Polyglot Programming

TruffleRuby allows you to interface with any other Truffle language to create polyglot programs – programs written in more than one language.

This guide describes how to load code written in foreign languages, how to export and import objects between languages, how to use Ruby objects from a foreign language, how to use foreign objects from Ruby, how to load Java types to interface with Java, and how to embed in Java.

If you are using the native configuration, you will need to use the --polyglot flag to get access to other languages. The JVM configuration automatically has access to other languages.

Running Ruby Code from Another Language

When you eval Ruby code from the Context API in another language and mark the Source as interactive, the same interactive top-level binding is used each time. This means that if you set a local variable in one eval, you will be able to use it from the next.

The semantics are the same as the Ruby semantics of calling INTERACTIVE_BINDING.eval(code) for every Context.eval() call with an interactive Source. This is similar to most REPL semantics.

Loading Code Written in Foreign Languages

Polyglot.eval(id, string) executes code in a foreign language identified by its ID.

Polyglot.eval_file(id, path) executes code in a foreign language from a file, identified by its language ID.

Polyglot.eval_file(path) executes code in a foreign language from a file, automatically determining the language.

Exporting Ruby Objects to Foreign Languages

Polyglot.export(name, value) exports a value with a given name.

Polyglot.export_method(name) exports a method, defined in the top-level object.

Importing Foreign Objects to Ruby

Polyglot.import(name) imports and returns a value with a given name.

Polyglot.import_method(name) imports a value, which should be IS_EXECUTABLE, with a given name, and defines it in the top-level object.

Using Ruby Objects from a Foreign Language

Using JavaScript as an example: the left example is JavaScript, the right one is the corresponding action it takes on the Ruby object expressed in Ruby code.

object[name/index] calls object[name/index] if the object has a method [], or reads an instance variable if the name starts with @, or returns a bound method with the name.

object[name/index] = value calls object[name/index] = value if the object has a method []=, or sets an instance variable if the name starts with @.

delete object.name calls object.delete(name).

delete object[name/index] calls object.delete(name).

object.length calls object.size.

Object.keys(hash) gives the hash keys as strings.

Object.keys(object) gives the methods of an object as functions, unless the object has a [] method, in which case it returns an empty array.

object(args...) calls a Ruby Proc, Method, UnboundMethod, etc.

object.name(args...) calls a method on the Ruby object.

new object(args...) calls object.new(args...).

"length" in obj returns true for a Ruby Array.

object == null calls object.nil?.

Notes on Creating Ruby Objects for Use in Foreign Languages

If you want to pass a Ruby object to another language for fields to be read and written, a good object to pass is usually a Struct, as this will have both the object.foo and object.foo = value accessors for you to use from Ruby, and they will also respond to object['foo'] and object['foo'] = value, which means they will work from other languages sending read and write messages.

Using Foreign Objects from Ruby

object[name/index] will read a member from the foreign object.

object[name/index] = value will write a value to the foreign object.

object.delete(name/index) will remove a value from the foreign object.

object.size will get the size or length of the foreign object.

object.keys will get an array of the members of the foreign object.

object.call(*args) will execute the foreign object.

object.name(*args) will invoke a method called name on the foreign object.

object.new(*args) will create a new object from the foreign object (as if it is some kind of class).

object.class on a Java Class object will give you an object on which you can call instance methods, rather than static methods.

object.respond_to?(:size) will tell you if the foreign object has a size or length.

object.nil? will tell you if the foreign object represents the language’s equivalent of null or nil.

object.respond_to?(:call) will tell you if a foreign object can be executed.

object.respond_to?(:new) will tell you if a foreign object can be used to create a new object (if it’s a class).

object.respond_to?(:keys) will tell you if a foreign object can give you a list of members.

object.respond_to?(:class) will tell you if an object is a Java class.

Polyglot.as_enumerable(object) will create a Ruby Enumerable from the foreign object, using its size or length, and reading from it.

Where boolean value is expected (e.g., in if conditions) the foreign value is converted to boolean if possible or considered to be true.

Accessing Java Objects

TruffleRuby’s Java interoperability interface is similar to the interface from the Nashorn JavaScript implementation, as also implemented by GraalVM’s JavaScript implementation.

It is easier to use Java interoperability in JVM mode (--jvm). Java interoperability is also supported in native mode but requires more setup. See here for more details.

Java.type('name') returns a Java class object, given a name such as java.lang.Integer or int[]. With the class object, .new will create an instance, .foo will call the static method foo, [:FOO] will read the field FOO, and so on. To access instance methods use .class, such as MyClass.class.getName.

To import a Java class as a top-level constant, use Java.import 'name'.

Java.synchronized(object) { } will use the Java object’s monitor to synchronize.

Embedding in Java

TruffleRuby is embedded via the Polyglot API, which is part of GraalVM. You will need to use GraalVM to use this API.

import org.graalvm.polyglot.*;

class Embedding {
    public static void main(String[] args) {
        Context polyglot = Context.newBuilder().allowAllAccess(true).build();
        Value array = polyglot.eval("ruby", "[1,2,42,4]");
        int result = array.getArrayElement(2).asInt();

Using Ruby Objects from Embedding Java

Ruby objects are represented by the Value class when embedded in Java.

Accessing Arrays

boolean hasArrayElements()
Value getArrayElement(long index)
void setArrayElement(long index, Object value)
boolean removeArrayElement(long index)
long getArraySize()

Accessing Methods in Objects

boolean hasMembers()
boolean hasMember(String identifier)
Value getMember(String identifier)
Set<String> getMemberKeys
void putMember(String identifier, Object value
boolean removeMember(String identifier)

Executing Procs, Lambdas, and Methods

boolean canExecute()
Value execute(Object... arguments)
void executeVoid(Object... arguments)

Instantiating Classes

boolean canInstantiate() {
Value newInstance(Object... arguments)

Accessing Primitives

boolean isString()
String asString()
boolean isBoolean()
boolean asBoolean()
boolean isNumber()
boolean fitsInByte()
byte asByte()
boolean fitsInShort()
short asShort()
boolean fitsInInt()
int asInt()
boolean fitsInLong()
long asLong()
boolean fitsInDouble()
double asDouble()
boolean fitsInFloat()
float asFloat()
boolean isNull()

The JRuby migration guide includes some more examples.


Ruby strings and symbols are converted to Java strings when they are passed to foreign languages, and Java strings are converted to Ruby strings when they are passed into Ruby.

Threading and Interop

Ruby is designed to be a multi-threaded language and much of the ecosystem expects threads to be available. This may be incompatible with other Truffle languages which do not support threading, so you can disable the creation of multiple threads with the option --single-threaded. This option is set by default unless the Ruby launcher is used, as part of the embedded configuration, described below.

When this option is enabled, the timeout module will warn that the timeouts are being ignored, and signal handlers will warn that a signal has been caught but will not run the handler, as both of these features would require starting new threads.

Embedded Configuration

When used outside of the Ruby launcher - such as from another language’s launcher via the polyglot interface, embedded using the native polyglot library, or embedded in a Java application via the GraalVM SDK - TruffleRuby will be automatically configured to work more cooperatively within another application. This includes options such as not installing an interrupt signal handler, and using the I/O streams from the Graal SDK. It also turns on the single-threaded mode, as described above.

It will also warn when you explicitly do things that may not work well when embedded, such as installing your own signal handlers.

This can be turned off even when embedded, with the embedded option (--ruby.embedded=false from another launcher, or -Dpolyglot.ruby.embedded=false from a normal Java application).

It is a separate option, but in an embedded configuration you may want to set allowNativeAccess(false) in your Context.Builder, or use the experimental --platform-native=false option, to disable use of the NFI for internal functionality.

Also, the experimental option --cexts=false can disable C extensions.

Note: Unlike for example pure JavaScript, Ruby is more than a self-contained expression language. It has a large core library that includes low-level I/O and system and native-memory routines which may interfere with other embedded contexts or the host system.