MetaBorg in Action

September 15, 2005

This morning Rene de Groot presented his thesis work in our Software Technology Colloquium. He has worked on two case studies of embedding domain-specific languages in Java — applying and refining our MetaBorg approach to `concrete syntax for objects’.

In the first case study Rene elaborated the JavaSwul extension of Java with the SWing Userinterface Language. Rather than the usual spaghetti code of Swing method invocations, the language allows you to follow the hierarchical structure of the Swing class hierarchy to create userinterface objects. For example, the following GUI

is produced by this Java program

import javax.swing.*;
import java.awt.*;
public class Test3 {
  public static void main(String[] ps) {
    JFrame frame = frame {
      title = "Welcome!"
      content = panel of border layout {
        center = label { text = "Hello World" }
        south = panel of grid layout {
          row = {
            button { text = "cancel" }
            button { text = "ok"}
The JavaSwul Examples page contains a number of other cool examples including the creation of menus with eventhandlers and gridbags with a semi-visual layout syntax.

The second case study is the extension of Java with concrete syntax for regular expressions. First it provides a syntax for regular expressions and checks regular expressions in a program against that syntax at compile-time. Second because of the explicit syntax extension, there is no need for escaping special characters as has to be done when encoding regular expressions using strings. Finally, the extension provides syntax for defining string rewrite rules and combining those into compound string rewrite operations (analogously to rewriting strategy combinators in Stratego). Here is an example, with some wiki-like rewrite rules:

public String publish(String page) {
  regex body = [/ <body[^>]*?> .* </body> /]

  regex amp = [/ & /] -> [/ &amp; /] ;
  regex lt  = [/ < /] -> [/ &lt;  /] ;
  regex gt  = [/ > /] -> [/ &gt;  /] ;

  regex escape = amp <+ lt <+ gt

  regex noattach  = [/ <a[^>]*?> \s* Attach \s* </a> /]
                 -> [/ <strike> Attach </strike> /] ;
  regex edittopic = [/ %EDITTOPIC% /]
                 -> [/ <a href="${editAction}"><b> Edit </b></a> /] ;
  input ~= one( body
                all( edittopic <+ noattach <+ escape  )
  return input ;

In addition to the syntactic extensions of Java and their definition using transformations, the implementations include an extension of Martin Bravenboer's typechecker for Java, so that Java programs using the extensions are typechecked before transformation.

A preview of the work is presented in MetaBorg in Action, a paper for the Summer School on Generative and Transformational Techniques in Software Engineering (GTTSE'05) that was held in Braga, Portugal last Summer. Rene's thesis is due soon.