Left corner

{{Short description|Part of a production rule in a context-free grammar}} {{one source |date=April 2024}} In formal language theory, the '''left corner''' of a production rule in a context-free grammar is the left-most symbol on the right side of the rule.<ref>[http://cs.union.edu/~striegnk/courses/nlp-with-prolog/html/node55.html 9.3 Using Left-corner Tables], Patrick Blackburn and Kristina Striegnitz, Natural Language Processing Techniques in Prolog</ref>

For example, in the rule ''A→Xα'', ''X'' is the left corner.

The '''left corner table''' associates to a symbol all possible left corners for that symbol, and the left corners of those symbols, etc.

Given the grammar :S → VP :S → NP VP :VP → V NP :NP → DET N

the left corner table is as follows.

{| !Symbol !Left corner(s) |- |S |VP, NP, V, DET |- |NP |DET |- |VP |V |}

Left corners are used to add bottom-up filtering to a top-down parser, or top-down filtering to a bottom-up parser.

==References== {{reflist}}

Category:Parsing