Grammars and Constituency Structure › Summary
Michael BrenndoerferMay 4, 20252 min read
Language AI HandbookAll chapters Grammars and Constituency Structure › SummarySection view · Sections in this chapter10 / 11 ↓
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- Constituency is the hierarchical grouping of words into phrases. Constituents can be identified with substitution, movement, coordination, and answer tests, though those tests are diagnostics rather than proofs.
- A context-free grammar is a tuple with rules . It is context-free because each rule rewrites a single nonterminal regardless of surrounding symbols.
- A parse tree records a derivation hierarchically. Its terminal leaves are tokens, its nonterminal nodes record licensed phrase expansions, and its yield is the terminal sequence.
- Ambiguity arises when one token sequence has multiple parse trees. Lexical category ambiguity comes from words with multiple categories. Structural ambiguity comes from different phrase groupings, such as prepositional phrase attachment.
- CKY parsing uses dynamic programming over spans. In Chomsky Normal Form, a span is built by splitting it into two subspans and applying a binary rule. For a fixed grammar, its compact recognition chart runs in time; enumerating every parse adds output-sensitive work.
- A PCFG assigns probabilities to rules and therefore to trees. The probability of a tree is the product of its rule probabilities. The inside algorithm sums over all parses of a span.
- CFGs capture recursion and nested constituency, including some unbounded dependencies. A particular grammar may need refined categories or feature constraints for agreement and lexical selection. Some dependency patterns motivate richer formalisms, while neural constituency parsers can supply learned structural scores.
- Symbolic grammars specify licensed trees. A parser can recognize or enumerate these structures without ranking them. A model or contextual information can then support the choice of a preferred or intended reading.
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