Transcript NLU:Frames
NLU: Frames • Frame KR is a good way to represent common sense – can define stereotypical aspects of some domain we are interested in analyzing – sentences about that domain can be parsed, and their meaning extracted in terms of what the frames expect • actions: activity of a sentence, specified by verb phrase • thematic role (of a noun phrase): how the object participates in the action of a sentence • sentence analysis: – what thematic roles are to be filled by a sentence – determining thematic roles of noun phrases • many theories define different thematic roles; common objective is to determine adequate constraints (logical limitations) between action and thematic roles What are semantic roles and what is their history? • A lot of forms of traditional grammar (Sanskrit, Japanese, …) analyze in terms of a rich array of semantically potent case ending or particles – They’re kind of like semantic roles • The idea resurfaces in modern generative grammar in work of Charles (“Chuck”) Fillmore, who calls them Case Roles (Fillmore, 1968, The Case for Case). – They’re quickly renamed to other words, but various: • Semantic roles • Thematic roles • Theta roles • A predicate and its semantic roles are often taken together as an argument structure Okay, but what are they? • An event is expressed by a predicate and various other dependents • The claim of a theory of semantic roles is that these other dependents can be usefully classified into a small set of semantically contentful classes • And that these classes are useful for explaining lots of things Common semantic roles • Agent: initiator or doer in the event • Patient: affected entity in the event; undergoes the action – Sue killed the rat. • Theme: object in the event undergoing a change of state or location, or of which location is predicated – The ice melted • Experiencer: feels or perceive the event – Bill likes pizza. Common semantic roles • Goal: – Bill ran to Copley Square. • Recipient (may or may not be distinguished from Goal): – Bill gave the book to Mary. • Benefactive (may be grouped with Recipient): – Bill cooked dinner for Mary. • Source: – Bill took a pencil from the pile. • Instrument: – Bill ate the burrito with a plastic spork. Common semantic roles Try for yourself! 1. The submarine sank a troop ship. 2. Doris hid the money in the flowerpot. 3. Emma noticed the stain. 4. We crossed the street. 5. The boys climbed the wall. 6. The chef cooked a great meal. 7. The computer pinpointed the error. 8. A mad bull damaged the fence on Jack’s farm. 9. The company wrote me a letter. 10.Jack opened the lock with a paper clip. Thematic roles • Agent: the passive or active entity that causes an action – Donald kicked the ball. • Co-agent: partner with agent – Donald kicked the ball with his friend Mickey. • Thematic object: the object undergoing change, to which the action is being applied – Donald kicked the ball. • Instrument: tool used by agent – Donald kicked the ball with his foot. • Location: where action occurs – Donald kicked the ball on the field. • and others Frame Based Formalism The kind man gave a bread to the beggar for his hungry child. To Whom? (Co-Agent) Who’s Giving? (Agent) Giving What? (Theme) GIVE For Whom (Beneficiary) Where? (@loc) When? (Time) Frame based formalism (contd.) • Partial instantiation of a frame is possible. • Different verbs can have different frames. • The domain of a role is dependent on the verb. • Frame is instantiated based on the input provided by the user. • Role can be a single concept or a composition based on qualifier-qualified structure. Filled thematic roles • Robbie made coffee for Suzie with a percolator. Thematic roles and parsing • In easy sentences, thematic roles are ascertainable by from grammar: – verb constrains thematic roles, and placement of noun phrases – propositions limit noun phrase role possibilities • eg. by : agent or conveyance or location • with : co-agent or instrument • to : destination – nouns may limit roles – main noun phrase determines roles of others – still lots of ambiguities: need a large lexicon! • Time flies like an arrow. • Fruit flies like a banana. Constraints • Constraint: an enforced problem-specific limitation – the main task of NLU is to exploit contraints of grammar, phrases, thematic roles on one another to determine an overall (hopefully unambiguous) meaning – without constraints, there are too many possible meanings and thematic roles – humans can quickly ascertain the constraints of a sentence – but you can write ambiguous ones too: • I gave the celery to the rabbit and then ate it. • We can predefine word meaning, and use to establish permissible constraints – but there can be a lot! (how many meanings of “take”) Constraints and frames • frame representations – slots: thematic roles – action frames: specify sentence action – state-change frames: resulting change of state of thematic roles that occur through action – result link: indicates change of state from an action, & points to state-change frame • meanings of sentences represented by state changes • primitive action systems: a frame system in which: – a primitive action slot is filled from a small set of actions – state change slots indicate result of primitive actions • can be used to determine what happened in sentence (explicitly and implicitly)