Feature: Generalized Void

Author: eernst@

Version: 0.10 (2018-07-10)

Status: This document is now background material. For normative text, please consult the language specification.

This document is a feature specification of the generalized support in Dart 2 for the type void.

The feature described here, generalized void, allows for using void as a type annotation and as a type argument.

The motivation for allowing the extended usage is that it helps developers state the intent that a particular value should be ignored. For example, a Future<void> may be awaited in order to satisfy ordering dependencies in the execution, but no useful value will be available at completion. Similarly, a Visitor<void> (where we assume the type argument is used to describe a value returned by the visitor) may be used to indicate that the visit is performed for its side-effects alone. The generalized void feature includes mechanisms to help developers avoid using such a value.

In general, situations where it may be desirable to use void as a type argument arise when the corresponding formal type variable is used covariantly. For instance, the class Future<T> uses return types like Future<T> and Stream<T>, and it uses T as a parameter type of a callback in the method then.

Note that using the value of an expression of type void is not technically dangerous, doing so does not violate any constraints at the level of the language semantics. By using the type void, developers indicate that the value of the corresponding expression evaluation is meaningless. Hence, there is no requirement for the generalized void mechanism to be strict and sound. However, it is the intention that the mechanism should be sufficiently sound to make the mechanism helpful and non-frustrating in practice.

No constraints are imposed on which values may be given type void, so in that sense void can be considered to be just another name for the type Object, flagged as useless. Note that this is an approximate rule in Dart 1.x, it fails to hold for function types; it does hold in Dart 2.

The mechanisms helping developers to avoid using the value of an expression of type void are divided into two phases. This document specifies the first phase.

The first phase uses restrictions which are based on syntactic criteria in order to ensure that direct usage of the value of an expression of type void is a compile-time error. A few exceptions are allowed, e.g., type casts, such that developers can explicitly make the choice to use such a value. The general rule is that for every expression of type void, its value must be ignored.

The second phase will deal with casts and preservation of voidness. Some casts will cause derived expressions to switch from having type void to having some other type, and hence those casts introduce the possibility that “a void value” will get passed and used. Here is an example:

class A<T> { T foo(); }
A<Object> a = new A<void>(); // Violates voidness preservation.
var x = a.foo(); // Use a "void value", now with static type Object.

We intend to introduce a voidness preservation analysis (which is similar to a small type system) to keep track of such situations. As mentioned, the second phase is not specified in this document. Voidness preservation is a purely static analysis, and there are no plans to introduce dynamic checking for it.


The reserved word void remains a reserved word, but it will now be usable in additional contexts. Below are the grammar rules affected by this change. New grammar rules are marked NEW, other grammar rules are modified. Unchanged alternatives in a rule are shown as .... The grammar rules used as a starting point for this syntax are taken from the language specification as of June 2nd, 2017 (git commit 0603b18).

typeNotVoid ::= // NEW
    typeName typeArguments?
    typeNotVoid | 'void'
redirectingFactoryConstructorSignature ::=
    'const'? 'factory' identifier ('.' identifier)?
    formalParameterList `=' typeNotVoid ('.' identifier)?
superclass ::=
    'extends' typeNotVoid
mixinApplication ::=
    typeNotVoid mixins interfaces?
typeParameter ::=
    metadata identifier ('extends' typeNotVoid)?
newExpression ::=
    'new' typeNotVoid ('.' identifier)? arguments
constObjectExpression ::=
    'const' typeNotVoid ('.' identifier)? arguments
typeTest ::=
    isOperator typeNotVoid
typeCast ::=
    asOperator typeNotVoid
onPart ::=
    catchPart block |
    'on' typeNotVoid catchPart? block
typeNotVoidList ::=
    typeNotVoid (',' typeNotVoid)*
mixins ::=
    'with' typeNotVoidList
interfaces ::=
    'implements' typeNotVoidList
functionSignature ::=
    metadata type? identifier formalParameterList
functionFormalParameter ::=
    metadata 'covariant'? type? identifier formalParameterList
operatorSignature ::=
    type? 'operator' operator formalParameterList
getterSignature ::=
    type? 'get' identifier
setterSignature ::=
    type? 'set' identifier formalParameterList
topLevelDefinition ::=
    type? 'get' identifier functionBody |
    type? 'set' identifier formalParameterList functionBody |
functionPrefix ::=
    type? identifier

The rule for returnType in the grammar is deleted.

This is because we can now use type, which derives the same expressions as returnType used to derive. In that sense, some of these grammar modifications are renames. Note that the grammar contains known mistakes, especially concerned with the placement of metadata. This document makes no attempt to correct those mistakes, that is a separate issue.

A complete grammar which includes support for generalized void is available in the file Dart.g from https://codereview.chromium.org/2688903004/.

Dynamic semantics

There are no values at run time whose dynamic type is the type void.

This implies that it is never required for the getter runtimeType in the built-in class Object to return a reified representation of the type void. Note, however, that apart from the fact that usage is restricted for values with the type void, it is possible for an expression of type void to evaluate to any value. In that sense, every value has the type void, it is just not the only type that it has, and loosely speaking it is not the most specific type.

There is no value which is the reified representation of the type void at run time.

Syntactically, void cannot occur as an expression, and hence expression evaluation cannot directly yield such a value. However, a formal type parameter can be used in expressions, and the actual type argument bound to that formal type parameter can be the type void. That case is specified explicitly below. Apart from the reserved word void and a formal type parameter, no other term can denote the type void.

There is no way for a Dart program at run time to obtain a reified representation of a return type or parameter type of a function type, even when the function type as a whole may be obtained (e.g., the function type could be passed as a type argument and the corresponding formal type parameter could be evaluated as an expression). A reified representation of such a return type is therefore not necessary.

For a composite type (a generic class instantiation or a function type), the reified representation at run time must be such that the type void and the built-in class Object are treated as equal according to ==, but they need not be identical.

For example, with typedef F<S, T> = S Function(T), the Type instance for F<Object, void> at run time is == to the one for F<void, void> and for F<void, Object>.

In case of a dynamic error, implementations are encouraged to emit an error message that includes information about such parts of types being void rather than Object. Developers will then see types which are similar to the source code declarations. This may be achieved using distinct Type objects to represent types such as F<void, void> and F<Object, void>, comparing equal using == but not identical.

This treatment of the reified representation of the type void reinforces the understanding that “voidness” is merely a statically known flag on the built-in class Object. However, for backward compatibility we need to treat return types differently in Dart 1.x.

It may be possible to use a reflective subsystem (mirrors) to deconstruct a function type whose return type is the type void, but the existing design of the system library dart:mirrors already handles this case by allowing for a type mirror that does not have a reflected type. All in all, the type void does not need to be reified at run time, and it is not reified.

Consider a type T where the type void occurs as an actual type argument to a generic class, or as a parameter type in a function type. Dynamically, the more-specific-than relation (<<) and the dynamic subtype relation (<:) between T and other types are determined by the following rule: the type void is treated as being the built-in class Object.

Dart 1.x does not support generic function types dynamically, because they are erased to regular function types during compilation. Hence there is no need to specify the the typing relations for generic function types. In Dart 2, the subtype relationship for generic function types follows from the rule that the type void is treated as Object.

Consider a function type T where the return type is the type void. In Dart 1.x, the dynamic more-specific-than relation, <<, and the dynamic subtype relation, <:, are determined by the existing rules in the language specification, supplemented by the above rule for handling occurrences of the type void other than as a return type. In Dart 2 there is no exception for return types: the type void is treated as being the built-in class Object.

This ensures backward compatibility for the cases where the type void can be used already today. It follows that it will be a breaking change to switch to a ruleset where the type void even as a return type is treated like the built-in class Object, i.e. when switching to Dart 2. However, the only situation where the semantics differs is as follows: Consider a situation where a value of type void Function(...) is assigned to a variable or parameter x whose type annotation is Object Function(...), where the argument types are arbitrary, but such that the assignment is permitted. In that situation, in checked mode, the assignment will fail with the current semantics, because the type of that value is not a subtype of the type of x. The rules in this document preserve that behavior. If we were to consistently treat the type void as Object at run time (as in Dart 2) then this assignment would be permitted (but we would then use voidness preservation to detect and avoid this situation at compile time).

The semantics of dynamic checks involving types where the type void occurs is determined by the semantics of subtype tests, so we do not specify that separately.

It is a compile-time error to use void as the bound of a type variable.

An instantiation of a generic class G is malbounded if it contains the type void as an actual type argument for a formal type parameter, unless that type parameter does not have a bound, or it has a bound which is the built-in class Object, or dynamic.

The treatment of malbounded types follows the current specification.

Static Analysis

For the static analysis, the subtype relation, <:, is determined by the same rules as described above for the dynamic semantics.

That is, the type void, for the purposes of subtyping, is considered to be equivalent to the built-in class Object. As mentioned, this document does not specify voidness preservation. However, when voidness preservation checks are added we will get (among other things) an effect which is similar to the special treatment of void as a return type which was used in Dart 1.x: In Dart 1.x, an implicit downcast from void Function() to Object Function() will fail at run time, but with voidness preservation it will be a compile-time error.

It is a compile-time error to evaluate an expression of type void, except for the following situations:

  • In an expressionStatement e;, e may have type void.
  • In the initialization and increment expressions of a for-loop, for (e1; e2; e3) {..}, e1 and e3 may have type void.
  • In a type cast e as T, e may have type void.
  • In a parenthesized expression (e), e may have type void.
  • In a conditional expression e ? e1 : e2, e1 and e2 may have the type void; the static type of the conditional expression is then the type void. (This is true even if one of the branches has a different type.)
  • In a null coalescing expression e1 ?? e2, e2 may have the type void; the static type of the null coalescing expression is then the type void.
  • If N1 and N2 are non-terminals in the Dart grammar, and there is a derivation of the form N1 --> N2, and N2 can have type void, then N1 can also have type void for such a derivation. In this derivation no additional tokens are included, it is only the non-terminal which changes.
  • In a return statement return e;, when the return type of the innermost enclosing function is the type void or dynamic, e may have type void.
  • In an arrow function body => e, when the return type is the type void or dynamic, the returned expression e may have type void.
  • An initializing expression for a variable of type void may have the type void.
  • An actual parameter expression corresponding to a formal parameter whose statically known type annotation is the type void may have the type void.
  • In an expression of the form e1 = e2 where e1 is an assignableExpression denoting a variable or parameter of type void, e2 may have the type void.
  • Assume that e is an expression ending in a cascadeSection of the form '..' S s = e1 where S is of the form (cascadeSelector argumentPart*) (assignableSelector argumentPart*)* and e1 is an expressionWithoutCascade. If s is an assignableSelector of the form '.' identifier or '?.' identifier where the static type of the identifier is the type void, e1 may have type void; otherwise, if s is an assignableSelector of the form '[' e0 ']' where the static type of the first formal parameter in the statically known declaration of operator []= is the type void, e0 may have type void; also, if the static type of the second formal parameter is the type void, e1 may have type void.

The rule about non-terminals is needed in order to allow, say, void x = b ? (y) : e2; where y has type void: y is an identifier which is derived from primary, which is derived from postfixExpression, from unaryExpression, from multiplicativeExpression, etc. Only if we allow such a (trivial) multiplicativeExpression can we allow the corresponding (trivial) unaryExpression, etc., all the way down to identifier, and all the way up to expression, which is needed for the initialization of x.

The general rule is that the value yielded by an expression of type void must be discarded (and hence ignored), except when explicitly subjected to a type cast, or when returned or assigned to a target of type void. This “makes it hard to use a meaningless value”, but leaves a small escape hatch open for the cases where the developer knows that the typing misrepresents the actual situation.

It is a compile-time error if a return statement return e; occurs such that the innermost enclosing function has return type void and the static type of e is not the type void.

It is a compile-time error if a function marked async*, or sync* has return type void.

Note that it is allowed for an async function to have return type void. This serves to indicate that said function performs a “fire-and-forget” operation, that is, it is not even useful for the caller to synchronize with the completion of that task.

It is a compile-time error for a for-in statement to have an iterator expression of type T such that Iterator<void> is the most specific instantiation of Iterator that is a superinterface of T, unless the iteration variable has type void.

It is a compile-time error for an asynchronous for-in statement to have a stream expression of type T such that Stream<void> is the most specific instantiation of Stream that is a superinterface of T, unless the iteration variable has type void.

Hence, for (Object x in <void>[]) {} and await for (int x in new Stream<void>.empty()) {} are errors, whereas for (void x in <void>[]) {...} and for (var x in <void>[]) {...} are OK. The usage of x in the loop body is constrained, though, because it has type void.

During bounds checking, it is possible that a bound of a formal type parameter of a generic class or function is statically known to be the type void. In this case, the bound is considered to be the built-in class Object.

It is a compile-time error when a method declaration D2 with return type void overrides a method declaration D1 whose return type is not void.

This rule is a special case of voidness preservation, which maintains the discipline which arises naturally from the function type subtype rules in Dart 1.x concerning void as a return type. It also matches the conceptual interpretation that a value of type void can be anything, but it should be discarded: This ensures that a subtype can be used where the supertype is expected (also known as Liskov substitutability), because it is always considered safe to ignore the value of an expression evaluation.


Expressions derived from typeCast and typeTest do not support void as the target type. We have omitted support for this situation because we consider it to be useless. If void is passed indirectly via a type variable T then e as T, e is T, and e is! T will treat T like Object. In general, the rationale is that the type void admits all values (because it is just Object plus a “static voidness flag”), but the value of expressions of type void should be discarded. So there is no point in obtaining the type void for a given expression which already has a different type.

The treatment of bounds is delicate. We syntactically prohibit void as a bound of a formal type parameter of a generic class or function. It is possible to pass the type void as an actual type argument to a generic class, and that type argument might in turn be used as the bound of another formal type parameter of the class, or of a generic function in the class. It would be possible to make it a compile-time error to pass void as a type argument to a generic class where it will be used as a bound, but this would require a transitive traversal of all generic classes and functions where the corresponding formal type parameter is passed on to other generic classes or functions, which would be highly brittle: A tiny change to a generic class or function could break code far away. So we do not wish to prevent formal type parameter bounds from indirectly becoming the type void. This motivated the decision to treat such a void-valued bound as Object.


  • July 10th 2018, v0.10: Added case to whitelist: It is not an error to return e; with an e of type void when the return type is dynamic.

  • February 22nd 2018, v0.9: Added several new contexts where an expression with static type void may be evaluated, such that pure data transfers to a target of type void are allowed. For instance, a void expression may be passed as an actual argument to a parameter of type void.

  • August 22nd 2017: Reworded specification of reified types to deal with only such values which may be obtained at run time (previously mentioned some entities which may not exist). Added one override rule.

  • August 17th 2017: Several parts clarified.

  • August 16th 2017: Removed exceptions allowing e is T and e is! T.

  • August 9th 2017: Transferred to SDK repo, docs/language/informal.

  • July 16th 2017: Reformatted as a gist.

  • June 13th 2017: Compile-time error for using a void value was changed to static warning.

  • June 12th 2017: Grammar changed extensively, to use typeNotVoid rather than voidOrType.

  • June 5th 2017: Added typeCast and typeTest to the locations where void expressions may occur.