Prolog

1
Prolog

• Overview
• Syntax
• Mechanism
• Summary

2
Overview

• PROLOG Programming with Logic
• Uses predicate (first-order) calculus
• History
• Roots J.A. Robinson (1965) created algorithms
• for resolution refutation.

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Resolution
P V Q P V A
Q V A
star(A75) V galaxy(andromeda)
star(X) V
bright(X)
galaxy(andromeda) V bright(A75) T X/A75
4
History

• First Prolog Program written in France early
  1970s
• as part of a project in natural language
  understanding.
• Theoretical background defined by Kowalski,
  Hayes,
• and others.
• Major development of the language done from
  1975-1979
• at the dept. of artificial intelligence of the
  University of
• Edinburgh.

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Applications

• Automatic code generation
• Program verification
• Design of high-level specification languages
• Example Mathematica

6
Examples
Everyone likes Susie likes(X,susie) George
likes Kate and George likes Susie likes(george,ka
te) , likes(george,susie) George likes Kate or
George likes Susie likes(george,kate)
likes(george,susie) George likes Susie if
George does not like Kate likes(george,susie) -
not(likes(george,kate))
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Prolog

• Overview
• Syntax
• Mechanism
• Summary

8
Syntax

• There are many dialects of Prolog.
• We will use C-Prolog (Clocksin and Mellish 1984)
• Prolog uses Predicate Calculus as its language.

9
Prolog

• Overview
• Syntax
• Mechanism
• Summary

10
Prologs Mechanism

• A Prolog program is a set of specifications in
  predicate
• calculus.
• The specification is known as the database of
  the system.
• Prolog is an interactive language (the user
  enters queries
• in response to a prompt).

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Example 1
likes(george,kate). likes(george,susie). likes(geo
rge,wine). likes(susie,wine). likes(kate,gin). lik
es(kate,susie). ?- likes(george,kate) yes ?-
likes(george,beer) no
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Example 2
likes(george,kate). likes(george,susie). likes(geo
rge,wine). ?- likes(george,X) X kate X
susie X wine no
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Points to Consider

• Successive user prompts cause the
  interpreter to
• return all terms that can be substituted for X.
• They are returned in the order found.
• ORDER IS IMPORTANT
• PROLOG adopts the closed-world assumption
• All knowledge of the world is present in the
  database.
• If a term is not in the database, assume it is
  false.

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Defining Rules
To define a rule use logical implication -

• Example
• friends(X,Y) - likes(X,Z), likes(Y,Z).
• Only one predicate is allowed on the left of the
  implication
• The predicate on the left cannot be negated
• This is Horn Clause Logic

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Example Rule
likes(george,kate). likes(george,susie). likes(geo
rge,wine). likes(susie,wine). likes(kate,gin). lik
es(kate,susie). friends(X,Y) - likes(X,Z),
likes(Y,Z). ?- friends(george,susie) yes
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Quantifiers

• When a variable appears in the specification of a
  database,
• the variable is universally quantified. Example
• likes(susie,Y) Susie likes everyone
• For the existentially quantifier one may do two
  things
• Enter the value directly into the database
• likes(george,Z) becomes likes(george,wine)
  
• b. Query the interpreter
• ?- likes(george,Z) returns a value for Z
  if one exists.

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Recursion in Prolog
Recursion is the primary control structure in
Prolog. Lets look at an example with
lists Elements in lists are enclosed by
brackets 1,2,3,4 george,kate,allen,amy,d
on,pat
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lists
The first element of a list can be separated from
the tail using operator Example Match the
list tom,dick,harry,fred to XY
then X tom and Y dick,harry,fred X,YZ
then X tom, Y dick, and Z
harry,fred V,W,X,Y,ZU will not
match tom,Xharry,fred gives X dick
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List membership
We want to write a function member that works as
follows ?- member(a,a,b,c,d,e) yes ?-
member(a,1,2,3,4) no ?- member(X,a,b,c) X
a X b X c no
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Function membership
Define two predicates member(X,XT). member(X,
YT) - member(X,T). Trace the following
call ?- member(c,a,b,c) yes
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Function membership
A more elegant definition uses anonymous
variables member(X,X,_). member(X,_T) -
member(X,T). The symbol _ indicates that the
contents of that variable is unimportant.
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Function write list
writelist(). writelist(HT) - write(H), nl,
writelist(T). A reverse write of a list is as
follows reverse_writelist(). reverse_writelist
(HT) - reverse_writelist(T), write(H), nl.
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Other functions
?- assertz(likes(david,sarah)) Adds this
predicate to the database. To remove a
predicate p use ?- retract(p)
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Other functions
To download a database of sentences ?-
consult(myfile) yes Or ?- myfile To read and
write from console ?- read(X) ?- write(X)
25
Prolog

• Overview
• Syntax
• Mechanism
• Summary

26
Summary

• There are many dialects of Prolog we will use
• C-Prolog (Clocksin and Mellish 1984). Prolog
  uses
• Predicate Calculus as its language.
• A Prolog program is a set of specifications in
  predicate
• calculus. The specification is known as the
  database
• of the system.
• Prolog is an interactive language (the user
  enters queries
• in response to a prompt).
• Recursion is the primary control structure in
  Prolog.
• PROLOG adopts the closed-world assumption
• All knowledge of the world is present in the
  database.
• If a term is not in the database assume it is
  false.