A New Top-Down Parsing Algorithm for Left-Recursive DCGs

Abs t rac t . In this paper we introduce a new parsing algorithm, called cancellation parsing. Deterministic cancellation parsing with lookahead k can handle the C(k) grammars, which include the LL(k) grammars and are contained in the LC(k) grammars. The C(k) property is of theoretical interest in that it shows how to formalize the intuitive notion of the LL(k) property extended with the possibility of left recursion. The top-down nature of cancellation parsing gives it advantages over many other parsing algorithms. No analysis is needed to construct a nondeterministic cancellation parser. This makes cancellation parsing particularly useful for the implementation of definite clause grammars in Prolog environments.