HPSGs How its done, Examples, Examples, Examples

CSE6339 3.0 Introduction to Computational LinguisticsTuesdays, Thursdays 14:30-16:00 – South Ross 101

Fall Semester, 2011

Instructor: Nick Cercone - 3050 CSEB - [email protected] 1

HPSGs

How its done, Examples, Examples, Examples


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HPSG Feature Structure Descriptions – recall for pizza’s


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Head Driven Phrase Structure Grammars (HPSGs)


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A type hierarchy....• ... states what kinds of objects we claim exist (the

types)

• ... organizes the objects hierarchically into classes

with shared properties (the type hierarchy)

• ... states what general properties each kind of object

has (the feature and feature value declarations).


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• Pizza Descriptions and Pizza Models

• How many pizza models (by definition, fully

resolved) satisfy this description?


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Answer: 2

{<CRUST , thick> , <TOPPINGS , { <OLIVES ,+ > , <ONIONS, +> , <MUSHROOMS, −>}>}

{<CRUST , thick> , <TOPPINGS , { <OLIVES ,+ > , <ONIONS, +> , <MUSHROOMS, +>}>}


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Pizza Descriptions and Pizza Models

How many pizzas-in-the-world do the pizzamodels correspond to?Answer: A large, constantly-changing number.


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‘type’/‘token’ distinction applies to sentences as well


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Combining Constraints


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A new theory of pizzas


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Identity Constraints (tags)


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Note


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Why combine constraints?

• The pizza example illustrates how unification can be used to combine information from different sources.

• In our grammar, information will come from lexical entries, grammar rules, and general principles.


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Linguistic Application of Feature Structures: Making the Mnemonic Meaningful

What do these CFG categories have in common?NP & VP: are both phrasesN & V: are both wordsNP & N: are both ‘nouny’VP & V: are both ‘verby’


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The Beginnings of our type hierarchy


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A Feature for Part of Speech


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Type Hierarchy for Parts of Speech I


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Type Hierarchy for Parts of Speech II


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A Feature for Valence


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Underspecification


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Another Valence Feature


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SPR and Verbs


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S and NP

• We created a monster

• our creation of a monster


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Type Hierarchy So Far


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Reformulating Grammar Rules 1Which rules do these correspond to?


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Reformulating Grammar Rules 2


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Reformulating the Grammar Rules 3


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Advantages of the New Formulation

• Subject-verb agreement is stipulated only once (where?)

• Common properties of verbs with different valences are expressed by common features

• Parallelisms across phrase types are captured


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Disadvantages of the New Formulation

• We still have three head complement rules

• We still have two head specifier rules

• We only deal with three verb valences (which ones? what are some others?)

• The non-branching rule does not really account for any empirical work

• Others?


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Heads

Intuitive idea: A phrase typically contains a word thatdetermines its most essential properties, including

– where it occurs in larger phrases, and– what is its internal structure

This is called the headThe term “head” is used both for the head word in aphrase and for all the intermediate phrases containingthat word

• NB: Not all phrases have heads


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Formalizing the Notion of Head

• Expressions have a feature HEAD

• HEAD’s values are of type pos

• For HEAD values of type agr-cat, HEAD’s value also includes the feature AGR

• Well-formed trees are subject to the Head Feature Principle


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The Head Feature Principle

• Intuitive idea: Key properties of phrases are shared with their heads

• The HFP: In any headed phrase, the HEAD value of the mother and the head daughter must be identical.

• Sometimes described in terms of properties “percolating up” or “filtering down”, but this is just metaphorical talk


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A Tree is Well-Formed if …

• It and each subtree are licensed by a grammar rule or lexical entry

• All general principles (like the HFP) are satisfied.

• NB: Trees are part of our model of the language, so all their features have values (even though we will often be lazy and leave out the values irrelevant to our current point).


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Question:

Do phrases that are not headed have HEAD features?


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Reminder: Where We Are

• Attempting to model English with CFG led to problems with the granularity of categories, e.g.

– Need to distinguish various subtypes of verbs– Need to identify properties common to all verbs

• So we broke categories down into feature structures and began constructing a hierarchy of types of feature structures.

• This allows us to schematize rules and state crosscategorial generalizations, while still making fine distinctions.


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But it’s still not quite right…

• There’s still too much redundancy in the rules.

• The rules and features encode the same information in different ways.


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Solution: More Elaborate Valence Feature Values• The rules just say that heads combine with

whatever their lexical entries say they can (or must) combine with.

• The information about what a word can or must combine with is encoded in list-valued valence features.

– The elements of the lists are themselves feature structures

– The elements are “cancelled” off the lists once heads combine with their complements and specifiers.


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Complements

• This allows for arbitrary numbers of complements, but only applies when there is at least one.

– Heads in English probably never have more than 3 or 4 complements

– This doesn’t apply where Head-Complement Rule 1 would.• This covers lots of cases not covered by the old Head-

Complement Rules 1-3.


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Question: What would the grammar change if English had postpositions instead of prepositions?


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Specifiers - Head Specifier Rule (version 1)

• Combines the rules expanding S and NP.• In principle also generalizes to other categories.• Question: Why is SPR list-valued?


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QUESTION: Why are these rightbranching?That is, what formal property of our grammar forces theCOMPS to be lower inthe tree than the SPR?


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Another Question…

What determines the VAL value of phrasal nodes?

ANSWER: The Valence Principle

Unless the rule says otherwise, the mother’s values for the VAL features (SPR and COMPS) are identical to those of the head daughter.


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More on the Valence Principle

• Intuitively, the VAL features list the contextual requirements that haven’t yet been found.

• This way of thinking about it (like talk of “cancellation”) is bottom-up and procedural.

• But formally, the Valence Principle (like the rest of our grammar) is just a well-formedness constraint on trees, without inherent directionality


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So far, we have:• Replaced atomic-valued VAL features with list-

valued ones.• Generalized Head-Complement and Head-

Specifier rules, to say that heads combine with whatever their lexical entries say they should combine with.

• Introduced the Valence Principle to “cancel” things off the COMPS and SPR lists.


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The Parallelism between S and NP• Motivation:

– pairs like Chris lectured about syntax and Chris’slecture about syntax.– both S and NP exhibit agreementThe bird sings/*sing vs. The birds sing/*singsthis/*these bird vs. these/*this birds

• So we treat NP as the saturated category of type noun and S as the saturated category of type verb.


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Question: Is there any other reason to treat V as the head of S?

• In standard English, sentences must have verbs.(How about non-standard English or otherlanguages?)

• Verbs taking S complements can influence the form of the verb in the complement:

I insist/*recall (that) you be here on time.• Making V the head of S helps us state such

restrictions formally


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Concluding Remarks

NAÏVE

Naive you are if you believe

life favours those who aren't naive.

HPSGs How its done, Examples, Examples, Examples

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