Left corner

The left corner of a production rule in a context-free grammar is the left-most symbol on the right side of the rule.[1]

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

The left corner table associates a symbol with 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
Symbol Left corner(s)
S VP, NP, V, DET
NP DET
VP V

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

You can use the left corners to do top-down filtering of a bottom-up parser.

References

  1. 9.3 Using Left-corner Tables, Patrick Blackburn and Kristina Striegnitz, Natural Language Processing Techniques in Prolog


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