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README.markdown 0.4.0

Copyright 2010-2011 Håkan Råberg - Released under the EPL license.

Ruby/Smalltalk style internal iterators for Java 5 using bytecode transformation to capture expressions as closures.


Enumerable allows you to write blocks in valid Java like this:

Fn1 square = λ( n, n * n);
List<Integer> result = collect(integers, square);

Which is expanded using the ASM Toolkit for Bytecode Manipulation into this:

Fn1 square = new Fn1() {
    public Object call(Object arg) {
        return (Integer) arg * (Integer) arg;
List<Integer> result = collect(integers, square);

Closure works as expected, by transforming local variables to arrays:

int i = 0;
λ( n, i += n).call(10);
assert i == 10;


final int[] i = new int[] { 0 };
new Fn1() {
    public Object call(Object arg) {
        return i[0] += (Integer) arg;
assert i[0] == 10;

Block parameters are defined using annotated static fields. For more examples see EnumerableExample which has plenty of comments.

Note: The actual blocks are limited to one expression.



Usage is packaged as a java agent. ASM has been moved to a local package (org.enumerable.lambda.weaving.asm).

java -javaagent:enumerable-java-<version>.jar [...]

Look at LamdaLoader if you have different class loading needs.

This file is also the actual library itself, and is needed as a compile time dependency.

The API is very similar to the Enumerabe module in Ruby. You will be mainly importing static methods and fields from Enumerable, Lambda and Parameters

If you're using Eclipse, you can add the agent as a default VM argument under Installed JREs. You can also add Lambda, Paramters and Enumerable as Favorites in the Java Content Assist settings. Finally, you can create a Java Editor Template to easier insert closures in your code, see org.enumerable.lambda.Lambda for an example. requires your classes to have local variable debugging info (-g:vars or -g in javac).

AOT Compilation

To avoid the use of the agent, or any other non standard class loading, you can compile your lambdas ahead of time like this:

java -cp <project class path> org.enumerable.lambda.weaving.LambdaCompiler project.jar

The jar will be compiled and rebuilt in place. The runtime dependency on enumerable-java-<version>.jar as a library remains after AOT compilation.

See the targets aot-compile-tests and aot-tests in build.xml for an example.

Binary Distribution

enumerable-java-<version>.jar is both the actual library, and the java agent enabling load time weaving of lambdas.

The binary distribution, when downloaded as a .tgz archive, or built using ant dist, doubles as an example project which can be directly imported into Eclipse. Open org.enumerable.lambda.enumerable.EnumerableExample to get started. The example bootstraps itself if needed, so you don't need to configure the javaagent. There's also a build.xml in the example folder, which includes targets for AOT compilation.

System Properties

  • lambda.weaving.debug - will log to System.out and write all generated classes to disk if set to true.
  • lambda.weaving.debug.classes.dir - where to write the classes. Defaults to target/generated-classes.
  • - will log lots of ASMified information about the transfromation to System.out if set to true.
  • lambda.weaving.skipped.packages - is a comma separated list of package prefixes to skip.
  • lambda.weaving.included.packages - is a comma separated list of packages to include. This overrides the skipped packages defined above, i.e. any package that is not included will be skipped.
  • lambda.weaving.exclude.pattern - is regexp to prevent transformation of classes based on the complete class name rather than by package prefix. This is applied after the package level filtering.


You probably want to use the @LambdaParameter annotation to mark fields of your own types to be used in blocks via static imports:

public class MyDomainLambdaParameters {
    public static Money m;

Accessing a static field marked with @LambdaParameter outside of a block will either start a new block or throw an exception depending on the situation. The fields are never really used, as all accesses are redirected.

NewLambda is not tied to the Fn0 hierarchy or function classes. Any single abstract method interface or class can be implemented as a Lambda using @NewLambda:

public class MyDomainLambdas {
    public static <R> Callable<R> callable(R block) {
        throw new LambdaWeavingNotEnabledException();

This allows you to create a new anonymous instance of a Callable like this:

Callable<String> c = callable("you called?");

The call to the method marked with @NewLambda will be replaced with the creation of a new anonymous instance at runtime, as seen in the beginning of this document.

Alternatively, you can create an instance of any single abstract method interface or class like this:

static ActionEvent event;

// ...

ActionListener a = delegate(event, out.printf(event + "\n"));

This approach works best for functions which always take the same non generic type, like ActionEvent here.


Once created, a lambda can be turned into an invocation handler for a proxy like this:

static KeyEvent event;

// ...

KeyListener listener = λ( event, out.printf(event + "\n")).as(KeyListener.class);

The version above forwards all calls to the lambda. You can also limit the calls to be forwarded like this:

KeyListener keyTyped = λ( event, out.printf(event + "\n")).as(KeyListener.class, ".*Typed", EventObject.class);

Now only calls matching the regular expression and the specidifed argument types will be forwarded.

Default Parameter Values

Parameters to Fn1, Fn2 and Fn3 can have default values:

Fn2<Double, Double, Double> nTimesMorPI = λ( n, m = Math.PI, n * m);
assert 2.0 * Math.PI ==;

The default value expression is captured as the expression assigned to the static field marked with @LambdaParameter, and can be more complex than just accessing a constant value like in this example.

External Library Closure Support

Concurrency JSR-166 (for Java 6)

LamdaOps allows you to create lambdas implementing interfaces from extra166y.Ops to be used with extra166y.ParallelArray. You need to have jsr166y.jar and extra166y.jar on your class path. They can be downloaded from the Concurrency JSR-166 Interest Site. They can also be found in this repository in lib.

The LambdaOps class is an example of a collection of static factory methods marked with @NewLambda as mentioned above. You can create your own factory classes in a similar way, has no special support for the interfaces in Ops.

Clojure Sequences and IFns

LambdaClojure allows you to create lambdas implementing the interface clojure.lang.IFn to be used directly with Clojure or via ClojureSeqs which acts as a facade for the Clojure Seq library. The tests run against clojure-1.3.0.jar. Download from

JRuby Blocks

LambdaJRuby allows you to create lambdas extending RubyProc. The tests run against jruby-1.6.5.jar. Download from

JavaScript Functions (for Java 6)

LambdaJavaScript allows you to create lambdas extending Function. Uses Rhino 1.6r2 which comes with Java 6.

Groovy Closures

LambdaGroovy allows you to create lambdas extending Closure. The tests run against groovy-all-1.8.3.jar. Download from

Scala Functions

LambdaScala allows you to create lambdas extending Scala's Function and also convert them to Fn0. Tested against scala-library.jar from Scala 2.9.1. Download from

Google Collections

LambdaGoogleCollections allows you to create lambdas implementing Function, Predicate and Supplier from Guava (a superset of Google Collections). The tests run against guava-10.0.1.jar. Download from Guava.

Functional Java

LambdaFunctionalJava allows you to create lambdas implementing F etc. from Functional Java. You need functionaljava-0.3.0.jar on your class path. Download from Functional Java.

Expression Trees (using JavaParser)

LambdaExpressionTrees is a facade for a simple decompiler that turns lambdas into Expression Trees represented by JavaParser's AST. You need javaparser-1.0.8.jar on your class path. Download from javaparser.

Note: The decompiler has several limitations, for example, it doesn't support nested ternary operators. It also doesn't support decompiling lambdas which are closures.

Modified trees can also be compiled using InMemoryCompiler in Java 6 (in Java 5 you can use Janino). The in-memory compiler defaults to using ToolProvider.getSystemJavaCompiler() (which is javac when using Sun's JDK).

By setting the system property to true and adding ecj-3.7.1.jar on your class path, you can use the Eclipse batch compiler instead.


Enumerable and EnumerableArrays act as a facades for the implementation in EnumerableModule and EMap. uses Ant to build. Run ant tests, ant example or ant agent-jar.

The transformation is implemented in two passes. The first pass identifies all blocks and their arities and which local variables they access, if any. The second pass does the actual transformation, which has three main elements:

  • Moving the actual block expression into a new inner class implementing the return type of the @Newlambda factory method, which is assumed to have one single abstract method for the lambda to override.
  • Passing any accessed local variables into the block constructor. Mutable variables are wrapped in arrays.
  • Replacing the original expression with code that constructs the new block.

To understand the transformation better, a good point to start is running ant example -Dlambda.weaving.debug=true.

How expressions are transformed into blocks

Take this block:

import static org.enumerable.lambda.enumerable.Enumerable.*;
import static org.enumerable.lambda.Lambda.*;

// ...

each(strings, λ( s, out.printf("Country: %s\n", s)));

The first pass starts by looking for any static fields marked with the @LambdaParameter annotation. Once it sees access to one, s in this case, it will start moving the code into a new Fn1 (or Fn2) implementation. A block ends with a call to a static method marked with @NewLambda: fn. (Remember when reading the code that all arguments are (obviously) evaluated before the method call, so s is accessed first, and fn called last.)

The first pass also keeps track of any local variable that is accessed from within a block, so that it can be wrapped in an array when initialized. This allows the block to properly close over local variables.

The Enumerable methods themselves are implemented using plain old Java. You can call them using normal anonymous inner classes (as seen in the beginning of this document).


Running ant rubyspec will run the RubySpecs for core/enumerable. uses it's own "platform", enumerable_java to skip specifications for features that aren't supported.

Notes on the Architecture has 3 layers:

org.enumerable.lambda.weaving - transforms expressions into anonymous inner classes

This layer uses ASM, and is directed by the annotations @LambdaParameter and @NewLambda. It's not coupled to the layer above, and you can build your own bridge layer by using these annotations.

org.enumerable.lambda - simple functional programming constructs

This layer is normal Java and can be used on it's own. It also uses the annotated class org.enumerable.lambda.Lambda to direct the weaving process, if enabled.

This layer mainly exists to simplify implemention of bridges from the user facing closures to an actual Java API. If you want to use closures for another library, you can wrap or implement its API using this layer as a starting point.

org.enumerable.lambda.enumerable - a port of Ruby's Enumerable module

This layer is also normal Java and has no knowledge of the bytecode weaving.

Embedding also provides a enumerable-java-weaver-<version>.jar which can be used to embed the Closure weaver in external frameworks. It looks for /org/enumerable/lambda/weaving/ and reads the following properties:

  • lambda.weaving.annotation.newlambda - an annotation with target method used to trigger lambda macro expansion. Not needed at runtime. Defaults to @NewLambda.
  • lambda.weaving.annotation.lambdaparameter - an annotation with target field used to mark static fields as placeholders for parameters in lambdas. Not needed at runtime. Defaults to @LambdaParameter.
  • lambda.weaving.annotation.lambdalocal - an annotation that with target field or parameter that is used to add runtime meta data to created lambdas. If empty, no meta data will be added, and the annotation won't be needed at runtime. Defaults to @LambdaLocal


Ruby Enumerable:

Closures for JDK7, a Straw-Man Proposal:

BlocksInJava, old c2 wiki article about everyone's favorite missing feature:

My blog post which outlined my final try using Dynamic Proxies:

A library that's quite close to what I suggested there is LambdaJ:

License is released under the EPL license.

ASM 3.3: Copyright (c) 2000-2005 INRIA, France Telecom, see ASM License.

jsr166y and extra166y: Written by Doug Lea with assistance from members of JCP JSR-166 Expert Group and released to the public domain, as explained at

Clojure: Copyright (c) Rich Hickey, released under the EPL license.

JRuby: Copyright (c) 2007-2010 The JRuby project, and is released under a tri CPL/GPL/LGPL license.

Groovy: Copyright 2003-2007 the original author or authors. Licensed under the Apache License, Version 2.0.

Rhino: Copyright (C) 1997-1999 Norris Boyd/Netscape Communications Corporation. The majority of Rhino is MPL 1.1 / GPL 2.0 dual licensed.

Google Collections: Copyright (C) 2008 Google Inc. Licensed under the Apache License, Version 2.0.

RubySpec and MSpec: Copyright (c) 2008 Engine Yard, Inc. All rights reserved. License.

JavaParser: Copyright (C) 2008 Júlio Vilmar Gesser. Released under the LGPL

Scala: Copyright (c) 2002-2010 EPFL, Lausanne, unless otherwise specified. Released under the SCALA LICENSE

Functional Java: Copyright (c) 2008-2011, Tony Morris, Runar Bjarnason, Tom Adams, Brad Clow, Ricky Clarkson, Jason Zaugg All rights reserved. Released under an open source BSD license


Ruby/Smalltalk style internal iterators for Java 5 using bytecode transformation to capture expressions as closures.



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