COS 333: Chapter 16, Part 2

Summary

This lecture delves into the foundations of Prolog, a logic programming language developed in the 1970s for applications in natural language processing and automated theorem proving. It covers the basic elements of Prolog, including terms, fact, rule, and goal statements, and discusses the inferencing process, highlighting its distinction from first-order predicate calculus. The lecture also touches on Prolog's development, its two main dialects, and provides insights into its programming constructs, illustrating how facts and rules are used to prove goals, and introducing the concepts of resolution, backtracking, and the tracing model for debugging.

Takeaways

• 📜 The lecture covers Prolog, its origins, basic elements, and inferencing process.
• 🏛️ Prolog was developed in the 1970s-1980s by the University of Aix-Marseille in France and the University of Edinburgh in Scotland.
• 🔍 Prolog is used for natural language processing and automated theorem proving.
• 🔧 Prolog programs are built from terms, which can be constants, variables, or structures.
• 📚 Fact statements in Prolog represent basic ground truths, using structures to express hypotheses.
• 📏 Rule statements express theorems and consist of an antecedent and a consequent.
• ❓ Goal statements, or queries, are used for theorem proving, asking Prolog to prove the truth of propositions.
• 🔄 Prolog uses top-down resolution (backward chaining) to match goals to facts.
• 🌐 Multiple sub-goals are proved using depth-first search, and Prolog handles backtracking if a sub-goal fails.
• 🔍 Prolog's tracing model helps debug programs, showing events like call, exit, redo, and fail.

Q & A

What are the main topics covered in the second lecture on logic programming languages?

The second lecture primarily focuses on Prolog, starting with its origins, moving on to basic elements like terms, facts, rules, and goals, and finally discussing how Prolog handles the inferencing process differently from theorem proving in first-order predicate calculus.

Which two universities were initially involved in the development of Prolog?

The development of Prolog began as a collaboration between the University of Marseille in France and the University of Edinburgh in Scotland.

What were the main interests of the University of Marseille and Robert Kowalski in the development of Prolog?

The University of Marseille aimed to use Prolog for natural language processing, while Robert Kowalski from the University of Edinburgh was interested in using it for automated theorem proving.

What are the three main kinds of terms in Prolog programming language?

The three main kinds of terms in Prolog are constants (which can be atoms or integers), variables, and structures.

How are atoms named in Prolog and what are the conventions for naming them?

Atoms in Prolog can be named using a string of letters, digits, and underscores beginning with a lowercase letter, or using any sequence of printable ASCII characters including spaces, as long as they are enclosed in single quotes. However, the first approach is standardized for better readability.

What is a variable in Prolog and how is it named?

A variable in Prolog is a term that can be instantiated with a value during the resolution process. It is named as a string of letters, digits, and underscores but must always start with an uppercase letter.

What is a fact statement in Prolog and how is it represented?

A fact statement in Prolog is an atomic proposition used to express a hypothesis or a basic ground truth. It is represented using Prolog structures as headless Horn clauses, starting with a functor followed by a pair of parentheses containing parameters.

What is the syntactic form of rule statements in Prolog and how do they differ from fact statements?

Rule statements in Prolog are represented by headed Horn clauses, consisting of an antecedent (the 'if' part) and a consequent (the 'then' part), separated by a colon and dash, indicating an implication. They differ from fact statements in that they include a rule for deriving new facts from existing ones.

How does Prolog handle the inferencing process when attempting to prove the truth of a proposition?

Prolog uses top-down resolution or backward chaining for its inferencing process. It starts with the goal and attempts to find a sequence of matches using rules that lead to facts, which are then used to prove the goal.

What are the two general approaches for proving multiple sub-goals in Prolog and which one does Prolog use?

The two general approaches for proving multiple sub-goals are breadth-first search and depth-first search. Prolog uses the depth-first search approach due to its lower computational load.

What is backtracking in Prolog and when does it occur?

Backtracking in Prolog occurs when the inferencing process fails to prove one of the sub-goals after proving a previous sub-goal. It involves moving back to the last successfully proved sub-goal and finding a different solution that could potentially satisfy the failed sub-goal.

What is the purpose of the trace structure in Prolog and what are the four events it provides?

The trace structure in Prolog is used for debugging Prolog programs by providing insight into the steps performed during the inferencing process. The four events it provides are call (beginning of an attempt to satisfy a goal), exit (goal has been satisfied), redo (backtracking occurs), and fail (goal fails).