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Value converters are registered to convert parsed text into a certain data type instance and back. The primary hook is called org.eclipse.xtext.conversion.IValueConverterService and the concrete implementation can be registered via the runtime Guice module (TODO reference to framework description).

Annotation based value converters

The most simple way to register additional value converters is to make use of org.eclipse.xtext.conversion.impl.AbstractAnnotationBasedValueConverterService, which allows to declaratively register IValueConverter via annotated methods.

The implementation for the default token grammar looks like so (org/eclipse/xtext/common/services/

public class DefaultTerminalConverters extends AbstractAnnotationBasedValueConverterService {
 private Grammar grammar;
 public void setGrammar(IGrammarAccess grammarAccess) {
   this.grammar = grammarAccess.getGrammar();
 protected Grammar getGrammar() {
   return grammar;
 @ValueConverter(rule = "ID")
 public IValueConverter<String> ID() {
   return new AbstractNullSafeConverter<String>() {
     protected String internalToValue(String string, AbstractNode node) {
       return string.startsWith("^") ? string.substring(1) : string;
     protected String internalToString(String value) {
       if (GrammarUtil.getAllKeywords(getGrammar()).contains(value)) {
         return "^"+value;
       return value;
      ...  some other value converter

If you use the common terminals grammar (org.eclipse.xtext.common.Terminals) you should subclass DefaultTerminalConverters and overwrite or add addition value converter by adding the respective methods.

Imagine, you would want to add a rule creating BigDecimals:

 @ValueConverter(rule = "BIG_DECIMAL")
 public IValueConverter<BigDecimal> BIG_DECIMAL() {
   return new AbstractToStringConverter<BigDecimal>() {
     protected BigDecimal internalToValue(String string, AbstractNode node) {
       return BigDecimal.valueOf(string);

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