Note: Some versions of Prolog allow the following.

• The word "and" for the comma ","
• The word "or" for the semicolon ";"
• The word "if" for the symbol ":-".

For historical reasons, the notes may use either notation.

Logic languages, such as Prolog and Datalog, are often used for expert systems applications, semi-formal specifications, and rapid prototyping, all of which are often found in areas of software engineering.

A specification describes what is to be done, but not how to do it.

Formal specification techniques get very complicated very quickly and are often of limited in their high cost to value ratio in many situations.

Rapid prototyping is very useful in many systems. A declarative system is a system where the implementation is automatically obtained from the specification.

What are some examples of declarative systems.

Examples:

• word processor
• spreadsheet

Logical implications can be derived from a precise model of facts and rules. Such an understanding that is obtained by studying expert systems and declarative specification techniques.

• Facts and rules are axioms of the system.
• Queries are the theorems to be proved.

A collection of facts (and rules) is called a database.

• Knowledge base of facts and rules
• Inference engine
• User interface

• Textual form for knowledge base (i.e., a program)
• Computational strategy (backward chaining using depth first search and resolution)
• Text or graphics user interface (implementation and platform dependent)

• tax preparation
• grammar checkers
• legal advice
• help desk
• medical diagnosis
• student advising