State of the Lambda

Version 2


Brian Goetz, 6 July 2010

This is an updated proposal to add lambda expressions (informally, "closures") to the Java programming language. This sketch is built on the straw-man proposal made by Mark Reinhold in December 2009.

1. Background; SAM classes

The Java programming language already has a form of closures: anonymous inner classes. There are a number of reasons these are considered imperfect closures, primarily:

It is not a goal of Project Lambda to address all of these issues.

The standard way for Java APIs to define callbacks is to use an interface representing the callback method, such as:

public interface CallbackHandler { 
    public void callback(Context c);

The CallbackHandler interface has a useful property: it has a single abstract method. Many common interfaces and abstract classes have this property, such as Runnable, Callable, EventHandler, or Comparator. We call these classes SAM classes.

The biggest pain point for anonymous inner classes is bulkiness. To call a method taking a CallbackHandler, one typically creates an anonymous inner class:

foo.doSomething(new CallbackHandler() { 
                    public void callback(Context c) { 

The anonymous inner class here is what some might call a "vertical problem": five lines of source code to encapsulate a single statement.

Astute readers will notice that the syntax used for examples in this document differ from that expressed in the straw-man proposal. This does not reflect a final decision on syntax; we are still experimenting with various candidate syntax options.

2. Lambda expressions

Lambda expressions are anonymous functions, aimed at addressing the "vertical problem" by replacing the machinery of anonymous inner classes with a simpler mechanism. One way to do that would be to add function types to the language, but this has several disadvantages: - Mixing of structural and nominal types; - Divergence of library styles (some libraries would continue to use callback objects, while others would use function types).

So, we have instead chosen to take the path of making it easier to create instances of callback objects.

Here are some examples of lambda expressions:

{ -> 42 }

{ int x -> x + 1 }

The first expression takes no arguments, and returns the integer 42; the second takes a single integer argument, named x, and returns x+1.

Lambda expressions are distinguished from ordinary statement blocks by the presence of a (possibly empty) formal parameter list and the -> token. The lambda expressions shown so far are a simplified form containing a single expression; there is also a multi-statement form that can contain one or more statements.

3. SAM conversion

One can describe a SAM type by its return type, parameter types, and checked exception types. Similarly, one can describe the type of a lambda expression by its return type, parameter types, and exception types.

Informally, a lambda expression e is convertible-to a SAM type S if an anonymous inner class that is a subtype of S and that declares a method with the same name as S's abstract method and a signature and return type corresponding to the lambda expressions signature and return type would be considered assignment-compatible with S.

The return type and exception types of a lambda expression are inferred by the compiler; the parameter types may be explicitly specified or they may be inferred from the assignment context (see Target Typing, below.)

When a lambda expression is converted to a SAM type, invoking the single abstract method of the SAM instance causes the body of the lambda expression to be invoked.

For example, SAM conversion will happen in the context of assignment:

CallbackHandler cb = { Context c -> System.out.println("pippo") };

In this case, the lambda expression has a single Context parameter, has void return type, and throws no checked exceptions, and is therefore compatible with the SAM type CallbackHandler.

4. Target Typing

Lambda expressions can only appear in context where it will be converted to a variable of SAM type; the type of 'this' inside the lambda expression is (a subtype of) the SAM type to which the lambda expression is being converted. So the following code will print "Yes":

Runnable r = { -> 
                 if (this instanceof Runnable) 

The following use of lambda expressions is forbidden because it does not appear in a SAM-convertible context:

Object o = { -> 42 };

In a method invocation context, the target type for a lambda expression used as a method parameter is inferred by examining the set of possible compatible method signatures for the method being invoked. This entails some additional complexity in method selection; ordinarily the types of all parameters are computed, and then the set of compatible methods is computed, and a most specific method is selected if possible. Inference of the target type for lambda-valued actual parameters happens after the types of the other parameters is computed but before method selection; method selection then happens using the inferred target types for the lambda-valued parameters.

The type of the formal parameters to the lambda expression can also be inferred from the target type of the lambda expression. So we can abbreviate our callback handler as:

CallbackHandler cb = { c -> System.out.println("pippo") };

as the type of the parameter c can be inferred from the target type of the lambda expression.

Allowing the formal parameter types to be inferred in this way furthers a desirable design goal: "Don't turn a vertical problem into a horizontal problem." We wish that the reader of the code have to wade through as little code as possible before arriving at the "meat" of the lambda expression.

The user can explicitly choose a target type by specifying a type name. This might be for clarity, or might be because there are multiple overloaded methods and the compiler cannot correctly chose the target type. For example:

executor.submit(Callable<String> { -> "foo" });

If the target type is an abstract class, it is an open question as to whether we want to permit an argument list so a constructor other than the no-arg constructor can be used.

5. Lambda bodies

In addition to the simplified expression form of a lambda body, a lambda body can also contain a list of statements, similar to a method body, with several differences: the break, return, and continue statements are not permitted, and a "yield" statement, whose form is similar to to the return statement, is permitted instead of a return statement. The type of a multi-statement lambda expression is inferred by unifying the type of the values yielded by the set of yield statements. As with method bodies, every control path through a multi-statement lambda expression must either yield a value, yield no value, or throw an exception. Expressions after a yield statement are unreachable.

The complete syntax is given by:

lambda-exp := "{" arg-list "->" lambda-body "}"
arg-list := "(" args ")" | args
args := arg | arg "," args
arg := [ type ] identifier
lambda-body := expression | statement-list [ ";" ]
statement-list := statement | statement ";" statement-list

6. Instance capture

Once the target type of a lambda expression is determined, the body of a lambda expression is treated largely the same way as an anonymous inner class whose parent is the target type. The 'this' variable refers to the SAM-converted lambda (whose type is a subtype of the target type). Variables of the form OuterClassName.this refer to the instances of lexically enclosing classes, just as with inner classes. Unqualified names may refer to members of the SAM class (if it is a class and not an interface), or to members of lexically enclosing classes, using the same rules as for inner classes.

For members of lexically enclosing instanaces, member capture is treated as if the references were desugared to use the appropriate "Outer.this" qualifier and Outer.this is captured as if it were a local final variable.

7. Local variable capture

The current rules for capturing local variables of enclosing contexts in inner classes are quite restrictive; only final variables may be captured. For lambda expressions (and for consistency, probably inner class instances as well), we relax these rules to also allow for capture of effectively final local variables. (Informally, a local variable is effectively final if making it final would not cause a compilation failure.)

It is likely that we will not permit capture of mutable local variables. The reason is that idioms like this:

int sum = 0;
list.forEach({ Element e -> sum += e.size(); });

are fundamentally serial; it is quite difficult to write lambda bodies like this that do not have race conditions. Unless we are willing to enforce (preferably statically) that such lambdas not escape their capturing thread, such a feature may likely cause more trouble than it solves.

8. Exception transparency

A separate document on exception transparency proposes our strategy for amending generics to allow abstraction over thrown checked exception types.

9. Method references

SAM conversion allows us to take an anonymous method body and treat it as if it were a SAM type. It is often desirable to do the same with an existing method (such as when a class has multiple methods that are signature-compatible with Comparable.compareTo().)

Method references are expressions which have the same treatment as lambda expressions (i.e., they can only be SAM-converted), but instead of providing a method body they refer to a method of an existing class or object instance.

For example, consider a Person class that can be sorted by name or by age:

class Person { 
    private final String name;
    private final int age;

    public static int compareByAge(Person a, Person b) { ... }

    public static int compareByName(Person a, Person b) { ... }

Person[] people = ...
Arrays.sort(people, #Person.compareByAge);

Here, the expression #Person.compareByAge is sugar for a lambda expression whose formal argument list is copied from the method Person.compareByAge, and whose body calls Person.compareByAge. This lambda expression will then get SAM-converted to a Comparator.

If the method being referenced is overloaded, it can be disambiguated by providing a list of argument types:

Arrays.sort(people, #Person.compareByAge(Person, Person));

Instance methods can be referenced as well, by providing a receiver variable:

Arrays.sort(people, #comparatorHolder.comparePersonByAge);

In this case, the implicit lambda expression would capture a final copy of the "comparatorHolder" reference and the body would invoke the comparePersonByAge using that as the receiver.

We may choose to restrict the forms that the receiver can take, rather than allowing arbitrary object-valued expressions like "#foo(bar).moo", when capturing instance method references.

10. Extension methods

A separate document on defender methods proposes our strategy for extending existing interfaces with virtual extension methods.