Beautiful Code

Chapter 9. Top Down Operator Precedence

Douglas Crockford

In 1973, vaughan pratt presented “top down operator precedence”^[17] at the first annual Principles of Programming Languages Symposium in Boston. In the paper, Pratt described a parsing technique that combines the best properties of Recursive Descent and the Operator Precedence syntax technique of Robert W Floyd.^[18]. He claimed that the technique is simple to understand, trivial to implement, easy to use, extremely efficient, and very flexible. I will add that it is also beautiful.

It might seem odd that such an obviously utopian approach to compiler construction is completely neglected today. Why is this the case? Pratt suggested in the paper that preoccupation with BNF grammars and their various offspring, along with their related automata and theorems, have precluded development in directions that are not visibly in the domain of automata theory.

Another explanation is that his technique is most effective when used in a dynamic, functional programming language. Its use in a static, procedural language would be considerably more difficult. In his paper, Pratt used LISP and almost effortlessly built parse trees from streams of tokens.

But parsing techniques are not greatly valued in the LISP community, which celebrates the Spartan denial of syntax. There have been many attempts since LISP’s creation to give the language a rich, ALGOL-like syntax, including:

Pratt’s CGOL

http://zane.brouhaha.com/~healyzh/doc/cgol.doc.txt

LISP ...