Author: eernst@.
Version: 0.1 (2017-08-15)
Status: Under discussion
This document is an informal specification of the optional new feature. The feature adds support for omitting the reserved word new in instance creation expressions.
This feature relies on optional const, and it is assumed that the reader knows the optional const feature. Otherwise, this informal specification is derived from a combined proposal which presents optional new and several other features.
In Dart without optional new, the reserved word new is present in every expression whose evaluation invokes a constructor (except constant expressions). These expressions are known as instance creation expressions. If new is removed from such an instance creation expression, the remaining phrase is still syntactically correct in almost all cases, and the required grammar update that makes them all syntactically correct is exactly the one that is specified for optional const.
Assuming the grammar update in optional const, all instance creation expressions can technically omit the new because tools (compilers, analyzers) are able to parse these expressions, and they are able to recognize that they denote instance creations (rather than, say, static function invocations), because the part before the left parenthesis is statically known to denote a constructor.
For instance, p.C.foo may resolve statically to a constructor named foo in a class C imported with prefix p. Similarly, D may resolve to a class, in which case D(42) is statically known to be a constructor invocation because the other interpretation is statically known to be incorrect (that is, cf. section ‘16.14.3 Unqualified Invocation’ in the language specification, evaluating (D)(42): (D) is an instance of Type which is not a function type and does not have a method named call).
For human readers, it may be helpful to document that a particular expression is guaranteed to yield a fresh instance, and this is the most common argument why new should not be omitted. However, Dart already allows instance creation expressions to invoke a factory constructor, so Dart developers never had any local guarantees that any particular expression would yield a fresh object.
Developers may thus prefer to omit new in order to obtain more concise code, and possibly also in order to achieve greater uniformity among invocations of constructors and other invocations, e.g., of static or global functions.
With that in mind, this proposal allows instance creation expressions to omit the new in all cases, but also preserves the permission to include new in all cases. It is a matter of style to use new in a manner that developers find helpful.
The syntax changes associated with this feature are the following:
postfixExpression ::=
assignableExpression postfixOperator |
constructorInvocation | // NEW
primary selector*
constructorInvocation ::= // NEW
typeName typeArguments '.' identifier arguments
assignableExpression ::=
SUPER unconditionalAssignableSelector |
typeName typeArguments '.' identifier arguments
(arguments* assignableSelector)+ | // NEW
identifier |
primary (arguments* assignableSelector)+
This grammar update is identical to the grammar update for optional const. For more information including a complete grammar, please consult that specification.
We specify a type directed source code transformation which eliminates the feature. The static analysis proceeds to work on the transformed program.
Similarly to optional const, this means that the feature is “static semantic sugar”. We do not specify the dynamic semantics for this feature, because the feature is eliminated in this transformation step.
We need to treat expressions differently in different locations, hence the following definition: An expression e is said to occur in a constant context,
We define new/const insertion as the following transformation:
e by const e.e by new eAn expression on one of the following forms must be modified to be or contain a constantObjectExpression or newExpression as described:
With a postfixExpression e,
constructorInvocation, i.e., typeName typeArguments '.' identifier arguments then perform new/const insertion on e.typeIdentifier arguments where typeIdentifier denotes a class then perform new/const insertion on e.identifier1 '.' identifier2 arguments where identifier1 denotes a class and identifier2 is the name of a named constructor in that class, or identifier1 denotes a prefix for a library L and identifier2 denotes a class exported by L, perform new/const insertion on e.identifier1 '.' typeIdentifier '.' identifier2 arguments where identifier1 denotes a library prefix for a library L, typeIdentifier denotes a class C exported by L, and identifier2 is the name of a named constructor in C, perform new/const insertion on e.There is no dynamic semantics to specify for this feature because it is eliminated by code transformation.
The optional new and optional const feature can easily be introduced at the same time: Just update the grammar as specified for optional const (and mentioned again here) and use the program transformation specified in this document. The program transformation in this document subsumes the program transformation specified for optional const, and hence this will provide support for both features.