Prolog

Programming in Logic

=====================
Software:

SWI-Prolog

Windows: www.cs.fit.edu/~pkc/classes/ai/
Linux/unix: pl 

Loading source files:

<filename>.pl

	[<filename>].

	or

	consult(<filename>).


Exit:

	halt.

=====================
Syntax:

- Predicates/functions and constants start with a lowercase letter  

- Variables start with an uppercase letter

- Universally qunatified Horn sentences.

- "Facts" and "Rules"

- "Facts" before "Rules" in the source file

- Must end with a period (.)

- Facts: single predicate (no atecendents in Horn sentences), no variables.

- Rules: consequent :- antecedent1, antecedent2, ..., antecedent_n.

- "Head:" consequent; "Body:" antecedents

- Queries return "yes" or "no"

- If the queries have variables, queries return the variable bindings

- One variable binding is displayed, addtional ones, if present, are
  displayed one by one when the user types a semicolon (;)

- comments start with a percent sign (%), or are delimited by /*   */

=====================
Sample source file

% facts
parent(mary, tom).
parent(mary, liz).
parent(tom, bob).

% rules

% Parent(x,z) ^ Parent(z,y) => Grandparent(x,y)

grandparent(X, Y) :-
    parent(X, Z),
    parent(Z, Y).

% Parent(x,y) => Ancestor(x,y)
% Parent(x,z) ^ Ancestor(z,y) => Ancestor(x,y)

ancestor(X, Y) :-
    parent(X, Y).

ancestor(X, Y) :-
    parent(X, Z),
    ancestor(Z, Y).


=====================
Sample queries

?- parent(mary, tom).
yes
?- parent(mary, X).
X=tom;
X=liz
?- grandparent(mary, bob).
yes
?- grandparent(mary, X).
X=bob
?- ancestor(mary, X).
yes

========================

- Starting from the query, perform depth first search

- search ordering in the body (antecedents): left to right

- Generally, start with the most restrictive predicate in the body