# Implicit Creation Author: eernst@. Version: 0.7 (2018-04-10) Status: Background material, normative language now in dartLangSpec.tex. **This document** is an informal specification of the *implicit creation* feature. **The feature** adds support for omitting some occurrences of the reserved words `new` and `const` in instance creation expressions. This feature specification was written with a [combined proposal](https://github.com/dart-lang/sdk/blob/main/docs/language/informal/optional-new-const.md) as the starting point. That proposal presents optional new and optional const together with several other features. ## Motivation In Dart without implicit creation, the reserved word `new` is present in almost all expressions whose evaluation invokes a constructor at run time, and `const` is present in the corresponding constant expressions. These expressions are known as *instance creation expressions*. If `new` or `const` is removed from such an instance creation expression, the remaining phrase is still syntactically correct in most cases. This feature specification updates the grammar to make them all syntactically correct. With that grammar update, all instance creation expressions can technically omit `new` or `const` because tools (compilers, analyzers) are able to parse these expressions. The tools are able to recognize that these expressions denote instance creations (rather than, say, static function invocations), because the part before the arguments 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`, so we cannot call `(D)`). In short, even without the keyword, we can still unambiguously recognize the expressions that create objects. In that sense, the keywords are superfluous. For human readers, however, it may be helpful to document that a particular expression will yield a fresh instance, and this is the most common argument why `new` should *not* be omitted: It can be good documentation. But Dart already allows instance creation expressions to invoke a factory constructor, which is not guaranteed to return a newly created object, so Dart developers never had any firm local guarantees that any particular expression would yield a fresh object. This means that it may very well be justified to have an explicit `new`, but it will never be a rigorous guarantee of freshness. Similarly, it may be important for developers to ensure that certain expressions are constant, because of the improved performance and the guaranteed canonicalization. This is a compelling argument in favor of making certain instance creation expressions constant: It is simply a bug for that same expression to have `new` because object identity is an observable characteristic, and it may be crucial for performance that the expression is constant. In summary, both `new` and `const` may always be omitted from an instance creation expression, but it is useful and reasonable to allow an explicit `new`, and it is necessary to allow an explicit `const`. Based on that line of reasoning, we've decided to make them optional. It will then be possible for developers to make many expressions considerably more concise, and they can still enforce the desired semantics as needed. Obviously, this underscores the importance of the default: When a given instance creation expression omits the keyword, should it be `const` or `new`? **As a general rule** `const` is used whenever it is required, and otherwise `new` is used. This requirement arises from the syntactic context, based on the fact that a non-constant expression would be a compile-time error. In summary, the implicit creation feature allows for concise construction of objects, and it still allows developers to explicitly specify `new` or `const`, whenever needed and whenever it is considered to be good documentation. ## Syntax The syntax changes associated with this feature are the following: ``` postfixExpression ::= assignableExpression postfixOperator | constructorInvocation selector* | // NEW primary selector* constructorInvocation ::= // NEW typeName typeArguments '.' identifier arguments assignableExpression ::= SUPER unconditionalAssignableSelector | constructorInvocation assignableSelectorPart+ | // NEW identifier | primary assignableSelectorPart+ assignableSelectorPart ::= argumentPart* assignableSelector ``` ## Static analysis We specify a type directed source code transformation which eliminates the feature by expressing the same semantics with different syntax. The static analysis proceeds to work on the transformed program. *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*, - if _e_ is an element of a constant list literal, or a key or value of an entry of a constant map literal. - if _e_ is an actual argument of a constant object expression or of a metadata annotation. - if _e_ is the initializing expression of a constant variable declaration. - if _e_ is a switch case expression. - if _e_ is an immediate subexpression of an expression _e1_ which occurs in a constant context, unless _e1_ is a `throw` expression or a function literal. *This roughly means that everything which is inside a syntactically constant expression or declaration is in a constant context. Note that a `const` modifier which is introduced by the source code transformation does not create a constant context, it is only the explicit occurrences of `const` in the program that create a constant context. Also note that a `throw` expression is currently not allowed in a constant expression, but extensions affecting that status may be considered. A similar situation arises for function literals.* *A formal parameter may have a default value, which must be a constant expression. We have chosen to not put such default values into a constant context. They must be constant, and it may be necessary to add the keyword `const` in order to make them so. This may seem inconvenient at times, but the rationale is that it allows for future generalizations of default value expressions allowing them to be non-constant. Still, there is no guarantee that such features will be added to Dart.* *For a class which contains a constant constructor and an instance variable which is initialized by an expression _e_, it is a compile-time error if _e_ is not constant. We have chosen to not put such initializers into a constant context, and hence an explicit `const` may be required. This may again seem inconvenient at times, but the rationale is that the reason for the constancy requirement is non-local (the constant constructor declaration may be many lines away from the instance variable declaration); it may break programs in surprising and confusing ways if a constructor is changed to be constant; and it may cause subtle bugs at run time due to the change in identity, if such a change is made and it does not cause any compile-time errors.* We define *new/const insertion* as the following transformation, which will be applied to specific parts of the program as specified below: - if the expression _e_ occurs in a constant context, replace _e_ by `const` _e_, - otherwise replace _e_ by `new` _e_. *Note that new/const insertion is just a syntactic transformation, it is specified below where to apply it, including which syntactic constructs may play the role of _e_.* *Also note that the outcome of new/const insertion may have static semantic errors, e.g., actual arguments to a constructor invocation may have wrong types because that's how the program was written, or a `const` list may have elements which are not constant expressions. In such cases, tools like analyzers and compilers should emit diagnostic messages that are meaningful in relation to the original source of the program, which might mean that the blame is assigned to a larger syntactic construct than the one that directly has a compile-time error after the transformation.* *We specify the transformation as based on a depth-first traversal of an abstract syntax tree (AST). This means that the program is assumed to be free of syntax errors, and when the current AST is, e.g., a `postfixExpression`, the program as a whole has such a structure that the current location was parsed as a `postfixExpression`. This is different from the situation where we just require that a given subsequence of the tokens of the program allows for such a parsing in isolation. For instance, an identifier like `x` parses as an `assignableExpression` in isolation, but if it occurs in the context `var x = 42;` or `var y = x;` then it will not be parsed as an `assignableExpression`, it will be parsed as a plain `identifier` which is part of a `declaredIdentifier` in the first case, and as a `primary` which is a `postfixExpression`, which is a `unaryExpression`, etc., in the second case. In short, we are transforming the AST of the program as a whole, not isolated snippets of code.* *In scientific literature, this kind of transformation is commonly specified as an inductive transformation where `[[e1 e2]] = [[e1]] [[e2]]` when the language supports a construct of the form `e1 e2`, etc. The reader may prefer to view the transformation in that light, and we would then say that we have omitted all the congruence rules.* For the purposes of describing the transformation on assignable expressions we need the following syntactic entity: ``` assignableExpressionTail ::= arguments assignableSelector assignableSelectorPart* ``` The transformation proceeds as follows, with three groups of situations where a transformation is applied: 1. With a `postfixExpression` _e_, - if _e_ is of the form `constructorInvocation selector*`, i.e., `typeName typeArguments '.' identifier arguments selector*` then perform new/const insertion on the initial `constructorInvocation`. - if _e_ is of the form `typeIdentifier arguments` where `typeIdentifier` denotes a class then perform new/const insertion on _e_. - if _e_ is of the form `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_. - if _e_ is of the form `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_. 2. With an `assignableExpression` _e_, - if _e_ is of the form `constructorInvocation assignableSelectorPart+` then perform new/const insertion on the initial `constructorInvocation`. - if _e_ is of the form `typeIdentifier assignableExpressionTail` where `typeIdentifier` denotes a class then perform new/const insertion on the initial `typeIdentifier arguments`. - if _e_ is of the form `typeIdentifier '.' identifier assignableExpressionTail` where `typeIdentifier` denotes a class and `identifier` is the name of a named constructor in that class, or `typeIdentifier` denotes a prefix for a library _L_ and `identifier` denotes a class exported by _L_ then perform new/const insertion on the initial `typeIdentifier '.' identifier arguments`. - if _e_ is of the form `typeIdentifier1 '.' typeIdentifier2 '.' identifier assignableExpressionTail` where `typeIdentifier1` denotes a library prefix for a library _L_, `typeIdentifier2` denotes a class _C_ exported by _L_, and `identifier` is the name of a named constructor in _C_ then perform new/const insertion on the initial `typeIdentifier1 '.' typeIdentifier2 '.' identifier arguments`. 3. If _e_ is a literal list or a literal map which occurs in a constant context and does not have the modifier `const`, it is replaced by `const` _e_. *In short, `const` is added implicitly in almost all situations where it is required by the context, and in other situations `new` is added on instance creations. It is easy to verify that each of the replacements can be derived from `postfixExpression` via `primary selector*` and similarly for `assignableExpression`. Hence, the transformation preserves syntactic correctness.* ## Dynamic Semantics There is no dynamic semantics to specify for this feature, because it is eliminated by the code transformation. ## Revisions - 0.7 (2018-04-10) Clarified the structure of the algorithm. Added commentary about cases where there is no constant context even though a constant expression is required, with a motivation for why it is so. - 0.6 (2018-04-06) Removed "magic const" again, due to the risks associated with this feature (getting it specified and implemented robustly, in time). - 0.5 (2018-01-04) Rewritten to use `const` whenever possible (aka "magic const") and adjusted to specify optional const as well as optional new together, because they are now very closely connected. This document was renamed to 'implicit-creation.md', and the document 'optional-const.md' was deleted. - 0.4 (2017-10-17) Reverted to use 'immediate subexpression' again, for correctness. Adjusted terminology for consistency. Clarified the semantics of the transformation. - 0.3 (2017-09-08) Included missing rule for transformation of composite literals (lists and maps). Eliminated the notion of an immediate subexpression, for improved precision. - 0.2 (2017-07-30) Updated the document to specify the previously missing transformations for `assignableExpression`, and to specify a no-magic approach (where no `const` is introduced except when forced by the syntactic context). - 0.1 (2017-08-15) Stand-alone informal specification for optional new created, using version 0.8 of the combined proposal [optional-new-const.md](https://github.com/dart-lang/sdk/blob/main/docs/language/informal/optional-new-const.md) as the starting point.