mirror of
https://github.com/dart-lang/sdk
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Integrated generalized-void.md into dartLangSpec.tex
Also marked the feature specification generalized-void.md as background material Change-Id: I08c9b54d956208db5aa8e695540b0c1fc941e46b Reviewed-on: https://dart-review.googlesource.com/c/86421 Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
This commit is contained in:
parent
2219b51b7e
commit
5b3b1996a8
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@ -163,6 +163,9 @@
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% parameterized type (e.g., `C<int>`).
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% - Integrate invalid_returns.md. This replaces the rules about when it is
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% an error to have `return;` or `return e;` in a function.
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% - Integrate generalized-void.md. Introduces syntactic support for using
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% `void` in many new locations, including variable type annotations and
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% actual type arguments; also adds errors for using values of type `void`.
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%
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% 1.15
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% - Change how language specification describes control flow.
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@ -1058,13 +1061,10 @@ Functions abstract over executable actions.
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\begin{grammar}
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<functionSignature> ::= \gnewline{}
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<metadata> <returnType>? <identifier> <formalParameterPart>
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<metadata> <type>? <identifier> <formalParameterPart>
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<formalParameterPart> ::= <typeParameters>? <formalParameterList>
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<returnType> ::= \VOID{}
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\alt <type>
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<functionBody> ::= \ASYNC{}? `=>' <expression> `;'
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\alt (\ASYNC{} | \ASYNC `*' | \SYNC `*')? <block>
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@ -1364,7 +1364,7 @@ It is a compile-time error if any default values are specified in the signature
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\alt <simpleFormalParameter>
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<functionFormalParameter> ::= \gnewline{}
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<metadata> \COVARIANT{}? <returnType>? <identifier> <formalParameterPart>
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<metadata> \COVARIANT{}? <type>? <identifier> <formalParameterPart>
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<simpleFormalParameter> ::= <declaredIdentifier>
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\alt <metadata> \COVARIANT{}? <identifier>
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@ -1681,7 +1681,7 @@ and $u$ is not a subtype of any function type which is a proper subtype of $t$.
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If we had omitted the last requirement then
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\code{f \IS{} int\,\FUNCTION([int])}
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could evaluate to \TRUE{} with the declaration
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\code{void f()\,\{\}},
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\code{\VOID{} f()\,\{\}},
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e.g., by letting $u$ be \code{Null}.
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}
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@ -1743,7 +1743,9 @@ Classes may be defined by class declarations as described below, or via mixin ap
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\gnewline{} `{' (<metadata> <classMemberDefinition>)* `}'
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\alt <metadata> \ABSTRACT{}? \CLASS{} <mixinApplicationClass>
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<mixins> ::= \WITH{} <typeList>
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<mixins> ::= \WITH{} <typeNotVoidList>
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<typeNotVoidList> ::= <typeNotVoid> (`,' <typeNotVoid>)*
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<classMemberDefinition> ::= <declaration> `;'
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\alt <methodSignature> <functionBody>
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@ -2001,7 +2003,7 @@ in a trade-off where some static type safety is lost.
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\begin{grammar}
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<operatorSignature> ::= \gnewline{}
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<returnType>? \OPERATOR{} <operator> <formalParameterList>
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<type>? \OPERATOR{} <operator> <formalParameterList>
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<operator> ::= `~'
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\alt <binaryOperator>
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@ -2418,7 +2420,7 @@ because this is an invocation of a function object
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Getters are functions (\ref{functions}) that are used to retrieve the values of object properties.
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\begin{grammar}
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<getterSignature> ::= <returnType>? \GET{} <identifier>
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<getterSignature> ::= <type>? \GET{} <identifier>
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\end{grammar}
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\LMHash{}%
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@ -2455,7 +2457,7 @@ are given elsewhere
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Setters are functions (\ref{functions}) that are used to set the values of object properties.
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\begin{grammar}
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<setterSignature> ::= <returnType>? \SET{} <identifier> <formalParameterList>
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<setterSignature> ::= <type>? \SET{} <identifier> <formalParameterList>
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\end{grammar}
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\commentary{
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@ -3099,7 +3101,7 @@ whenever the redirecting constructor is called.
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\begin{grammar}
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<redirectingFactoryConstructorSignature> ::= \gnewline{}
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\CONST{}? \FACTORY{} <identifier> (`.' <identifier>)? <formalParameterList> `='
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\gnewline{} <type> (`.' <identifier>)?
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\gnewline{} <typeNotVoid> (`.' <identifier>)?
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\end{grammar}
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Assume that
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@ -3422,7 +3424,7 @@ It is a compile-time error to specify an \EXTENDS{} clause
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for class \code{Object}.
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\begin{grammar}
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<superclass> ::= \EXTENDS{} <type>
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<superclass> ::= \EXTENDS{} <typeNotVoid>
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\end{grammar}
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%The superclass clause of a class C is processed within the enclosing scope of the static scope of C.
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@ -3609,7 +3611,7 @@ and the interfaces of the classes specified in
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the \IMPLEMENTS{} clause of the class.
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\begin{grammar}
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<interfaces> ::= \IMPLEMENTS{} <typeList>
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<interfaces> ::= \IMPLEMENTS{} <typeNotVoidList>
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\end{grammar}
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\LMHash{}%
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@ -4216,7 +4218,7 @@ or type \code{FutureOr<$T$>} for any $T$ (\ref{typeFutureOr}).
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<mixinApplicationClass> ::= \gnewline{}
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<identifier> <typeParameters>? `=' <mixinApplication> `;'
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<mixinApplication> ::= <type> <mixins> <interfaces>?
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<mixinApplication> ::= <typeNotVoid> <mixins> <interfaces>?
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\end{grammar}
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\LMHash{}%
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@ -4551,7 +4553,7 @@ where each formal type parameter has been replaced by the corresponding actual t
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}
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\begin{grammar}
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<typeParameter> ::= <metadata> <identifier> (\EXTENDS{} <type>)?
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<typeParameter> ::= <metadata> <identifier> (\EXTENDS{} <typeNotVoid>)?
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<typeParameters> ::= `<' <typeParameter> (`,' <typeParameter>)* `>'
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\end{grammar}
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@ -7133,7 +7135,7 @@ or is an enumerated type (\ref{enums}).
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The \Index{new expression} invokes a constructor (\ref{constructors}).
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\begin{grammar}
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<newExpression> ::= \NEW{} <type> (`.' <identifier>)? <arguments>
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<newExpression> ::= \NEW{} <typeNotVoid> (`.' <identifier>)? <arguments>
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\end{grammar}
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\LMHash{}%
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@ -7317,7 +7319,7 @@ A \Index{constant object expression} invokes a constant constructor
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(\ref{constantConstructors}).
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\begin{grammar}
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<constObjectExpression> ::= \CONST{} <type> (`.' <identifier>)? <arguments>
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<constObjectExpression> ::= \CONST{} <typeNotVoid> (`.' <identifier>)? <arguments>
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\end{grammar}
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\LMHash{}%
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@ -10430,7 +10432,7 @@ and there is no declaration $d$ with name \id{} in the lexical scope enclosing t
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The \Index{is-expression} tests if an object is a member of a type.
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\begin{grammar}
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<typeTest> ::= <isOperator> <type>
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<typeTest> ::= <isOperator> <typeNotVoid>
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<isOperator> ::= \IS{} `!'?
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\end{grammar}
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@ -10499,7 +10501,7 @@ The static type of an is-expression is \code{bool}.
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The \Index{cast expression} ensures that an object is a member of a type.
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\begin{grammar}
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<typeCast> ::= <asOperator> <type>
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<typeCast> ::= <asOperator> <typeNotVoid>
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<asOperator> ::= \AS{}
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\end{grammar}
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@ -11500,7 +11502,7 @@ The try statement supports the definition of exception handling code in a struct
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<tryStatement> ::= \TRY{} <block> (<onPart>+ <finallyPart>? | <finallyPart>)
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<onPart> ::= <catchPart> <block>
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\alt \ON{} <type> <catchPart>? <block>
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\alt \ON{} <typeNotVoid> <catchPart>? <block>
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<catchPart> ::= \CATCH{} `(' <identifier> (`,' <identifier>)? `)'
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@ -11671,6 +11673,12 @@ unless \flatten{T}
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(\ref{functionExpressions})
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is \VOID{}, \DYNAMIC{}, or \code{Null}.
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%
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\rationale{%
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An asynchronous non-generator always returns a future of some sort.
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If no expression is given, the future will be completed with the null object
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(\ref{null})
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which motivates this rule.%
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}
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% Returning with a value in an void async function only ok
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% when that value is async-"voidy".
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It is a compile-time error if $s$ is \code{\RETURN{} $e$;},
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@ -11744,22 +11752,18 @@ which can otherwise be very hard to recognize.
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There is no real downside to it,
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as returning a value from a generative constructor is meaningless.
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}
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\rationale{
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An asynchronous non-generator always returns a future of some sort.
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If no expression is given, the future will be completed with the null object (\ref{null}) and this motivates the requirement above.
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}
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\EndCase
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\LMHash{}%
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Executing a return statement \code{\RETURN{} $e$;} proceeds as follows:
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\LMHash{}%
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First the expression $e$ is evaluated, producing an object $o$.
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Let $T$ be the run-time type of $o$ and
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let $S$ be the actual return type of $f$
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Let $S$ be the run-time type of $o$ and
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let $T$ be the actual return type of $f$
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(\ref{actualTypeOfADeclaration}).
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If the body of $f$ is marked \ASYNC{} (\ref{functions})
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and $T$ is a subtype of \code{Future<\flatten{S}>}
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and $S$ is a subtype of \code{Future<\flatten{T}>}
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then let $r$ be the result of evaluating \code{await $v$}
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where $v$ is a fresh variable bound to $o$.
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Otherwise let $r$ be $o$.
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@ -11783,12 +11787,12 @@ Then the return statement returns the value $r$ (\ref{statementCompletion}).
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%%
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%% Object /* or dynamic, etc. */ foo() async {
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%% Object o;
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%% if (someCondition) o = 42; else o = new Future<T>.value(42);
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%% if (someCondition) o = 42; else o = new Future<S>.value(42);
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%% return o;
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%% }
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%%
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%% Here we will interject an `await` on the returned value whenever
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%% we return a future (for all `T`). This means that we will `await o`
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%% we return a future (for all `S`). This means that we will `await o`
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%% in some situations and not in others. It may surprise developers
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%% that we can have this dynamic variation at a location in the code
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%% where there is no static justification for expecting a future.
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@ -11796,16 +11800,11 @@ Then the return statement returns the value $r$ (\ref{statementCompletion}).
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\LMHash{}%
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Let $U$ be the run-time type of $r$.
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\begin{itemize}
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\item If the body of $f$ is marked \ASYNC{} (\ref{functions})
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it is a dynamic type error if $o$ is not the null object (\ref{null}),
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% TODO(lrn): Remove the next line when void is a proper supertype of all types.
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$S$ is not \VOID,
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and \code{Future<$U$>} is not a subtype of $S$.
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\item Otherwise,
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it is a dynamic type error if $r$ is not the null object (\ref{null}),
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% TODO(lrn): Remove the next line when void is a proper supertype of all types.
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$S$ is not \VOID{},
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and $U$ is not a subtype of $S$.
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\item
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If the body of $f$ is marked \ASYNC{} (\ref{functions})
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it is a dynamic type error if \code{Future<$U$>} is not a subtype of $T$.
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\item
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Otherwise, it is a dynamic type error if $U$ is not a subtype of $T$.
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\end{itemize}
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\LMHash{}%
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@ -12131,8 +12130,8 @@ The members of a library $L$ are those top level declarations given within $L$.
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\alt \EXTERNAL{}? <getterSignature> `;'
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\alt \EXTERNAL{}? <setterSignature> `;'
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\alt <functionSignature> <functionBody>
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\alt <returnType>? \GET{} <identifier> <functionBody>
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\alt <returnType>? \SET{} <identifier> <formalParameterList> <functionBody>
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\alt <type>? \GET{} <identifier> <functionBody>
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\alt <type>? \SET{} <identifier> <formalParameterList> <functionBody>
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\alt (\FINAL{} | \CONST{}) <type> <staticFinalDeclarationList> `;'
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\alt <variableDeclaration> `;'
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@ -12759,7 +12758,9 @@ Type annotations are used in variable declarations (\ref{variables}) (including
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Type annotations are used during static checking and when running programs.
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\begin{grammar}
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<type> ::= <typeName> <typeArguments>?
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<type> ::= <typeNotVoid> | \VOID{}
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<typeNotVoid> ::= <typeName> <typeArguments>?
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<typeName> ::= <qualified>
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@ -12928,8 +12929,8 @@ declaring a name for the same function type are equal:
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}
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\begin{dartCode}
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\TYPEDEF{} F = void Function<X>(X);
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\TYPEDEF{} G = void Function<Y>(Y);
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\TYPEDEF{} F = \VOID{} \FUNCTION{}<X>(X);
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\TYPEDEF{} G = \VOID{} \FUNCTION{}<Y>(Y);
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\\
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\VOID{} main() \{
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assert(F == G);
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|
@ -12973,7 +12974,7 @@ A \Index{type alias} declares a name for a type expression.
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<functionTypeAlias> ::= \gnewline{}
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<functionPrefix> <typeParameters>? <formalParameterList> `;'
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|
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<functionPrefix> ::= <returnType>? <identifier>
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<functionPrefix> ::= <type>? <identifier>
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\end{grammar}
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% TODO(eernst): Introduce new type aliases and new function type syntax, then
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|
@ -13886,8 +13887,6 @@ or they are very, very likely to be developer mistakes.
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\subsection{Type FutureOr}
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\LMLabel{typeFutureOr}
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%% TODO(eernst): We should make this a separate section, or change the title of this section.
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\LMHash{}%
|
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The built-in type declaration \code{FutureOr},
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which is declared in the library \code{dart:async},
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|
@ -13948,46 +13947,318 @@ only allow invocations of members inherited from \code{Object}.
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\subsection{Type Void}
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\LMLabel{typeVoid}
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|
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%% TODO(eernst): Adjust everything in this section when specifying generalized-void.
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\LMHash{}%
|
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The special type \VOID{} is used to indicate
|
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that the value of an expression is meaningless and intended to be discarded.
|
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|
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\commentary{%
|
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A typical case is that the type \VOID{} is used as the return type
|
||||
of a function that ``does not return anything''.
|
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Technically, there will always be \emph{some} object
|
||||
which is the return value
|
||||
(\ref{functions}).
|
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But it is perfectly meaningful to have a function
|
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whose sole purpose is to create side-effects,
|
||||
such that \emph{any} use of the returned object
|
||||
would be misguided.
|
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%
|
||||
This does not mean that there is anything wrong
|
||||
with the returned object as such.
|
||||
It could be any object whatsoever.
|
||||
But the developer who chose the return type \VOID{}
|
||||
did that to indicate that it is a misunderstanding to
|
||||
ascribe any meaning to that object,
|
||||
or to use it for any purpose.
|
||||
}
|
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|
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\commentary{%
|
||||
The type \VOID{} is a top type
|
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(\ref{superBoundedTypes}),
|
||||
so \VOID{} and \code{Object} are subtypes of each other
|
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(\ref{subtypes}),
|
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which also implies that any object can be
|
||||
the value of an expression of type \VOID.
|
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%
|
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Consequently, any instance of type \code{Type} which reifies the type \VOID{}
|
||||
must compare equal (according to the \code{==} operator \ref{equality})
|
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to any instance of \code{Type} which reifies the type \code{Object}
|
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(\ref{dynamicTypeSystem}).
|
||||
It is not guaranteed that \code{identical(\VOID, Object)} evaluates to true.
|
||||
In fact, it is not recommended that implementations strive to achieve this,
|
||||
because it may be more important to ensure that diagnostic messages
|
||||
(including stack traces and dynamic error messages)
|
||||
preserve enough information to use the word `void' when referring to types
|
||||
which are specified as such in source code.
|
||||
}
|
||||
|
||||
\LMHash{}%
|
||||
The special type \VOID{} may only be used as the return type of a function: it is a compile-time error to use \VOID{} in any other context.
|
||||
In support of the notion
|
||||
that the value of an expression with static type \VOID{} should be discarded,
|
||||
such values can only be used in specific situations:
|
||||
The occurrence of an expression of type \VOID{} is a compile-time error
|
||||
unless it is permitted according to one of the following rules.
|
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|
||||
\commentary{
|
||||
For example, as a type argument, or as the type of a variable or parameter
|
||||
|
||||
Void is not an interface type.
|
||||
|
||||
The only subtype relations that pertain to void are therefore:
|
||||
\begin{itemize}
|
||||
\item[$\bullet$]
|
||||
$\VOID{} <: \VOID{}$ (by reflexivity)
|
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\item[$\bullet$]
|
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$\bot <: \VOID{}$ (as bottom is a subtype of all types).
|
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\item[$\bullet$]
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$\VOID{} <: \DYNAMIC{}$ (as \DYNAMIC{} is a supertype of all types)
|
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\item
|
||||
In an \synt{expressionStatement} \code{$e$;}, $e$ may have type \VOID.
|
||||
\rationale{The value of $e$ is discarded.}
|
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\item
|
||||
In the initialization and increment expressions of a for-loop,
|
||||
\code{\FOR{} ($e_1$; $e_2$; $e_3$) {\ldots}},
|
||||
$e_1$ may have type \VOID,
|
||||
and each of the expressions in the expression list $e_3$ may have type \VOID.
|
||||
\rationale{The values of $e_1$ and $e_3$ are discarded.}
|
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\item
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||||
In a type cast \code{$e$ as $T$}, $e$ may have type \VOID.
|
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\rationale{%
|
||||
Developers thus obtain the ability to \emph{override} the constraints
|
||||
on usages of values with static type \VOID.
|
||||
This means that it is not enforced that such values are discarded,
|
||||
but they can only be used when the wish to do so
|
||||
has been indicated explicitly.%
|
||||
}
|
||||
\item
|
||||
In a parenthesized expression \code{($e$)}, $e$ may have type \VOID.
|
||||
\rationale{%
|
||||
Note that \code{($e$)} itself has type \VOID,
|
||||
which implies that it must occur in some context
|
||||
where it is not an error to have it.%
|
||||
}
|
||||
\item
|
||||
In a conditional expression \code{$e$\,?\,$e_1$\,:\,$e_2$},
|
||||
$e_1$ and $e_2$ may have type \VOID.
|
||||
\rationale{%
|
||||
The static type of the conditional expression is then \VOID,
|
||||
even if one of the branches has a different type,
|
||||
which means that the conditional expression must again occur
|
||||
in some context where it is not an error to have it.%
|
||||
}
|
||||
\item
|
||||
In a null coalescing expression \code{$e_1$\,??\,$e_2$},
|
||||
$e_2$ may have type \VOID.
|
||||
\rationale{%
|
||||
The static type of the null coalescing expression is then \VOID,
|
||||
which in turn restricts where it can occur.%
|
||||
}
|
||||
\item
|
||||
\commentary{%
|
||||
In a return statement \code{\RETURN\,$e$;},
|
||||
$e$ may have type \VOID{} in a number of situations
|
||||
(\ref{return}).%
|
||||
}
|
||||
\item
|
||||
\commentary{%
|
||||
In an arrow function body \code{=> $e$},
|
||||
the returned expression $e$ may have type \VOID{}
|
||||
in a number of situations
|
||||
(\ref{functions}).%
|
||||
}
|
||||
\item
|
||||
An initializing expression for a variable of type \VOID{}
|
||||
may have type \VOID.
|
||||
\rationale{Usages of that variable are constrained.}
|
||||
\item
|
||||
An actual parameter expression corresponding to a formal parameter
|
||||
whose statically known type annotation is \VOID{}
|
||||
may have type \VOID.
|
||||
\rationale{%
|
||||
Usages of that parameter in the body of the callee
|
||||
are statically expected to be constrained by having type \VOID.
|
||||
See the discussion about soundness below
|
||||
(\ref{voidSoundness}).
|
||||
}
|
||||
\item
|
||||
In an expression of the form \code{$e_1$\,=\,$e_2$}
|
||||
where $e_1$ is an \synt{assignableExpression}
|
||||
denoting a variable or formal parameter of type \VOID{},
|
||||
$e_2$ may have type \VOID.
|
||||
\rationale{%
|
||||
Usages of that variable or formal parameter
|
||||
are statically expected to be constrained by having type \VOID.
|
||||
See the discussion about soundness below
|
||||
(\ref{voidSoundness}).
|
||||
}
|
||||
\item
|
||||
Let $e$ be an expression ending in a \synt{cascadeSection}
|
||||
of the form \code{..\,$S$\,$s$\;=\;$e_1$},
|
||||
where $S$ is of the form
|
||||
|
||||
\noindent
|
||||
\syntax{(<cascadeSelector> <argumentPart>*)
|
||||
(<assignableSelector> <argumentPart>*)*}
|
||||
|
||||
\noindent
|
||||
and $e_1$ is of the form \synt{expressionWithoutCascade}.
|
||||
|
||||
If $s$ is an \synt{assignableSelector} of the
|
||||
form \code{.\id} or \code{?.\id}
|
||||
where the static type of the identifier \id{} is \VOID,
|
||||
$e_1$ may have type \VOID.
|
||||
Otherwise, if $s$ is an \synt{assignableSelector} of the form
|
||||
\code{[$\,e_0\,$]} where the static type of
|
||||
the first formal parameter in the statically known declaration
|
||||
of operator \code{[]=} is \VOID,
|
||||
$e_0$ may have type \VOID.
|
||||
Also, if the static type of the second formal parameter is \VOID,
|
||||
$e_1$ may have type \VOID.
|
||||
\end{itemize}
|
||||
|
||||
The analogous rules also hold for the $<<$ relation for similar reasons.
|
||||
\LMHash{}%
|
||||
Finally, we need to address situations involving implicit usage of
|
||||
a value whose static type can be \VOID{}.
|
||||
%
|
||||
It is a compile-time error for a for-in statement to have an iterator
|
||||
expression of type $T$ such that \code{Iterator<\VOID{}>}
|
||||
is the most specific instantiation of \code{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 \code{Stream<\VOID{}>} is the most specific
|
||||
instantiation of \code{Stream} that is a superinterface of $T$,
|
||||
unless the iteration variable has type \VOID.
|
||||
|
||||
Hence, the static checker will issue errors if one attempts to access a member of the result of a void method invocation (even for members of the null object (\ref{null}), such as \code{==}).
|
||||
Likewise, passing the result of a void method as a parameter or assigning it to a variable will raise an error unless the variable/formal parameter has type dynamic.
|
||||
|
||||
On the other hand, it is possible to return the result of a void method from within a void method.
|
||||
One can also return the null object (\ref{null}); or a value of type \DYNAMIC{}.
|
||||
Returning any other result will cause a compile-time error.
|
||||
%% TODO(eernst): Correct this when integrating generalized-void.md.
|
||||
A dynamic type error would arise if a non-null object was returned from a void method (since no object has run-time type \DYNAMIC{}).
|
||||
\commentary{%
|
||||
Here are some examples:
|
||||
}
|
||||
|
||||
\commentary{
|
||||
The name \VOID{} does not denote a \code{Type} object.
|
||||
\begin{dartCode}
|
||||
\FOR{} (Object x in <\VOID>[]) \{\} // \comment{Error.}
|
||||
\AWAIT{} \FOR{} (int x \IN{} new Stream<\VOID{}>.empty()) \{\} // \comment{Error.}
|
||||
\FOR{} (\VOID{} x \IN{} <\VOID{}>[]) \{\ldots\} // \comment{OK.}
|
||||
\FOR (\VAR{} x \IN{} <\VOID{}>[]) \{\ldots\} // \comment{OK, type of x inferred.}
|
||||
\end{dartCode}
|
||||
|
||||
\commentary{%
|
||||
However, in the examples that are not errors
|
||||
the usage of \code{x} in the loop body is constrained,
|
||||
because it has type \VOID.%
|
||||
}
|
||||
|
||||
\rationale{
|
||||
It is syntacticly illegal to use \VOID{} as an expression, and it would make no sense to do so.
|
||||
Type objects reify the run-time types of instances.
|
||||
No instance ever has type \VOID{}.
|
||||
\paragraph{Void Soundness}
|
||||
\LMLabel{voidSoundness}
|
||||
|
||||
\LMHash{}%
|
||||
The constraints on usage of a value obtained from the evaluation of
|
||||
an expression with static type \VOID{}
|
||||
are not strictly enforced.
|
||||
|
||||
\commentary{%
|
||||
The usage of a ``void value'' is not a soundness issue, that is,
|
||||
no invariants needed for correct execution of a Dart program
|
||||
can be violated because of such a usage.
|
||||
}
|
||||
|
||||
\rationale{%
|
||||
It could be said that the type \VOID{} is used
|
||||
to help developers maintain a certain self-imposed discipline
|
||||
about the fact that certain objects are not \emph{intended} to be used.
|
||||
%
|
||||
Because of the fact that enforcement is not necessary,
|
||||
and because of the treatment of \VOID{} in earlier versions of Dart,
|
||||
the language uses a \emph{best effort} approach to ensure
|
||||
that the value of an expression of type \VOID{}
|
||||
will not be used.%
|
||||
}
|
||||
|
||||
\commentary{%
|
||||
In fact, there are numerous ways in addition to the type cast
|
||||
in which a developer can get access to such a value:%
|
||||
}
|
||||
|
||||
\begin{dartCode}
|
||||
\ABSTRACT{} \CLASS A<X> \{
|
||||
final X x;
|
||||
A(this.x);
|
||||
Object foo(X x);
|
||||
\}
|
||||
\\
|
||||
\CLASS{} B<X> \EXTENDS{} A<X> \{
|
||||
B(X x): super(x);
|
||||
Object foo(Object x) => x;
|
||||
\}
|
||||
\\
|
||||
Object f<X>(X x) => x;
|
||||
\\
|
||||
\VOID{} main() \{
|
||||
\VOID x = 42;
|
||||
print(f(x)); // \comment{(1)}
|
||||
\\
|
||||
A<\VOID{}> a = B<\VOID{}>(x);
|
||||
A<Object> aObject = a;
|
||||
print(aObject.x); // \comment{(2)}
|
||||
print(a.foo(x)); // \comment{(3)}
|
||||
\}
|
||||
\end{dartCode}
|
||||
|
||||
\commentary{%
|
||||
At (1), a variable \code{x} of type \VOID{} is passed to
|
||||
a generic function \code{f},
|
||||
which is allowed because the actual type argument \VOID{} is inferred,
|
||||
and it is allowed to pass an actual argument of type \VOID{} to
|
||||
a formal parameter with the same type.
|
||||
%
|
||||
However, no special treatment is given when an expression has a type
|
||||
which is or contains a type variable whose value could be \VOID{},
|
||||
so we are allowed to return \code{x} in the body of \code{f},
|
||||
even though this means that we indirectly get access to the value
|
||||
of an expression of type \VOID{}, under the static type \code{Object}.
|
||||
|
||||
At (2), we indirectly obtain access to the value of
|
||||
the variable \code{x} with type \VOID{},
|
||||
because we use an assignment to get access to the instance of \code{B}
|
||||
which was created with type argument \VOID{} under the type
|
||||
\code{A<Object>}.
|
||||
Note that \code{A<Object>} and \code{A<\VOID{}>} are subtypes of each other,
|
||||
that is, they are equivalent according to the subtype rules,
|
||||
so neither static nor dynamic type checks will fail.
|
||||
|
||||
At (3), we indirectly obtain access to the value of
|
||||
the variable \code{x} with type \VOID{}
|
||||
under the static type \code{Object},
|
||||
because the statically known method signature of \code{foo}
|
||||
has parameter type \VOID{},
|
||||
but the actual implementation of \code{foo} which is invoked
|
||||
is an override whose parameter type is \code{Object},
|
||||
which is allowed because \code{Object} and \VOID{} are both top types.%
|
||||
}
|
||||
|
||||
\rationale{%
|
||||
Obviously, the support for preventing developers from using values
|
||||
obtained from expressions of type \VOID{} is far from sound,
|
||||
in the sense that there are many ways to circumvent the rule
|
||||
that such a value should be discarded.
|
||||
|
||||
However, we have chosen to focus on the simple, first-order usage
|
||||
(where an expression has type \VOID, and the value is used),
|
||||
and we have left higher-order cases largely unchecked,
|
||||
relying on additional tools such as linters to perform an analysis
|
||||
which covers indirect data flows.
|
||||
|
||||
It would certainly have been possible to define sound rules,
|
||||
such that the value of an expression of type \VOID{}
|
||||
would be guaranteed to be discarded after some number of transfers
|
||||
from one variable or parameter to the next one, all with type \VOID{},
|
||||
explicitly, or as the value of a type parameter.
|
||||
In particular, we could require that method overrides should
|
||||
never override return type \code{Object} by return type \VOID{},
|
||||
or parameter types in the opposite direction;
|
||||
parameterized types with type argument \VOID{} could not be assigned
|
||||
to variables where the corresponding type argument is anything other than
|
||||
\VOID, etc.\ etc.
|
||||
|
||||
But this would be quite impractical.
|
||||
In particular, the need to either prevent a large number of type variables
|
||||
from ever having the value \VOID{},
|
||||
or preventing certain usages of values whose type is such a type variable,
|
||||
or whose type contains such a type variable,
|
||||
that would be severely constraining on a very large part of all Dart code.
|
||||
|
||||
So we have chosen to help developers maintain this self-imposed discipline
|
||||
in simple and direct cases,
|
||||
and leave it to ad-hoc reasoning or separate tools to ensure
|
||||
that the indirect cases are covered as closely as needed in practice.%
|
||||
}
|
||||
|
||||
|
||||
|
|
|
@ -4,7 +4,8 @@
|
|||
|
||||
**Version**: 0.10 (2018-07-10)
|
||||
|
||||
**Status**: Implemented.
|
||||
**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`.
|
||||
|
|
Loading…
Reference in a new issue