Modular and Efficient Top-Down Parsing for Ambiguous Left-Recursive Grammars

In functional and logic programming, parsers can be built as modular executable specifications of grammars, using parser combinators and definite clause grammars respectively. These techniques are based on top-down backtracking search. Commonly used implementations are inefficient for ambiguous languages, cannot accommodate left-recursive grammars, and require exponential space to represent parse trees for highly ambiguous input. Memoization is known to improve efficiency, and work by other researchers has had some success in accommodating left recursion. This paper combines aspects of previous approaches and presents a method by which parsers can be built as modular and efficient executable specifications of ambiguous grammars containing unconstrained left recursion.