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‎specification/dart.sty

Lines changed: 37 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -119,10 +119,10 @@
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\newenvironment{commentary}[1]{{\color{commentaryColor}\sf{#1}}}{}
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% Auxiliary functions.
122-
\newcommand{\flatten}[1]{\ensuremath{\mbox{\it flatten}({#1})}}
123-
\newcommand{\futureOrBase}[1]{\ensuremath{\mbox{\it futureOrBase}({#1})}}
124-
\newcommand{\overrides}[1]{\ensuremath{\mbox{\it overrides}({#1})}}
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\newcommand{\inherited}[1]{\ensuremath{\mbox{\it inherited}({#1})}}
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\newcommand{\flatten}[1]{\ensuremath{\metavar{flatten}({#1})}}
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\newcommand{\futureOrBase}[1]{\ensuremath{\metavar{futureOrBase}({#1})}}
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\newcommand{\overrides}[1]{\ensuremath{\metavar{overrides}({#1})}}
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\newcommand{\inherited}[1]{\ensuremath{\metavar{inherited}({#1})}}
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% Used as a mini-section marker, indicating visibly that a range of
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% text (usually just a couple of paragraphs) are concerned with one
@@ -393,6 +393,39 @@
393393
\ensuremath{{#2}}%
394394
}
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396+
\newcommand{\FlattenName}{\metavar{flatten}}
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\newcommand{\Flatten}[1]{\ensuremath{\FlattenName(\code{#1})}}
398+
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\newcommand{\NnbdTopMergeName}{\metavar{topMerge}}
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\newcommand{\NnbdTopMerge}[2]{\ensuremath{\NnbdTopMergeName(\code{{#1},\,\,{#2}})}}
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\newcommand{\NonNullTypeOfName}{\metavar{nonNullTypeOf}}
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\newcommand{\NonNullTypeOf}[1]{\ensuremath{\NonNullTypeOfName(\code{#1})}}
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\newcommand{\IsTopTypeName}{\metavar{isTopType}}
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\newcommand{\IsTopType}[1]{\ensuremath{\IsTopTypeName(\code{#1})}}
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\newcommand{\IsObjectTypeName}{\metavar{isObjectType}}
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\newcommand{\IsObjectType}[1]{\ensuremath{\IsObjectTypeName(\code{#1})}}
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\newcommand{\IsBottomTypeName}{\metavar{isBottomType}}
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\newcommand{\IsBottomType}[1]{\ensuremath{\IsBottomTypeName(\code{#1})}}
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\newcommand{\IsNullTypeName}{\metavar{isNullType}}
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\newcommand{\IsNullType}[1]{\ensuremath{\IsNullTypeName(\code{#1})}}
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\newcommand{\IsMoreTopTypeName}{\metavar{isMoreTopType}}
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\newcommand{\IsMoreTopType}[2]{\ensuremath{\IsMoreTopTypeName(\code{{#1},\,\,{#2}})}}
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\newcommand{\IsMoreBottomTypeName}{\metavar{isMoreBottomType}}
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\newcommand{\IsMoreBottomType}[2]{\ensuremath{\IsMoreBottomTypeName(\code{{#1},\,\,{#2}})}}
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\newcommand{\NormalizedTypeOfName}{\metavar{normalizedTypeOf}}
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\newcommand{\NormalizedTypeOf}[1]{\ensuremath{\NormalizedTypeOfName(\code{#1})}}
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\newcommand{\FutureValueTypeOfName}{\metavar{futureValueTypeOf}}
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\newcommand{\FutureValueTypeOf}[1]{\ensuremath{\FutureValueTypeOfName(\code{#1})}}
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% ----------------------------------------------------------------------
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% Support for hash valued Location Markers
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‎specification/dartLangSpec.tex

Lines changed: 107 additions & 94 deletions
Original file line numberDiff line numberDiff line change
@@ -2000,11 +2000,13 @@ \subsubsection{Required Formals}
20002000
an instance method, instance setter, or instance operator.
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20022002

2003-
\subsubsection{Optional Formals}
2003+
\subsubsection{Optional and Named Formals}
20042004
\LMLabel{optionalFormals}
20052005

20062006
\LMHash{}%
2007-
Optional parameters may be specified and provided with default values.
2007+
Optional formal parameters may be specified and provided with default values.
2008+
Named formal parameters can be optional,
2009+
but if the modifier \REQUIRED{} is present then it is required.
20082010

20092011
\begin{grammar}
20102012
<defaultFormalParameter> ::= <normalFormalParameter> (`=' <expression>)?
@@ -2025,7 +2027,10 @@ \subsubsection{Optional Formals}
20252027
It is a compile-time error if the default value of an optional parameter is
20262028
not a constant expression (\ref{constants}).
20272029
If no default is explicitly specified for an optional parameter
2030+
whose declared type is nullable,
20282031
an implicit default of \NULL{} is provided.
2032+
Otherwise, when the declared type is not nullable,
2033+
it is a compile-time error if an optional parameter has no default value.
20292034

20302035
\LMHash{}%
20312036
It is a compile-time error if the name of a named optional parameter
@@ -10245,7 +10250,45 @@ \subsubsection{Set and Map Literal Disambiguation}
1024510250
\LMLabel{setAndMapLiteralDisambiguation}
1024610251

1024710252
\LMHash{}%
10248-
Some terms like \code{\{\}} and \code{\{\,...\id\,\}} are ambiguous:
10253+
Some terms are syntactically ambiguous because they contain a \lit{?}
10254+
which may be considered part of a nullable type
10255+
or part of a \syntax{conditionalExpression}.
10256+
In all such cases,
10257+
the ambiguity is resolved in favor of the set literal
10258+
and not the map literal.
10259+
10260+
\commentary{%
10261+
In other words, conditional expressions are given preference.
10262+
For example, \code{\{ a as bool ? - 3 : 3 \}} and
10263+
\code{\{ a is int ? - 3 : 3 \}} are syntactically ambiguous
10264+
in this manner.
10265+
The former can be parsed as a set literal or as a map literal,
10266+
depending on whether the \lit{?} is considered to make \code{bool} nullable,
10267+
or it is considered as part of a conditional expression.
10268+
The other expression has a similar ambiguity.
10269+
They are both resolved to be set literals
10270+
containing a conditional expression.%
10271+
}
10272+
10273+
\LMHash{}%
10274+
Similarly, null aware subscripts in conditional expressions
10275+
can be syntactically ambiguous.
10276+
Whenever there is a sequence of tokens
10277+
which may be parsed either as a conditional expression
10278+
or as two expressions separated by a colon,
10279+
the first of which is a null aware subscript,
10280+
parsers shall choose to parse as a conditional expression.
10281+
10282+
\commentary{%
10283+
For example, \code{\{ a?[b]:c \}} can be parsed either as a set
10284+
literal or a map literal, depending on whether the \lit{?} is
10285+
interpreted as part of a null aware subscript or as part of a
10286+
conditional expression.
10287+
It is resolved as the latter.%
10288+
}
10289+
10290+
\LMHash{}%
10291+
Some terms like \code{\{\}} and \code{\{\,...\id\,\}} are still ambiguous:
1024910292
they may be either a set literal or a map literal.
1025010293
This ambiguity is eliminated in two steps.
1025110294
The first step uses only the syntax and context type,
@@ -16329,8 +16372,7 @@ \subsection{Assignable Expressions}
1632916372
from the end of $e$.
1633016373
It is easy to see that only some \synt{assignableExpression}s
1633116374
have a receiver term.
16332-
For instance, a plain \synt{identifier} does not.
16333-
%
16375+
For instance, a plain \synt{identifier} does not.%
1633416376
}
1633516377

1633616378
\LMHash{}%
@@ -19069,7 +19111,7 @@ \subsection{Imports}
1906919111
\ref{exports}).
1907019112

1907119113
\LMHash{}%
19072-
The dart core library \code{dart:core}
19114+
The Dart core library \code{dart:core}
1907319115
is implicitly imported into every dart library other than itself
1907419116
via an import clause of the form
1907519117
\code{\IMPORT{} 'dart:core';}
@@ -21336,6 +21378,63 @@ \subsubsection{Additional Subtyping Concepts}
2133621378
}
2133721379

2133821380

21381+
\subsection{Type \code{Never}}
21382+
\LMLabel{typeNever}
21383+
21384+
\LMHash{}%
21385+
The Dart core library \code{dart:core} exports a type named \code{Never}.
21386+
No object has dynamic type \code{Never}.
21387+
21388+
\commentary{%
21389+
In other words, \code{Never} is the empty type.
21390+
\code{Never} is a subtype of every other type in Dart
21391+
(\ref{subtypeRules}).%
21392+
}
21393+
21394+
\rationale{%
21395+
\code{Never} is a useful type for several reasons:
21396+
It can be used to indicate that
21397+
the evaluation of an expression will never complete normally,
21398+
e.g., because it calls a function that always throws.
21399+
This allows for a more precise control flow analysis.
21400+
21401+
Also, the object \code{\CONST\,\,<Never>[]} can be used
21402+
in any situation where a \code{List<$T$>} is required,
21403+
for any $T$ whatsoever,
21404+
and the value of a variable of type \code{Object?\,\,\FUNCTION(Never)}
21405+
can be any function that accepts a single, positional argument.%
21406+
}
21407+
21408+
21409+
\subsection{Type \code{Null}}
21410+
\LMLabel{typeNull}
21411+
21412+
\LMHash{}%
21413+
The Dart core library \code{dart:core} exports a type named \code{Null},
21414+
There is exactly one object whose dynamic type is \code{Null},
21415+
and that is the null object.
21416+
21417+
\commentary{%
21418+
In other words, \code{Null} is a singleton type.
21419+
\code{Null} is a subtype of all other types
21420+
other than \code{Never} and any type which is
21421+
a subtype of \code{Never}
21422+
(\ref{subtypeRules}).%
21423+
}
21424+
21425+
\rationale{%
21426+
\code{Null} is mainly used implicitly in
21427+
a type of the form \code{$T$?} for some $T$.
21428+
If $e$ is an expression of type \code{$T$?} that evaluates to an object $o$,
21429+
then $o$ can be an instance of a subtype of $T$,
21430+
or it could be the null object.
21431+
The latter situation is commonly interpreted to mean that
21432+
the computation did not yield a result.
21433+
In this sense, \code{Null} can be considered to model
21434+
the property of being optional.%
21435+
}
21436+
21437+
2133921438
\subsection{Function Types}
2134021439
\LMLabel{functionTypes}
2134121440

@@ -22398,94 +22497,8 @@ \subsection{Operator Precedence}
2239822497

2239922498
\section{Null safety} %% !!!TODO!!!
2240022499

22401-
\newcommand{\FlattenName}{\metavar{flatten}}
22402-
\newcommand{\Flatten}[1]{\ensuremath{\FlattenName(\code{#1})}}
22403-
22404-
\newcommand{\NnbdTopMergeName}{\metavar{topMerge}}
22405-
\newcommand{\NnbdTopMerge}[2]{\ensuremath{\NnbdTopMergeName(\code{{#1},\,\,{#2}})}}
22406-
22407-
\newcommand{\NonNullTypeOfName}{\metavar{nonNullTypeOf}}
22408-
\newcommand{\NonNullTypeOf}[1]{\ensuremath{\NonNullTypeOfName(\code{#1})}}
22409-
22410-
\newcommand{\IsTopTypeName}{\metavar{isTopType}}
22411-
\newcommand{\IsTopType}[1]{\ensuremath{\IsTopTypeName(\code{#1})}}
22412-
22413-
\newcommand{\IsObjectTypeName}{\metavar{isObjectType}}
22414-
\newcommand{\IsObjectType}[1]{\ensuremath{\IsObjectTypeName(\code{#1})}}
22415-
22416-
\newcommand{\IsBottomTypeName}{\metavar{isBottomType}}
22417-
\newcommand{\IsBottomType}[1]{\ensuremath{\IsBottomTypeName(\code{#1})}}
22418-
22419-
\newcommand{\IsNullTypeName}{\metavar{isNullType}}
22420-
\newcommand{\IsNullType}[1]{\ensuremath{\IsNullTypeName(\code{#1})}}
22421-
22422-
\newcommand{\IsMoreTopTypeName}{\metavar{isMoreTopType}}
22423-
\newcommand{\IsMoreTopType}[2]{\ensuremath{\IsMoreTopTypeName(\code{{#1},\,\,{#2}})}}
22424-
22425-
\newcommand{\IsMoreBottomTypeName}{\metavar{isMoreBottomType}}
22426-
\newcommand{\IsMoreBottomType}[2]{\ensuremath{\IsMoreBottomTypeName(\code{{#1},\,\,{#2}})}}
22427-
22428-
\newcommand{\NormalizedTypeOfName}{\metavar{normalizedTypeOf}}
22429-
\newcommand{\NormalizedTypeOf}[1]{\ensuremath{\NormalizedTypeOfName(\code{#1})}}
22430-
22431-
\newcommand{\FutureValueTypeOfName}{\metavar{futureValueTypeOf}}
22432-
\newcommand{\FutureValueTypeOf}[1]{\ensuremath{\FutureValueTypeOfName(\code{#1})}}
22433-
22434-
22435-
\subsection{Syntax}
22436-
22437-
The precise changes to the syntax are given in an accompanying set of
22438-
modifications to the grammar in the formal specification. This section
22439-
summarizes in prose the grammar changes associated with this feature.
22440-
22441-
The grammar of types is extended to allow any type to be suffixed with a \code{?}
22442-
(e.g. \code{int?}) indicating the nullable version of that type.
22443-
22444-
A new primitive type \code{Never}. This type is denoted by the built-in type
22445-
declaration \code{Never} declared in \code{dart:core}.
22446-
22447-
The grammar of expressions is extended to allow any expression to be suffixed
22448-
with a \code{!}.
22449-
22450-
The modifier \LATE{} is added as a built-in identifier. The grammar of top level
22451-
variables, static fields, instance fields, and local variables is extended to
22452-
allow any declaration to include the modifer \LATE.
22453-
22454-
The modifier \REQUIRED{} is added as a built-in identifier. The grammar of
22455-
function types is extended to allow any named parameter declaration to be
22456-
prefixed by the \REQUIRED{} modifier
22457-
(e.g. \code{int \FUNCTION(int, {int? y, \REQUIRED\ int z})}).
22458-
22459-
The grammar of selectors is extended to allow null-aware subscripting using the
22460-
syntax \code{$e_1$?[$e_2$]} which evaluates to \code{null} if \code{$e_1$} evaluates to \code{null} and
22461-
otherwise evaluates as \code{$e_1$[$e_2$]}.
22462-
22463-
The grammar of cascade sequences is extended to allow the first cascade of a
22464-
sequence to be written as \code{?..} indicating that the cascade is null-shorting.
22465-
22466-
\subsubsection{Grammatical ambiguities and clarifications}
22467-
22468-
\paragraph{Conditional expression ambiguities}
22469-
22470-
Conditional expressions inside of braces are ambiguous between sets and maps.
22471-
That is, \code{\{ a as bool ? - 3 : 3 \}} can be parsed as a set literal \code{\{ (a as bool)
22472-
? - 3 : 3 \}} or as a map literal \code{\{ (a as bool ?) - 3 : 3 \}}. Parsers will
22473-
prefer the former parse over the latter.
22474-
22475-
The same is true for \code{\{ a is int ? - 3 : 3 \}}.
22476-
22477-
The same is true for \code{\{ int ? - 3 : 3 \}} if we allow this.
22478-
22479-
\paragraph{Null aware subscript}
22480-
22481-
Certain uses of null aware subscripts in conditional expressions are ambiguous.
22482-
For example, \code{\{ a?[b]:c \}} can be parsed either as a set literal or a map
22483-
literal, depending on whether the \code{?} is interpreted as part of a null aware
22484-
subscript or as part of a conditional expression. Whenever there is a sequence
22485-
of tokens which may be parsed either as a conditional expression or as two
22486-
expressions separated by a colon, the first of which is a null aware
22487-
subscript, parsers shall choose to parse as a conditional expression.
22488-
22500+
%% !!!At the end: Search Null, change to Never where appropriate
22501+
%% !!!Search all `TODO`.*null
2248922502

2249022503
\subsection{Static semantics}
2249122504

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