Memoization of Top Down Parsing

In a paper published in this journal, Norvig (1991) pointed out that memoization of a topdown recognizer program produces a program that behaves similiarly to a chart parser. This is not surprising to anyone familiar with logic-programming approaches to NLP. For example, the Earley deduction proof procedure is essentially a memoizing version of the top-down SLD proof procedure employed by Prolog. Pereira and Warren (1983) showed that the steps of the Earley Deduction proof procedure proving the wellformedness of a string S from the standard ‘top-down’ DCG axiomatization of a CFG G correspond directly to those of Earley’s algorithm recognizing S using G.