Theory, examples of reversible parsers?

Question

Does anyone out there know about examples and the theory behind parsers that will take (maybe) an abstract syntax tree and produce code, instead of vice-versa. Mathematicall

Answer 1

I've been doing these forever, and calling them "DeParse".

It only gets tricky if you also want to recapture whitespace and comments. You have to tuck them into the parse tree so you can regenerate them on output.

Answer 2

That sounds a lot like the back end of a non-optimizing compiler that has it's target language the same as it's source language.

One question would be whether you require the "unparsed" code to be identical to the original, or just functionally equivalent.

For example, would it be OK for the output to use a different indentation style than the original? That information wouldn't normally be stored in the AST because it's not semantically important.

One thing to look at would be automatic code refactoring tools.

Answer 3

There are several "lens languages" that allow bidirection transformation of source code.

It is also possible to implement reversible parsers using definite clause grammars in Prolog. In SWI-Prolog, the phrase/3 predicate converts parse trees into text and vice-versa. This book provides some additional examples of reversible parsing in Prolog.

Answer 4

The "Visitor Pattern" idea is good. But, I should consider "Visitor" pattern as a lineal list pattern, or, as a generic pattern, and add patterns for more specific cases like Lists, Matrices, and Trees.

Look for a "Hierarchical Visitor Pattern" or "Tree Visitor Pattern" on the web.

You have a tree data structure ("Collection") and want to do something with the data, each time you "visit", "iterate" or "read" an item from the tree.

In your case, you have a tree data structure, that represents the result of scanning/parsing some source code. Then you have read each item's data, and transform it into destination code.

Answer 5

I rather like lewap's response:

find a mathematical way to express a visitor and you have a dual to the parser

But you asked for a sample, so try this on for size: Visual Studio contains a UML editor with excellent symmetry. The way both it and the editors are implemented, all constitute views of the model, and editing either modifies the model resulting in all remaining in synch.