations that are guaranteed to be non-null. For example, under suitable assumptions about the class hierarchy above it, a method which returns a non-nullable int can be compiled to return it unboxed if it is guaranteed to never return null.

Programmers benefit from soundness because the checking is robust. In the absence of soundness, they either must choose to behave as if the system is sound and never check for null at the risk of getting unexpected nulls flowing into their code, or else defensively check for null despite the purported "non-nullability" provided by the type system. Compiler inserted null checking can help to lighten this burden, but only at the cost of performance.

Why not soundness?

Costs of soundness also fall into two categories: tooling costs and programmability costs. Unsoundness is a continuum ranging from very unsound systems which provide limited benefit to mostly sound systems that provide much of the benefit. Where exactly one lands on this spectrum defines the tradeoff between tooling cost and programmer cost/benefit. In general, making the system "sounder" requires either more programmer effort to satisfy the static analysis, or more tooling effort to make the static analysis understand more idioms.

Unsound systems are generally lower cost for tools because rather than doing more difficult analysis to understand programmer idioms (e.g. definite assignment analysis), they can choose to simply allow the potential unsoundness. Consider this code:

int test() {
  int x; // non-nullable
  if (something) {
    x = 3;
  } else {
    x = 4;
  return x;

In a sound system, either we must reject this code (at the programmer's expense) or we must implement a suitably sophisticated analysis to understand that x is always initialized before it is used. In an unsound system, we can simply accept this code on the assumption that the programmer knows what they are doing. The downside of this of course is that if the programmer fails to initialize the variable on some path, they may get an unexpected null pointer error somewhere else in the program.

Unsound systems can also require less effort from the programmer, since idioms which are difficult to understand in the static analysis can simply be allowed unsoundly. If the programmer finds that the static analysis cannot determine the safety of their code, they can simply opt-out.

Soundness in other languages

It's worth considering how other languages deal with null safety, and in particular with the question of soundness. Here we briefly examine how Kotlin, Swift, C# and Typescript deal with nullability.


Kotlin in general aims to have a sound type system. Interop with Java can subvert this, but the core language experience is intended to be that types can be relied on. There are two small exceptions to this which mean that Kotlin cannot actually provide full null soundness.

First, Kotlin provides an unsafe cast operator that can be used to escape the type system. This primarily arises because Kotlin is implemented over the JVM and hence have no way to implement the reification necessary for safe casts at composite types.

var nnf : (String) -> Int = { s -> s.length};
var nf : (String ?) -> Int = nnf as (String?) -> Int; // Compiler warning.
nf(null);  // Will cause a NPE

The compiler will emit a warning on the unsafe cast.

Secondly, the Java constructor semantics make it very difficult to provide a good experience for programmers in constructors, since this can leak or virtual methods can be called before the instance is initialized. Rather than cripple constructors, Kotlin has chosen to allow potential nullability violations to be introduced via constructors.

In general though, from the tooling and programmer standpoint, the intention seems to be to present the experience of a sound system. Kotlin does not seem to have compromised on issues like the soundness of "smart casts" (its version of type promotion) and definite assignment analysis.

class B {
    var f : String? = null;
void test() {
    var b : B = B();
    if (b.f != null) {
        // They do not promote b.f to a non-nullable type, since to do so
        // would be unsound.
        b.f.length; // Error, failed promotion

    var ns : String? = null;
    if (ns != null) {
      // This promotion succeeds, since it is sound
    ns.length; // Error, possibly null
    ns = "hello";
    ns.length;  // No error, definite assignment has promoted it

In general, Kotlin seems to have chosen to be unsound only in places where the limitations of the JVM leave them with no good alternatives. Notably, Kotlin does not seem to have done so in order to save tooling effort (since it has in general specified quite strong static analysis rules), and it does not seem to have done so out of concern that statisfying the static analysis would be too burdensome to programmers (since the Kotlin system is sound even in places where it might be considered inconvenient, such as failing to promote on fields).

Since Dart does not share the same limitations as Kotlin in terms of living over the JVM, we should not expect to need to be unsound in the places where it is unsound because of JVM limitations. The success of the Kotlin approach provides good evidence that a sound system can be attractive to programmers since almost the entire experience of programming in Kotlin is that of programming in a sound system.


Swift non-nullability appears to be sound, again excepting interoperation with Objective C. Unlike Kotlin, I have not been able to find any exceptions to this. This is perhaps unsurprising since as with Dart, Swift does not have to live within the constraints of running on the JVM.

Technically, Swift takes a somewhat different approach than most object-oriented languages in that Swift "nullable" values are actually instances of the built-in Optional type. This is observable in that the type T?? is different from T?.

let maybeMaybeString : String?? = Optional.some(nil)
let maybeString : String? = maybeMaybeString // Error

The swift syntax for the most part hides this distinction by building in close syntactic support for nullable values. The result is a system that largely feels to the programmer like a more standard language with null values inhabiting every nullable type.

As with Kotlin, Swift provides a rich set of operators for dealing with nullable types conveniently. However, Swift takes a slightly different approach to tackling the problem of exposing to the static analysis the results of runtime checks and operations. Kotlin primarily deals with this by allowing definite assignment analysis and null checks to "promote" or smart cast the type of variables. Swift on the other hand seems to primarily deal with this via additional language constructs making it easy to reflect the results of runtime tests into the program.

Definite assignment does not promote from nullable type to a non-nullable type in Swift.

var str : String?;
str = "Hello, playground"
str.sorted() // Error

Even guarding a use with a null check doesn't promote the type.

if str != nil {
  str.sorted() // Error

Instead, Swift provides convenient ways to both check that a value is non-nil and bind the value to a new variable in one step. The simplest version is the if let construct.

var str : String?;
// Execute the body if `str` is non-null, binding a new variable (also named
// str here) in the inner scope with a non-nullable type
if let str = str {
  str.sorted() // Ok
str.sorted() // Still an error

This is not always convenient, since the new variable goes out of scope after the body of the if, so Swift also provides a guard construct.

var str : String?;
guard let nnstr = str else {
  return;  // Must exit the enclosing scope in some way
nnstr.sorted() // No error

Swift does do some amount of definite assignment analysis in order to allow late initialization patterns.

var str : String? = "test"
var nnstr : String;
if (str != nil) {
    nnstr = "hello"
} else {
    nnstr = "world"
nnstr.sorted() // No error

Finally, Swift provides an explicit way to mark a variable as "trusted" non-null, which means that an implicit null-check is done on each reference.

var nnstr : String!;
nnstr = nil;

Swift's approach then is to provide a sound type system based around optional values, with heavy investment in additional language constructs to make working with potentially null values convenient to the programmer, but less investment in sophisticated static analysis. Swift also provides a convenient mechanism for falling back to runtime checking with low syntactic overhead where needed.


C# recently added a prototype for non-nullability for default. Unlike Swift and Kotlin, C# explicitly does not aim for soundness.

There is no guaranteed null safety, even if you react to and eliminate all the warnings. There are many holes in the analysis by necessity, and also some by choice."

There reasoning behind this is sketched out in this blog post. The high level point is that the C# designers want to avoid a "sea of errors" on existing code. They are open to at some point providing a stronger option to give sound checking, but don't believe that this is the right default for them.

I believe there are three issues in play here for C#:

  • In general, they cannot achieve soundness and backwards compatibility, so clearly they need to support unsound code at least initially. Moreover, because of the binary installed code base, it's not clear that they ever could get away from having to deal with unsoundness.
  • Some very common constructs in C# do not play with will non-nullability, and would result in pervasive errors in existing code.
  • They feel that some programming idioms are too painful to work around if checked soundly.

On the first issue, there is a massive installed base of C# code, a fair bit of it seems to be only available in a compiled form. This is essentially the interop problem, but at a much larger scale both because of the large user base, and because of the large installed base.

On the second issue, the main constructs that they call out are arrays allocated with uninitialized elements and default constructors of structs, which leave things uninitialized.

On the third issue, they specifically call out promoting a nullably typed field to a non-nullable type based on a null check as an unsound pattern that they feel is too painful to disallow.

The first of these is mostly not an issue for Dart at this point.

On the second point, arrays are likely to be much less of an issue for Dart, since the structure of the List API encourages incremental building, whereas the core C# Array type does not support this. Default constructors may be a place where some amount of Dart code will need to be modified, but Dart programming style encourages objects that are initialized in constructors, and provides good support for doing so.

This leaves the third issue. On the face of it, this flies in the face of the experience of other languages which have simply chosen to take the sound route on this with apparently no repercussions (Kotlin programmers seem to live fine with this, for example). It's possible that their sense was that this made the migration story of large amounts of existing C# code easier, or that they simply felt that since they were unsound elsewhere, they might as well be here. The migration story may come into play for Dart, but the scale of the problem is much smaller.


Java provides nullability annotations that can be used by external tools to provide sound or near-sound checking. The annotations are not interpreted with the language at all. The Java documentation suggests not relying on nullability annotations, saying "Optional Type Annotations are not a substitute for runtime validation".


Typescript is unsound by design. In addition to the core unsoundness of the type system, it specifically allows unsound nullable promotion of dotted paths.

class Ref<T> {
  v : T;
  constructor(_v : T) {this.v = _v;}

function main() {
	var rn: Ref<Ref<number | null>> = new Ref(new Ref(3));
	var rn2 = rn;

	if (rn.v.v != null) {
		rn2.v.v = null;
		var x : number = rn.v.v;  // Assigns null to a non-nullable type

As with other languages, Typescript provides an assertion operator e!.foo which asserts that e is non-null. Unlike Swift and Kotlin, this operator is not dynamically checked.

Typescript also provides an option to do definite assignment analysis in constructors to help enforce proper object construction.

class IntRef {
  v : number; // Error, not definitely assigned in constructor
	constructor(_v: number) { }

As with Kotlin and Swift, Typescript does not seem to have skimped on the static analysis work, but instead has invested fairly heavily in providing robust checking. Since the Typescript type system is unsound there is essentially no way for them to provide null-soundness. As with C# however, Typescript does seem to have chosen to be unsound in a few places out of choice in addition to necessity, to relieve the programmer of some of the burden of proving safety to the type checker. Since Typescript is purely a static layer on top of Javascript, there is also no performance benefit to null-soundness for it.

Issues affecting migration paths

In any of the possible paths, we have to consider what happens in the following scenarios.

Runtime type tests

The essential questions that needs to be answered in any proposal are the following:

  • How are the runtime types of objects from opted-in libraries viewed in non-opted-in libraries?
  • How are the runtime types of objects from non-opted-in libraries viewed in opted-in libraries?

The following examples illustrate this question.

Consider an opted in library with the following code:

library opted_in;
// A function taking and returning a non-null int
int f(int x) => x;

void test(Object f) {
  print(f is int Function(int?));

In a post-migration world where nullable types are reified, this should return false. Any migration path must decide what to do about this code.

  • Return false?
  • Return true during the migration, then switch to false later?
    • If so, then there are two migration steps
  • Don't allow the question? Difficult in the presence of implicit checks.

In either a waterfall or an unconstrained model, we need to consider what happens with the following:

library opted_out;

import "opted_in.dart" as oi;

// A function taking a non-null int and return null
int f(int x) {
  assert(x != null);
  return null;

// prints true or false?
// prints true or false?
print(oi.f is int Function(int));

In an unconstrained model, we must also consider the following.

library not_converted;
// A function taking a non-null int and return null
int f(int x) {
  assert(x != null);
  return null;

library opted_in;
import "not_converted.dart" as nc;

void test(Object f) {
  print(f is int Function(int?));
// Prints true or false?

Cross library static checks

Consider an opted in library with the following code:

library opted_in;

int Function(int) fnn;
int Function(int?) fyn;
int? Function(int) fny;
int? Function(int?) fyy;

// A non-null top level variable
int x = 3;
// A nullable top level variable
int? y = null;

In a waterfall model, an un-opted-in client of this library that worked before conversion should still work after the library has converted. In particular, the following code would have been statically allowed pre-conversion, and hence should still be statically allowed posts-conversion (and should arguably dynamically continue to work at least as well as it did pre-conversion).

library opted_out;

import "opted_in.dart" as oi;

int f(int x) => null;

void test() {
  // Assigning between nullable and non-nullable fields
  oi.x = null;
  int y = oi.y;
  oi.x = oi.y;
  oi.y = oi.x;

  // Calling methods on nullable fields

  int _ = oi.fny(3);

  // Assigning between higher order objects with nullable types embedded
  oi.fnn = f;
  oi.fyn = f;
  oi.fny = f;
  oi.fyy = f;
  f = oi.fnn;
  f = oi.fyn;
  f = oi.fny;
  f = oi.fyy;

In an unconstrained model, we must also consider opted-in libraries that import non-opted-in libraries.

library not_converted;

int f(int x) => null;

int x = null;
library opted_in;

import "not_converted.dart" as nc;

int Function(int) fnn;
int Function(int?) fyn;
int? Function(int) fny;
int? Function(int?) fyy;

// A non-null top level variable
int x = 3;
// A nullable top level variable
int? y = null;

void test() {

  nc.x = null;
  nc.x = 3;
  x = nc.x;

  int y = nc.y;
  nc.x = y;


  int _ = nc.f(3);

  // Assigning between higher order objects with nullable types embedded
  fnn = nc.f;
  fyn = nc.f;
  fny = nc.f;
  fyy = nc.f;
  nc.f = fnn;
  nc.f = fyn;
  nc.f = fny;
  nc.f = fyy;
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