This document is now obsolete, please refer to the current version.
Author: eernst@
Status: Under implementation.
This document is an informal specification of the generalized support in
Dart 1.x for the type void. Dart 2 will have a very similar kind of
generalized support for void, without the function type subtype exception
that this feature includes for backward compatibility in Dart 1.x. This document
specifies the feature for Dart 1.x and indicates how Dart 2 differs at relevant
points.
The feature described here, generalized void, allows for using the type
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.
Note that is not technically dangerous to use a value of type void, it does
not violate any constraints at the level of the language semantics.
Developers just made the decision to declare that the value is useless, based
on the program logic. 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 strict 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.
The mechanisms helping developers to avoid using values of type void are
divided into two phases. This document specifies the first phase. The
second phase may be added to Dart 1.x later, or it may only be provided in
Dart 2.0, depending on the schedule.
The first phase uses restrictions which are based on syntactic criteria
in order to ensure that direct usage of a value of type void is a static
warning (in Dart 2: an 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 all values of type void must be
discarded.
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", with static type Object.We plan 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, and it is possible that voidness preservation will be checked in Dart 2 and strong mode, but that it is never added to Dart 1.x. In any case, 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?
type ::= // ENTIRE RULE MODIFIED
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/.
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.
Conversely, void cannot denote any other entity than the type void: void
cannot occur as the declared name of any declaration (including library
prefixes, types, variables, parameters, etc.). This implies that void is
not subject to scoped lookup, and the name is not exported by any system
library. Similarly, it can never be accessed using a prefixed expression
(p.void). Hence, void has a fixed meaning everywhere in all Dart
programs, and it can only occur as a stand-alone word.
When void is passed as an actual type argument to a generic class or a
generic function, and when the type void occurs as a parameter type in a
function type, the reified representation is equal (according to ==) to the
reified representation of the built-in class Object.
It is encouraged for an implementation to use a reified representation for
void as a type argument and as a parameter type in a function type which is
not identical to the reified representation of the built-in class Object,
but they must be equal. This allows implementations to produce better
diagnostic messages, e.g., in case of a runtime error.
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, it is not a separate type. However, for backward
compatibility we need to treat return types differently.
When void is specified as the return type of a function, the reified
representation of the return type is left unspecified.
There is no way for a Dart program at run time to obtain a reified representation of that return type alone, even when the function type as a whole may be obtained (e.g., the function type could be evaluated as an expression). It is therefore not necessary to reified representation of such a return type.
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.
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 void is treated as Object.
Consider a function type T where the return type is the type void. 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.
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 2.0. 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 checked mode checks involving types where the type void occurs is determined by the semantics of subtype tests, so we do not specify that separately.
An instantiation of a generic class G is malbounded if it contains 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.
For the static analysis, the more-specific-than relation, <<, and the
subtype relation, <:, are determined by the same rules as described above
for the dynamic semantics.
That is, the type void is considered to be equivalent to the built-in class
Object, except when used as a return type, in which case it is effectively
considered to be a proper supertype of Object. As mentioned, voidness
preservation is a separate analysis which is not specified by this document,
but it is intended to be used in the future to track "voidness" in types and
flag implicit casts wherein information about voidness may indirectly be
lost. With voidness preservation in place, we expect to be able to treat the
type void as Object in all cases during subtype checks.
It is a static warning for an expression to have 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) {..},e1ande3may have type void. - In a typeCast
e as T,emay have type void. - In a typeTest
e is Tore is! T,emay have type void. - In a parenthesized expression
(e),emay have type void. - In a return statement
return e;, when the return type of the innermost enclosing function is the type void,emay have type void.
Note that the parenthesized expression itself has type void, so it is again subject to the same constraints. Also note that we may not allow return statements returning an expression of type void in the future, but it is allowed here for backward compatibility.
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.
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 values of type void should
be discarded.
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
presumably 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 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
typeNotVoidrather thanvoidOrType. -
June 5th 2017: Added
typeCastandtypeTestto the locations where void expressions may occur.