trans-objects.md

Model-driven translator

In the previous syntax-directed translation lab, we built a translator that collected strings representing output phrases, one for declaration. We're going to move the abstraction level up in this lab by creating objects that represent phrases rather than strings.

Model objects

First, copy all of the code from the previous lab into a new module or project.

Now, let's create classes that represent the various kinds of output phrases. At the root, let's place an abstract OutputModelObject class so that we can create data structures of generic output objects:

/** A generic root for any object representing an output component */
public abstract class OutputModelObject {
	/** The method used to generate code for a specific object */
	@Override
	public String toString() { return super.toString();	}
}

After walking the tree with our code generator, we will have access to an object representing an output file. Our "emitter" is then just:

System.out.println(listener.file.toString());

Now create the following class hierarchy:

• OutputFile has a list of OutputModelObject objects and its toString() method just returns a string with all of those objects rendered to string;each one is followed by newline. You can also add a handy method to add declarations:
  

public void addDecl(OutputModelObject decl) { decls.add(decl); }

• PrimitiveDecl has two strings, type and idand its toString() simply returns "type id ;"
• ObjectRefDecl is the same really except that its toString() returns "type *id ;"

Now that we have the appropriate output model objects, we have to alter the code generator, Gen, so that it does not collect a list of strings. Instead, it should create and fill an OutputFile object:

public class Gen extends LangBaseListener {
	public OutputFile file;
		
	@Override
	public void enterFile(LangParser.FileContext ctx) {
		// set file field
	}
	
	@Override
	public void enterDecl(LangParser.DeclContext ctx) {
		// very much like last lab except add
		// ObjectRefDecl or PrimitiveDecl not strings
		// and add to file
	}
	
	/** Pretend we have type information */
	public boolean isClassName(String typename) {
		return Character.isUpperCase(typename.charAt(0));
	}
}

Try that out with your main program printing out the listeners file object. You should get the same output as in the previous lab:

(file (decl (typename int) x ;) (decl (typename A) b ;))
int x;
A *b;

Unfortunately, our translator is still not the best we can do. (Dramatic foreshadowing of the next lab!)