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NoSuchMethod Forwarding

Author: eernst@

Status: Background material, normative language now in dartLangSpec.tex.

Version: 0.7 (2018-07-10)

This document is an informal specification of the support in Dart 2 for invoking noSuchMethod in situations where an attempt is made to invoke a method that does not exist.

The feature described here, noSuchMethod forwarding, is a particular approach whereby an implementation of noSuchMethod in a class C causes C to be extended with a set of compiler generated forwarding methods, such that an invocation of any method in the static interface of C will become a regular method invocation, which in turn invokes noSuchMethod.

Motivation

In Dart 1.x, noSuchMethod will be invoked whenever an attempt is made to call a method that does not exist.

In other words, consider an instance method invocation of a member named m on a receiver o whose class C does not have a member named m (or it has a member named m, but it does not admit the given invocation, e.g., because the number of arguments is wrong). The properties of the invocation are then specified using an instance i of Invocation, and noSuchMethod is then invoked with i as the actual argument. Among other things, i specifies whether the invocation was a method call or an invocation of a getter or a setter, and it specifies which actual arguments were passed.

One difficulty with this design is that it requires developers to take both method invocations and getter invocations into account, in order to support a given method using noSuchMethod:

class Foo {
  foo(x) {}
}

class MockFoo implements Foo {
  // PS: Make sure that a tear-off of `_mockFoo` has the same type
  // as a tear-off of `Foo.foo`.
  _mockFoo(x) {
    // ... implement mock behavior for `foo` here.
  }

  noSuchMethod(Invocation i) {
    if (i.memberName == #foo) {
      if (i.isMethod &&
          i.positionalArguments.length == 1 &&
          i.namedArguments.isEmpty) {
        return _mockFoo(i.positionalArguments[0]);
      } else if (i.isGetter) {
        return _mockFoo;
      }
    }
    return super.noSuchMethod(i);
  }
}

The reason why the type of a tear-off of _mockFoo should be the same as the type of a tear-off of foo is that the former should be able to emulate the properties of the latter faithfully, including the response it gives rise to when subjected to type tests, either explicitly or implicitly.

Obviously, this is verbose, tedious, and difficult to maintain if the claimed superinterfaces (implements ...) in the mock class introduce a large number of methods with complex signatures. It is particularly inconvenient if the mock behavior is simple and largely independent of all those types.

The noSuchMethod forwarding approach eliminates much of this tedium by means of compiler generated forwarding methods corresponding to all the unimplemented methods. The example could then be expressed as follows:

class Foo {
  foo(x) {}
}

class MockFoo implements Foo {
  noSuchMethod(Invocation i) {
    if (i.memberName == #foo) {
      if (i.isMethod &&
          i.positionalArguments.length == 1 &&
          i.namedArguments.isEmpty) {
        // ... implement mock behavior for `foo` here.
      }
    }
    return super.noSuchMethod(i);
  }
}

With noSuchMethod forwarding, this causes a foo forwarding method to be generated, with the signature declared in Foo and with the necessary code to create and initialize a suitable Invocation which will be passed to noSuchMethod.

Syntax

The grammar remains unchanged.

Static Analysis

We say that a class C has a non-trivial noSuchMethod if C declares or inherits a concrete method named noSuchMethod which is distinct from the declaration in the built-in class Object.

Note that such a declaration cannot be a getter or setter, and it must accept one positional argument of type Invocation, due to the requirement that it must correctly override the declaration of noSuchMethod in the class Object. For instance, in addition to the obvious choice noSuchMethod(Invocation i) it can be noSuchMethod(Object i, [String s]), but not noSuchMethod(Invocation i, String s).

If a concrete class C has a non-trivial noSuchMethod then each method signature (including getters and setters) which is a member of C's interface and for which C does not have a concrete declaration is noSuchMethod forwarded.

A concrete class C that does not have a non-trivial noSuchMethod implements its interface (it is a compile-time error not to do so), but there may exist superclasses of C declared in other libraries whose interfaces include some private methods for which C has no concrete declaration (such members are by definition omitted from the interface of C, because their names are inaccessible). Similarly, even if a class D does have a non-trivial noSuchMethod, there may exist abstract declarations of private methods with inaccessible names in superclasses of D for which D has no concrete declaration. In both of these situations, such inaccessible private method signatures are noSuchMethod forwarded.

No other situations give rise to a noSuchMethod forwarded method signature.

This means that whenever it is stated that a class C has a noSuchMethod forwarded method signature, it is guaranteed to be a concrete class with a non-trivial noSuchMethod, or the signature is guaranteed to be inaccessible. In the former case, the developer expressed the intent to obtain implementations of "missing methods" by having a non-trivial noSuchMethod declaration, and in the latter case it is impossible to write declarations in C that implement the missing private methods, but they will then be provided as generated forwarders.

If a class C has a noSuchMethod forwarded signature then an implicit method implementation implementing that method signature is induced in C. In the case where C already contains an abstract declaration with the same name, the induced method implementation replaces the abstract declaration.

It is a compile-time error if a concrete class C has a non-trivial noSuchMethod, and a name m has a set of method signatures in the superinterfaces of C where none is most specific, and there is no declaration in C which provides such a most specific method signature.

This means that even in the situation where everything else implies that a noSuchMethod forwarder should be induced, signature ambiguities must still be resolved by a developer-written declaration, it cannot be a consequence of implicitly inducing a noSuchMethod forwarder. However, that developer-written declaration could be an abstract method in the concrete class itself.

Note that there is no most specific method signature if there are several method signatures which are equally specific with respect to the argument types and return type, but an optional formal parameter in these signatures has different default values in different signatures.

It is a compile-time error if a class C has a noSuchMethod forwarded method signature S for a method named m, as well as an implementation of m.

This can only happen if that implementation is inherited and satisfies some, but not all requirements of the noSuchMethod forwarded method signature. In the example below, a foo(int i) implementation is inherited and a superinterface declares foo([int i]). This is a compile-time error because C does not have a method implementation with signature foo([int]), but if one were to be implicitly induced it would override A.foo (which is capable of accepting some but not all of the argument lists that an implementation of foo([int]) would allow). We have made this an error because it would be error prone to induce a forwarder in C which will silently override an A.foo which "almost" satisfies the requirement in the superinterface. In particular, developers are likely to be surprised if A.foo is not called even when it is passed a single int argument, which precisely matches the declaration of A.foo.

class A {
  foo(int i) => null;
}

abstract class B {
  foo([int i]);
}

class C extends A implements B {
  noSuchMethod(Invocation i) => ...;
  // Error on `foo`: Forwarder would override `A.foo`.
}

Note that this makes it a breaking change, in situations where such a signature conflict exists in some subtype like C, to change an abstract method declaration to a method implementation: If A had been an abstract class and A.foo an abstract method which was replaced by an A.foo declaration which implements the method, the error on foo in class C would be introduced because A.foo was implemented. There is a reasonably practical workaround, though: implement C.foo with a signature that resolves the conflict. That implementation might invoke A.foo in a superinvocation, or it might forward to noSuchMethod, or some times one and some times the other, that is up to the developer who writes C.foo.

Note that it is not a compile-time error if the interface of C has a noSuchMethod forwarded method signature S with name m, and a superclass of C also has a noSuchMethod forwarded method signature named m, such that the implicitly induced implementation of the former overrides the implicitly induced implementation of the latter. In other words, it is OK for a generated forwarder to override another generated forwarder.

Note that when a class C has an implicitly induced implementation of a method, superinvocations in subclasses are allowed, just like they would have been for a developer-written implementation.

abstract class D { baz(); }
class E implements D {
  noSuchMethod(Invocation i) => null;
}
class F extends E { baz() { super.baz(); }} // OK

Dynamic Semantics

Assume that a class C has an implicitly induced implementation of a method m with positional formal parameters T1 a1..., Tk ak and named formal parameters Tk+1 n1..., Tk+m nm. Said implementation will then create an instance i of the predefined class Invocation such that its

  • isGetter evaluates to true iff m is a getter, isSetter evaluates to true iff m is a setter, isMethod evaluates to true iff m is a method.
  • memberName evaluates to the symbol for the name m.
  • positionalArguments evaluates to an immutable list whose values are a1..., ak.
  • namedArguments evaluates to an immutable map with the same keys and values as {n1: n1..., nm: nm}

Note that the number of named arguments can be zero, in which case some of the positional parameters can be optional. We do not need to mention optional positional arguments separately, because they receive the same treatment as required parameters (which are of course always positional).

Finally the induced method implementation will invoke noSuchMethod with i as the actual argument, and return the result obtained from there.

This determines the dynamic semantics of implicitly induced methods: The declared return type and the formal parameters, with type annotations and default values, are uniquely determined by the noSuchMethod forwarded method signatures, and invocation of an implicitly induced method has the same semantics of invocation of other methods. In particular, dynamic type checks are performed on the actual arguments upon invocation when the corresponding formal parameter is covariant.

This ensures, relying on the heap soundness and expression soundness of Dart (which ensures that every expression of type T will evaluate to an entity of type T), that all statically type safe invocations will invoke a method implementation, user-written or implicitly induced. In other words, with statically checked calls there is no need for dynamic support for noSuchMethod at all.

For a dynamic invocation of a member m on a receiver o that has a non-trivial noSuchMethod, the semantics is such that an attempt to invoke m with the given actual arguments (including possibly some type arguments) is made at first. If that fails (because o has no implementation of m which can be invoked with the given argument list shape, be it a developer-written method or an implicitly induced implementation) noSuchMethod is invoked with an actual argument which is an Invocation describing the actual arguments and invocation.

This implies that dynamic invocations on receivers having a non-trivial noSuchMethod will simply invoke the forwarders whenever possible. Similarly, it will work for dynamic invocations as well as statically checked ones to tear off a method which is in the interface of the receiver and implemented as a generated forwarder.

The only remaining situation is when a dynamic invocation invokes a method which is not present in the static interface of the receiver, or when a method with that name is present, but its signature does not allow for the given invocation (e.g., because some required arguments are omitted). In this situation, the regular instance method invocation has failed (there is no such regular method, and no such generated forwarder). Such a dynamic invocation will then invoke noSuchMethod. In this situation, a developer-written implementation of noSuchMethod should also support both method invocations and tear-offs explicitly (as it should before this feature was added), because there is no generated forwarder to do that.

This approach may incur a certain performance penalty, but only for these invocations (which are dynamic, and have already failed to invoke an existing method, regular or generated).

In return, this approach enforces the following simple invariant, for both statically checked and dynamic invocations: Whenever an instance method is invoked, and no such method exists, noSuchMethod will be invoked.

One special case to be aware of is where a forwarder is torn off and then invoked with an actual argument list which does not match the formal parameter list. In that situation we will get an invocation of Object.noSuchMethod rather than the noSuchMethod in the original receiver, because this is an invocation of a function object (and they do not override noSuchMethod):

class A {
  dynamic noSuchMethod(Invocation i) => null;
  void foo();
}

main() {
  A a = new A();
  dynamic f = a.foo;
  // Invokes `Object.noSuchMethod`, not `A.noSuchMethod`, so it throws.
  f(42);
}

Updates

  • Jul 10th 2018, version 0.7: Added requirement to generate forwarders for inaccessible private methods even in the case where there is no non-trivial noSuchMethod.

  • Mar 22nd 2018, version 0.6: Added example to illustrate the case where a torn-off method invokes Object.noSuchMethod, not the one in the receiver, because of a non-matching actual argument list.

  • Nov 27th 2017, version 0.5: Changed terminology to use 'implicitly induced method implementations'. Helped achieving a major simplifaction of the dynamic semantics.

  • Nov 22nd 2017, version 0.4: Removed support for explicitly requesting generated forwarder in conflict case. Improved the clarity of many parts.

  • Oct 5th 2017, version 0.3: Clarified that generated forwarders must pass an Invocation to noSuchMethod which specifies the bindings of formal arguments to actual arguments. Clarified the treatment of default values for optional arguments.

  • Sep 20th 2017, version 0.2: Many smaller adjustments, based on review feedback.

  • Sep 18th 2017, version 0.1: Created the first version of this document.

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