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Feature: Generic Method Syntax

Feature: Generic Method Syntax

This document is an informal specification of the support for generic methods and functions which has been implemented in dart2js with option --generic-method-syntax, starting with commit acc5f59. In SDK 1.21 this feature is available by default (i.e., also without the option) in the virtual machine and the analyzer, as well as in dart2js.

The motivation for having this feature is that it enables partial support for generic methods and functions, thus providing a bridge between not having generic methods and having full support for generic methods. In particular, code declaring and using generic methods may be type checked and compiled in strong mode, and the same code will now be acceptable in standard (non-strong) mode as well. The semantics is different in certain cases, but standard mode analysis will emit diagnostic messages (e.g., errors) for that.

In this document, the word routine will be used when referring to something which can be a method, a top level function, a local function, or a function literal expression.

With this feature it is possible to compile code where generic methods and functions are declared, implemented, and invoked. The runtime semantics does not include reification of type arguments. Evaluations of the runtime value of a routine type parameter is a runtime error or yields dynamic, depending on the context. No type checking takes place at usages of a method or function type parameter in the body, and no type checking regarding explicitly specified or omitted type arguments takes place at call sites.

In short, generic methods and functions are supported syntactically, and the runtime semantics prevents dynamic usages of the type argument values, but it allows all usages where that dynamic value is not required. For instance, a generic routine type parameter, T, cannot be used in an expression like x is T, but it can be used as a type annotation. In a context where other tools may perform type checking, this allows for a similar level of expressive power as do language designs where type arguments are erased at compile time.

The motivation for this document is that it serves as an informal specification for the implementation of support for the generic method syntax feature in all Dart tools.


The syntactic elements which are added or modified in order to support this feature are as follows, based on grammar rules given in the Dart Language Specification (Aug 19, 2015).

	typeParameters? formalParameterList
	metadata returnType? identifier formalParameterPart
	metadata identifier (('extends'|'super') type)?
	formalParameterPart functionBody
	metadata finalConstVarOrType? 'this' '.' identifier
	typeArguments? arguments
	assignableSelector | argumentPart
	primary (argumentPart* assignableSelector)+ |
	'super' unconditionalAssignableSelector |
	'..' (cascadeSelector argumentPart*)
	(assignableSelector argumentPart*)*
	(assignmentOperator expressionWithoutCascade)?

In a draft specification of generic methods from June 2015, the number of grammar changes is significantly higher, but that form can be obtained via renaming.

This extension to the grammar gives rise to an ambiguity where the same tokens may be angle brackets of a type argument list as well as relational operators. For instance, foo(a<b,c>(d))[^1] may be parsed as a postfixExpression on the form primary arguments where the arguments are two relational expressions (a<b and c>(d)), and it may also be parsed such that there is a single argument which is an invocation of a generic function (a<b,c>(d)). The ambiguity is resolved in favor of the generic function invocation whenever the primary is followed by a balanced pair of angle brackets where the next token after the final > is a left parenthesis (in short, we are "looking at < .. >(").

This implies that an expression like foo(a<b,2>(d)) will be rejected because it is parsed such that foo gets one argument which must be a generic function invocation, but 2 cannot parse correctly as a type. This is a breaking change, because the same expression used to parse correctly as an invocation of foo with two arguments.

The reason why the generic function invocation is favored over the relational expressions is that it is considered to be a rare exception that this ambiguity arises: It requires a balanced set of angle brackets followed by a left parenthesis, which is already an unusual form. On top of that, the style guide recommendation to use named parameters for boolean arguments helps making this situation even less common.

If it does occur then there is an easy workaround: an extra set of parentheses (as in foo(a<b,(2>(d)))) will resolve the ambiguity in the direction of relational expressions; or we might simply be able to remove the parentheses around the last expression (as in foo(a<b,2>d)), which will also eliminate the ambiguity.

It should be noted that parsing techniques like recursive descent seem to conflict with this approach to disambiguation: Determining whether the remaining input starts with a balanced expression on the form < .. > seems to imply a need for an unbounded lookahead. However, if some type of "diet" parsing is used and various kinds of bracket tokens are matched up during the lexical analysis then it takes only a simple O(1) check in the parser to perform the required check.

Scope of the Mechanism

With the syntax in place, it is obvious that certain potential extensions have not been included.

For instance, constructors, setters, getters, and operators cannot be declared as generic. Actual type arguments cannot be passed at invocation sites for setters, getters, and operators, and for constructors there is a need to find a way to distinguish between type arguments for the new instance and type arguments for the constructor itself. It is possible that some of these restrictions will be lifted in a full-fledged version of this extension.

Conversely, the inclusion of lower bounds for type parameters (using the keyword super) serves to demonstrate that lower bounds fit well into the syntax. There is no guarantee that a final version of generic methods will support lower bounds, and it is not required that an implementation must support them.

In general, the support for generic methods offered by this feature is not intended to be complete, it is intended to allow for experiments such that a final version of the generic method support can be designed well, and it is intended to allow for the subset of usages where reification is not required.

Resolution and Type Checking

In order to be useful, the support for generic methods and functions must be sufficiently complete and consistent to avoid spurious diagnostic messages. In particular, even though no regular type checks take place at usages of routine type parameters in the body where they are in scope, those type parameters should be resolved. If they had been ignored then any usage of a routine type parameter X would give rise to a Cannot resolve type X error message, or the usage might resolve to other declarations of X in enclosing scopes such as a class type parameter, both of which is unacceptable.

In dart2js resolution, the desired behavior has been achieved by adding a new type parameter scope and putting the type parameters into that scope, giving each of them the bound dynamic. The type parameter scope is the current scope during resolution of the routine signature and the type parameter bounds, it encloses the formal parameter scope of the routine, and the formal parameter scope in turn encloses the body scope.

This implies that every usage of a routine type parameter is treated during type checking as if it had been an alias for the type dynamic.

Static checks for invocations of methods or functions where type arguments are passed are omitted entirely: The type arguments are parsed, but no checks are applied to certify that the given routine accepts type arguments, and no checks are applied for bound violations. Similarly, no checks are performed for invocations where no type arguments are passed, whether or not the given routine is statically known to accept type arguments.

Certain usages of a routine type parameter X give rise to errors: It is a compile-time error if X is used as a type literal (e.g., foo(X)), or in an expression on the form e is X or e is! X.

It could be argued that it should be a warning or an error if a routine type parameter X is used in an expression on the form e as X. The blind success of this test at runtime may introduce bugs into correct programs in situations where the type constraint is violated; in particular, this could cause "wrong" objects to propagate through local variables and parameters and even into data structures (say, when a List<T> is actually a List<dynamic>, because T is not present at runtime when the list is created). However, considering that these type constraint violations are expected to be rare, and considering that it is common to require that programs compile without warnings, we have chosen to omit this warning. A tool is still free to emit a hint, or in some other way indicate that there is an issue.

Dynamic semantics

If a routine invocation specifies actual type arguments, e.g., int in the invocation f<int>(42), those type arguments will not be evaluated at runtime, and they will not be passed to the routine in the invocation. Similarly, no type arguments are ever passed to a generic routine due to call-site inference. This corresponds to the fact that the type arguments have no runtime representation.

When the body of a generic routine is executed, usages of the formal type parameters will either result in a run-time error, or they will yield the value yielded by an evaluation of dynamic, following the treatment of malformed types in Dart. There are the following cases:

When X is a routine type parameter, the evaluation of e is X, e is! X, and X used as a type literal proceeds as if X had been a malformed type, producing a dynamic error if the type test itself is reached; the evaluation of e as X has the same outcome as the evaluation of e.

Note that the forms containing is are compile-time errors, which means that implementations are free to reject the program, or to compile the program with a different runtime semantics for these expressions. The rationale for dart2js to allow the construct and compile it to a run time error is that (1) this allows more programs using generic methods to be compiled, and (2) an is expression that blindly returns true every time (or false every time) may silently introduce a bug into an otherwise correct program, so the expression must fail if it is ever evaluated.

When X is a routine type parameter which is passed as a type argument to a generic class instantiation G which is itself used in e is G, e is! G, e as G, and G used as a type literal, evaluation again proceeds as if X were a malformed type, which in this case means that it is treated like dynamic.

This may be surprising, so let us consider a couple of examples: When X is a routine type parameter, 42 is X raises a dynamic error, <int>[42] is List<X> yields the value true, and 42 as X yields 42, no matter whether the syntax for the invocation of the routine included an actual type argument, and, if so, no matter which value the actual type argument would have had at the invocation.

Object construction is similar: When X is a routine type parameter which is a passed as a type argument to a constructor invocation, the value passed to the constructor will be the value yielded by an evaluation of dynamic.

In checked mode, when X is a routine type parameter which is used as a type annotation or in a generic type G used as a type annotation, no checked mode checks will ever fail for initialization or assignment to a local variable or parameter whose type annotation is X, and if the type annotation is a generic type G that contains X, checked mode checks will succeed or fail as if X had been the type dynamic.


2017-Jan-04: Changed 'static error' to 'compile-time error', which is the phrase that the language specification uses.


[^1]: These expressions violate the common style in Dart with respect to spacing and capitalization. That is because the ambiguity implies conflicting requirements, and we do not want to bias the appearance in one of the two directions.

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