Another DSL Framework - tutorial 2

The BNF like language we used to define the calculator is a language created in the Language Processor itself. Actually you can define your language in the object model. This is what you would do to define E -> E + E

            Terminal terminal = digit();

            Terminal plus = new RegExpTerminal("\\+");

     

            Grammer additonGrammer = new Grammer();

           

            NonTerminal expr = new NonTerminal("E");

            NonTerminal rest = new NonTerminal("T");

           

            Production exprProd = new Production(new ProductionElement[] { terminal, rest }, new Semantic() {

                  public Object evaluate(List<Object> childEvals) { 

                        if (childEvals.get(1).equals(Collections.EMPTY_LIST)) 

                              return childEvals.get(0); 

                        return new Integer(((Integer) childEvals.get(0)).intValue() + ((Integer) childEvals.get(1)).intValue()); 

                  } 

            }); 

     

            Production restProd = new Production(new ProductionElement[] { plus, expr }, new Semantic() { 

                  public Object evaluate(List<Object> childEvals) { 

                        return childEvals.get(1); 

                  } 

            });  

            expr.addProduction(exprProd); 

            rest.addProduction(restProd); 

            rest.addProduction(new EmptyProduction(){ 

                  @Override

                  public Object evaluate(List childResults) {

                        return new Integer(0);

                  } 

            }); 

           

            additonGrammer.addProductionElement(expr); 

            additonGrammer.addProductionElement(rest); 

            additonGrammer.addProductionElement(terminal);

The grammar is actually defined as

E -> TE'
E' -> +TE' | e
 

as the Language Processor is a recursive descent parser and you need to left factorize your grammar to consume tokens as you recurse.

This is a boring subject and the BNF like grammar sorts it out for you. If anyone is really interested, buy me a beer, I will tell you all about it.