🔠Intro to Semantics and Pragmatics Unit 4 – Compositionality and Semantic Roles

Key Concepts

• Compositionality involves understanding how the meaning of a complex expression is determined by the meanings of its parts and how they are combined
• Semantic roles represent the semantic relationships between a predicate (verb) and its arguments (noun phrases) in a sentence
• Thematic roles (agent, patient, theme, etc.) capture the general semantic roles that arguments can play in relation to a predicate
• Propositional meaning refers to the literal, context-independent meaning of a sentence determined by the meanings of its parts and their syntactic arrangement
• Argument structure specifies the number and types of arguments a predicate requires and the semantic roles they play
• Valency refers to the number of arguments a predicate takes (intransitive, transitive, ditransitive)
• Selectional restrictions constrain the semantic properties of the arguments a predicate can take (animate subject, abstract object, etc.)

Principles of Compositionality

• The Principle of Compositionality states that the meaning of a complex expression is a function of the meanings of its parts and the way they are syntactically combined
  • Allows for productivity and systematicity in language by enabling the creation and understanding of novel expressions
• Frege's Principle asserts that the meaning of a complex expression remains the same if a constituent expression is replaced by another with the same meaning (substitutivity)
• The Principle of Context Independence holds that the meaning of an expression is determined by its parts and their arrangement, regardless of the context in which it occurs
• Compositionality operates at various levels of linguistic structure, including morphology (word formation), syntax (sentence structure), and semantics (meaning)
• The interpretation of a complex expression proceeds incrementally, with the meanings of smaller parts combining to form the meaning of larger constituents
  • Enables efficient processing and avoids the need to store the meanings of all possible complex expressions
• Compositionality is a key assumption in formal semantics and is often modeled using lambda calculus and type theory

Types of Semantic Roles

• Agent: The initiator or doer of an action (John in "John kicked the ball")
• Patient: The entity affected by an action (the ball in "John kicked the ball")
• Theme: The entity undergoing a change of location or possession (the book in "Mary gave the book to John")
• Experiencer: The entity experiencing a psychological state or sensation (Mary in "Mary loves John")
• Stimulus: The entity or event that triggers a psychological state (John in "Mary loves John")
• Instrument: The means by which an action is performed (the key in "John opened the door with the key")
• Location: The place where an event occurs or an entity is situated (the park in "The children played in the park")
  • Source: The starting point of a motion (home in "John went from home to school")
  • Goal: The endpoint of a motion (school in "John went from home to school")

Semantic Role Labeling

• Semantic Role Labeling (SRL) is the task of automatically identifying the semantic roles of arguments in a sentence with respect to a given predicate
• Involves two main steps: predicate identification and argument classification
  • Predicate identification determines which words in a sentence are predicates (typically verbs)
  • Argument classification assigns semantic roles to the arguments of each identified predicate
• SRL systems often rely on syntactic parsing to identify the grammatical relations between predicates and arguments
• Machine learning approaches, such as sequence labeling with conditional random fields (CRFs) or neural networks, are commonly used for SRL
• Annotated corpora, such as PropBank and FrameNet, provide training data for SRL systems by annotating predicates and arguments with semantic roles
• Challenges in SRL include dealing with implicit arguments, disambiguating role labels, and handling long-range dependencies and discontinuous arguments
• SRL has applications in information extraction, question answering, machine translation, and other natural language understanding tasks

Compositional Analysis Techniques

• Syntactic parsing is often the first step in compositional analysis, providing the hierarchical structure of a sentence
  • Constituency parsing identifies the phrase structure of a sentence using context-free grammars (CFGs)
  • Dependency parsing captures the binary asymmetric relations between words in a sentence
• Type-logical semantics uses formal logic, such as lambda calculus and type theory, to model the composition of meaning
  • Lambda calculus allows for the representation of functions and the application of functions to arguments
  • Type theory specifies the semantic types of expressions and the rules for combining them
• Montague Grammar is a formal system that combines syntactic analysis with model-theoretic semantics to derive the meaning of a sentence compositionally
• Discourse Representation Theory (DRT) extends compositional semantics to handle discourse-level phenomena, such as anaphora and presupposition
• Vector-based compositional models represent the meaning of words and phrases as vectors in a high-dimensional space and use mathematical operations (addition, multiplication) to combine them
• Neural network approaches, such as recursive neural networks (RNNs) and transformers, learn compositional representations from data by encoding the structure and meaning of sentences

Challenges and Limitations

• The Principle of Compositionality is an idealization and does not always hold in natural language
  • Idioms, metaphors, and other non-literal expressions have meanings that are not strictly compositional
  • Contextual factors, such as pragmatics and world knowledge, can influence the interpretation of expressions
• Ambiguity and underspecification pose challenges for compositional analysis
  • Lexical ambiguity arises when a word has multiple meanings (bank as a financial institution or a river edge)
  • Structural ambiguity occurs when a sentence has multiple possible syntactic analyses ("I saw the man with the telescope")
• Some linguistic phenomena, such as ellipsis and anaphora, involve dependencies that cross sentence boundaries and require discourse-level analysis
• Compositionality does not account for the creative and dynamic aspects of language use, such as coercion, metonymy, and meaning shifts
• Compositional models often struggle with capturing the fine-grained nuances of meaning, such as connotations, emotions, and social implications
• Scaling compositional analysis to large, open-domain corpora remains a computational challenge, requiring efficient algorithms and resources

Applications in Natural Language Processing

• Semantic parsing: Mapping natural language utterances to formal meaning representations, such as logical forms or SQL queries
• Natural language inference: Determining the entailment or contradiction relations between sentences based on their compositional semantics
• Sentiment analysis: Identifying the overall sentiment (positive, negative, neutral) of a text by composing the sentiments of its constituent expressions
• Machine translation: Preserving the compositional structure and meaning of sentences across languages
• Dialogue systems: Interpreting user utterances and generating appropriate responses based on their compositional meaning
• Information retrieval: Matching queries with relevant documents by comparing their compositional semantic representations
• Text summarization: Generating concise summaries of texts by identifying and composing the most important information
• Creative language generation: Producing novel and coherent texts, such as stories or poems, by composing meaningful elements according to structural and semantic constraints

Further Reading and Resources

• "Semantics in Generative Grammar" by Irene Heim and Angelika Kratzer: A comprehensive introduction to formal semantics and its integration with syntactic theory
• "Semantic Role Labeling" by Martha Palmer, Daniel Gildea, and Nianwen Xue: A book-length treatment of the theory and practice of semantic role labeling
• "Compositional Semantics: An Introduction to the Syntax/Semantics Interface" by Pauline Jacobson: A textbook on the principles and techniques of compositional semantic analysis
• "Representation and Inference for Natural Language: A First Course in Computational Semantics" by Patrick Blackburn and Johan Bos: An introduction to computational semantics using logic-based methods
• "The Oxford Handbook of Compositionality" edited by Markus Werning, Wolfram Hinzen, and Edouard Machery: A collection of articles on various aspects of compositionality from philosophical, linguistic, and cognitive perspectives
• PropBank (https://propbank.github.io/): A corpus annotated with semantic roles, providing training data and resources for semantic role labeling
• FrameNet (https://framenet.icsi.berkeley.edu/): A lexical database of semantic frames and their roles, useful for frame-semantic analysis and role labeling
• Universal Dependencies (https://universaldependencies.org/): A framework for cross-linguistically consistent grammatical annotation, including semantic roles and dependencies