Compiling an AST back to source code

I'm currently in the process of building a PHP Parser written in PHP, as no existing parser came up in my previous question . The parser itself works fairly well.

Now obviously a parser by itself does little good (apart from static analysis). I would like to apply transformations to the AST and then compile it back to source code. Applying the transformations isn't much of a problem, a normal Visitor pattern should do.

What my problem currently is, is how to compile the AST back to source. There are basically two possibilities I see:

1. Compile the code using some predefined scheme
   


2. Keep the formatting of the original code and apply 1. only on Nodes that were changed.
   

For now I would like to concentrate on 1. as 2. seems pretty hard to accomplish (but if you got tips concerning that, I would like to hear them).

But I'm not really sure which design pattern can be used to compile the code. The easiest way I see to implement this, is to add a ->compile method to all Nodes. The drawback I see here, is that it would be pretty hard to change the formatting of the generated output. One would need to change the Nodes itself in order to do that. Thus I'm looking for a different solution.

I have heard that the Visitor pattern can be used for this, too, but I can't really imagine how this is supposed to work. As I understand the visitor pattern you have some NodeTraverser that iterates recursively over all Nodes and calls a ->visit method of a Visitor . This sounds pretty promising for node manipulation, where the Visitor->visit method could simply change the Node it was passed, but I don't know how it can be used for compilation. An obvious idea would be to iterate the node tree from leaves to root and replace the visited nodes with source code. But this somehow doesn't seem a very clean solution?

Source: Tips4all