An overview of Prolog

1
An overview of Prolog

• This lecture will cover
• the execution of a basic non-deterministic
  program showing
• the role of the stack
• exploration of alternative solutions
• binding of variables
• a discussion of the consequences of Prologs
  execution strategy.

2
Components need to run a Prolog program - 1

• a Prolog program
• consisting of clauses
• clauses are either
• facts or
• rules
• a query
• which is also a clause

3
Components need to run a Prolog program - 2

• a Prolog system (compiler or interpreter)
• includes

• an internal stack
• a matching algorithm
• a mechanism for placing and removing items on and
  from the stack (scheduling algorithm).

4
The workings of a simple program

• Consider the following simple program
• likes(max, julia).
• and the query
• ?- likes(max, julia).

• The Prolog system
• examines the query
• selects all matching clauses in the program and
  places them on the stack
• pops one clause at a time from the stack and
  decides if it satisfies the query.

5
Using variables in queries

• Consider the following simple program
• likes(max, julia).
• and the query
• ?- likes(max, Who).

• The Prolog system
• examines the query
• selects all matching clauses in the program and
  places them on the stack
• pops one clause at a time from the stack and
  decides if it satisfies the query and reports the
  variable bindings.

6
What happens when there is more than one
solution?

• Consider the following simple program
• likes(max, julia).
• likes(max, amabel).
• and the query
• ?- likes(max, Who).

• The Prolog system
• examines, selects, pops and decides as before,
• but
• what does it do to recognise that there is more
  than one solution?

7
Defining more complicated situations - rules

• Consider the following simple program
• likes(max, julia).
• likes(max, amabel).
• jealous(Jealous, Victim) -
• likes(Person, Jealous),
• likes(Person, Victim).
• and the query ?- jealous(julia, Who).

• The Prolog system
• examines and selects as before, but
• it needs a way of deciding about the lines in the
  body of rules

8
Some consequences of Prologs execution method -
1

• Determinism and non-determinism
• Prolog can easily compute solutions to problems
  where there is more than one solution.
• Search process
• As Prolog runs on a digital computer, its search
  must be implemented as a program. This must work
  predictably under any situation and must give all
  possible results.
• In other words it is an algorithm.

9
Some consequences of Prologs execution method -
2

• All variables have local scope
• Imagine how difficult it would be to use global
  variables with non-determinism.

10
Prolog needs a brilliant matching algorithm

• Fortunately, it has one - unification
• Unification is the name for the process that
• matches all Prolog data types
• instantiates empty variables.
• unifies one empty variable with another to
  create chains of instantiation.
• An understanding of unification is vital to be
  able to program in Prolog.

11
Summary

• Prolog has three parts

• unification 60
• search - 30
• the rest - 10

12
Self-help

• Clearly, you need to concentrate on learning
  unification.
• Use the WWW page
• http//www.cs.bham.ac.uk/mgl/prolog/
• to view the suggested individual study associated
  with this lecture.