Prolog sample written for learning

prolog_db.pl

% loves - predicate
% romeo - atoms
% juliet - atoms
% romeo and juliet are constants and arguments passed to predicate function
loves(romeo, juliet).

loves(juliet, romeo) :-
    loves(romeo, juliet).

male(albert).
male(bob).
male(bill).
male(carl).
male(charlie).
male(dan).
male(edward).

female(alice).
female(betsy).
female(diana).

happy(albert).
happy(alice).
happy(bob).
happy(bill).

with_albert(alice).

% albert is running if albert is happy
runs(albert) :-
    happy(albert).

% alice is dancing if alice is happy and alice is with albert
dances(alice) :-
    happy(alice),
    with_albert(alice).

% check statement and write
does_alice_dance :-
    dances(alice),
    write('When alice is happy and alice is with albert then alice is dancing').

% wrong case, because near_water does not exists
swims(bob) :-
    happy(bob),
    near_water(bob).

% different cases
swims(bill) :-
    happy(bill).

swims(bill) :-
    near_water(bill).
% parent(X, betsy).

% who is a parent of betsy and dances?
% parent(X, betsy), dances(X).

% who is a parent of carl and is a parent of parent of carl
% in other words: who is a parent of carl and who is grandparent of carl
% parent(X, carl), parent(Y, X).

% who is a child of albert and child of albert child
% in other words: who is albert child and albert grandchild
% parent(albert, X), parent(X, Y).

parent(albert, bob).
parent(albert, betsy).
parent(albert, bill).

parent(alice, bob).
parent(alice, betsy).
parent(alice, bill).

parent(bob, carl).
parent(bob, charlie).

% get results and write nice message
get_albert_grandchild :-
    parent(albert, X),
    parent(X, Y),
    write('Albert grandchild is '),
    write(Y), nl.