34.6 Earley 1970

Of Jay Earley’s papers on his general parsing algorithm, the most readily available is “An efficient context-free parsing algorithm”, Communications of the Association for Computing Machinery, 13:2:94-102, 1970.

Ordinarily, I’d not bother pointing out 35-year old nits in a brilliant and historically important article. But more than a few people treat this article as not just the first word in Earley parsing, but the last as well. Many implementations of Earley’s algorithm come, directly and unaltered, from his paper. These implementers and their users need to be aware of two issues.

First, the recognition engine itself, as described, has a serious bug. There’s an easy fix, but one that greatly slows down an algorithm whose main problem, in its original form, was speed. This issue is well laid out by Aycock and Horspool in their article. See Aycock and Horspool 2002.

Second, according to Tomita there is a mistake in the parse tree representation. See page 153 of Grune and Jacobs 1990, page 210 of Grune and Jacobs 2008, and the bibliography entry for Earley 1970 in Grune and Jacobs 2008. In the printed edition of the 2008 bibliography, the entry is on page 578, and on the web (ftp://ftp.cs.vu.nl/pub/dick/PTAPG_2nd_Edition/CompleteList.pdf), it’s on pp. 583-584. My methods for producing parse results from Earley sets do not come from Earley 1970, so I am taking Tomita’s word on this one.