Illustrated here is a program written in the logical language of Prolog.
This particular program defines facts as the simple relationships between an example family, followed by the rules that represent the organization of a family in general.
female(diana). female(margaret). female(julie). female(mary). male(jacob). male(mark). male(bob). male(jeff). male(tim). father(jacob, margaret). father(jacob, bob). father(mark, mary). father(mark, jeff). father(bob, tim). mother(diana, margaret). mother(diana, bob). mother(margaret, mary). mother(margaret, jeff). mother(julie, tim). offspring(Y, X) :- father(X, Y). offspring(Y, X) :- mother(X, Y). grandparent(X, Z) :- offspring(Z, Y), offspring(Y, X). mother-in-law(X, Y) :- mother(Y, W), father(Z, W), mother(X, Z). mother-in-law(X, Y) :- father(Y, W), mother(Z, W), mother(X, Z). father-in-law(X, Y) :- mother(Y, W), father(Z, W), father(X, Z). father-in-law(X, Y) :- father(Y, W), mother(Z, W), father(X, Z). aunt(X, Y) :- offspring(Y, Z), offspring(Z, A), offspring(X, A), female(X). uncle(X, Y) :- offspring(Y, Z), offspring(Z, A), offspring(X, A), male(X). cousin(X, Y) :- offspring(Y, Z), uncle(Z, X). cousin(X, Y) :- offspring(Y, Z), aunt(Z, X).
In logical programming, the programs are not written to solve a particular problem. They are instead given information such as facts and rules and the problem is stated at runtime. Take a look at the following input and output.
1 ?- trace, cousin(jeff, Who). Call: (7) cousin(jeff, _G0) ? creep Call: (8) offspring(_G0, _G58) ? creep Call: (9) father(_G57, _G0) ? creep Exit: (9) father(jacob, margaret) ? creep Exit: (8) offspring(margaret, jacob) ? creep Call: (8) uncle(jacob, jeff) ? creep Call: (9) offspring(jeff, _G58) ? creep Call: (10) father(_G57, jeff) ? creep Exit: (10) father(mark, jeff) ? creep Exit: (9) offspring(jeff, mark) ? creep Call: (9) offspring(mark, _G58) ? creep Call: (10) father(_G57, mark) ? creep Fail: (10) father(_G57, mark) ? creep Redo: (9) offspring(mark, _G58) ? creep Call: (10) mother(_G57, mark) ? creep Fail: (10) mother(_G57, mark) ? creep Fail: (9) offspring(mark, _G58) ? creep Redo: (10) father(_G57, jeff) ? creep Fail: (10) father(_G57, jeff) ? creep Redo: (9) offspring(jeff, _G58) ? creep Call: (10) mother(_G57, jeff) ? creep Exit: (10) mother(margaret, jeff) ? creep Exit: (9) offspring(jeff, margaret) ? creep Call: (9) offspring(margaret, _G58) ? creep Call: (10) father(_G57, margaret) ? creep Exit: (10) father(jacob, margaret) ? creep Exit: (9) offspring(margaret, jacob) ? creep Call: (9) offspring(jacob, jacob) ? creep Call: (10) father(jacob, jacob) ? creep Fail: (10) father(jacob, jacob) ? creep Redo: (9) offspring(jacob, jacob) ? creep Call: (10) mother(jacob, jacob) ? creep Fail: (10) mother(jacob, jacob) ? creep Fail: (9) offspring(jacob, jacob) ? creep Redo: (10) father(_G57, margaret) ? creep Fail: (10) father(_G57, margaret) ? creep Redo: (9) offspring(margaret, _G58) ? creep Call: (10) mother(_G57, margaret) ? creep Exit: (10) mother(diana, margaret) ? creep Exit: (9) offspring(margaret, diana) ? creep Call: (9) offspring(jacob, diana) ? creep Call: (10) father(diana, jacob) ? creep Fail: (10) father(diana, jacob) ? creep Redo: (9) offspring(jacob, diana) ? creep Call: (10) mother(diana, jacob) ? creep Call: (10) mother(diana, jacob) ? creep Fail: (10) mother(diana, jacob) ? creep Fail: (9) offspring(jacob, diana) ? creep Redo: (10) mother(_G57, margaret) ? creep Fail: (10) mother(_G57, margaret) ? creep Fail: (9) offspring(margaret, _G58) ? creep Redo: (10) mother(_G57, jeff) ? creep Fail: (10) mother(_G57, jeff) ? creep Fail: (9) offspring(jeff, _G58) ? creep Fail: (8) uncle(jacob, jeff) ? creep Redo: (9) father(_G57, _G0) ? creep Exit: (9) father(jacob, bob) ? creep Exit: (8) offspring(bob, jacob) ? creep Call: (8) uncle(jacob, jeff) ? creep Call: (9) offspring(jeff, _G58) ? creep Call: (10) father(_G57, jeff) ? creep Exit: (10) father(mark, jeff) ? creep Exit: (9) offspring(jeff, mark) ? creep Call: (9) offspring(mark, _G58) ? creep Call: (10) father(_G57, mark) ? creep Fail: (10) father(_G57, mark) ? creep Redo: (9) offspring(mark, _G58) ? creep Call: (10) mother(_G57, mark) ? creep Fail: (10) mother(_G57, mark) ? creep Fail: (9) offspring(mark, _G58) ? creep Redo: (10) father(_G57, jeff) ? creep Fail: (10) father(_G57, jeff) ? creep Redo: (9) offspring(jeff, _G58) ? creep Call: (10) mother(_G57, jeff) ? creep Exit: (10) mother(margaret, jeff) ? creep Exit: (9) offspring(jeff, margaret) ? creep Call: (9) offspring(margaret, _G58) ? creep Call: (10) father(_G57, margaret) ? creep Exit: (10) father(jacob, margaret) ? creep Exit: (9) offspring(margaret, jacob) ? creep Call: (9) offspring(jacob, jacob) ? creep Call: (10) father(jacob, jacob) ? creep Fail: (10) father(jacob, jacob) ? creep Redo: (9) offspring(jacob, jacob) ? creep Call: (10) mother(jacob, jacob) ? creep Fail: (10) mother(jacob, jacob) ? creep Fail: (9) offspring(jacob, jacob) ? creep Redo: (10) father(_G57, margaret) ? creep Fail: (10) father(_G57, margaret) ? creep Redo: (9) offspring(margaret, _G58) ? creep Call: (10) mother(_G57, margaret) ? creep Exit: (10) mother(diana, margaret) ? creep Exit: (9) offspring(margaret, diana) ? creep Call: (9) offspring(jacob, diana) ? creep Call: (10) father(diana, jacob) ? creep Fail: (10) father(diana, jacob) ? creep Redo: (9) offspring(jacob, diana) ? creep Call: (10) mother(diana, jacob) ? creep Fail: (10) mother(diana, jacob) ? creep Fail: (9) offspring(jacob, diana) ? creep Redo: (10) mother(_G57, margaret) ? creep Fail: (10) mother(_G57, margaret) ? creep Fail: (9) offspring(margaret, _G58) ? creep Redo: (10) mother(_G57, jeff) ? creep Fail: (10) mother(_G57, jeff) ? creep Fail: (9) offspring(jeff, _G58) ? creep Fail: (8) uncle(jacob, jeff) ? creep Redo: (9) father(_G57, _G0) ? creep Exit: (9) father(mark, mary) ? creep Exit: (8) offspring(mary, mark) ? creep Call: (8) uncle(mark, jeff) ? creep Call: (9) offspring(jacob, diana) ? creep Call: (10) father(diana, jacob) ? creep Fail: (10) father(diana, jacob) ? creep Redo: (9) offspring(jacob, diana) ? creep Call: (10) mother(diana, jacob) ? creep Fail: (10) mother(diana, jacob) ? creep Fail: (9) offspring(jacob, diana) ? creep Redo: (10) mother(_G57, margaret) ? creep Fail: (10) mother(_G57, margaret) ? creep Fail: (9) offspring(margaret, _G58) ? creep Redo: (10) mother(_G57, jeff) ? creep Fail: (10) mother(_G57, jeff) ? creep Fail: (9) offspring(jeff, _G58) ? creep Fail: (8) uncle(jacob, jeff) ? creep Redo: (9) father(_G57, _G0) ? creep Exit: (9) father(mark, mary) ? creep Exit: (8) offspring(mary, mark) ? creep Call: (8) uncle(mark, jeff) ? creep Call: (9) offspring(jeff, _G58) ? creep Call: (10) father(_G57, jeff) ? creep Exit: (10) father(mark, jeff) ? creep Exit: (9) offspring(jeff, mark) ? creep Call: (9) offspring(mark, _G58) ? creep Call: (10) father(_G57, mark) ? creep Fail: (10) father(_G57, mark) ? creep Redo: (9) offspring(mark, _G58) ? creep Call: (10) mother(_G57, mark) ? creep Fail: (10) mother(_G57, mark) ? creep Fail: (9) offspring(mark, _G58) ? creep Redo: (10) father(_G57, jeff) ? creep Fail: (10) father(_G57, jeff) ? creep Redo: (9) offspring(jeff, _G58) ? creep Call: (10) mother(_G57, jeff) ? creep Exit: (10) mother(margaret, jeff) ? creep Exit: (9) offspring(jeff, margaret) ? creep Call: (9) offspring(margaret, _G58) ? creep Call: (10) father(_G57, margaret) ? creep Exit: (10) father(jacob, margaret) ? creep Exit: (9) offspring(margaret, jacob) ? creep Call: (9) offspring(mark, jacob) ? creep Call: (10) father(jacob, mark) ? creep Fail: (10) father(jacob, mark) ? creep Redo: (9) offspring(mark, jacob) ? creep Call: (10) mother(jacob, mark) ? creep Fail: (10) mother(jacob, mark) ? creep Fail: (9) offspring(mark, jacob) ? creep Redo: (10) father(_G57, margaret) ? creep Fail: (10) father(_G57, margaret) ? creep Redo: (9) offspring(margaret, _G58) ? creep Call: (10) mother(_G57, margaret) ? creep Exit: (10) mother(diana, margaret) ? creep Exit: (9) offspring(margaret, diana) ? creep Call: (9) offspring(mark, diana) ? creep Call: (10) father(diana, mark) ? creep Fail: (10) father(diana, mark) ? creep Redo: (9) offspring(mark, diana) ? creep Call: (10) mother(diana, mark) ? creep Fail: (10) mother(diana, mark) ? creep Fail: (9) offspring(mark, diana) ? creep Redo: (10) mother(_G57, margaret) ? creep Fail: (10) mother(_G57, margaret) ? creep Fail: (9) offspring(margaret, _G58) ? creep Redo: (10) mother(_G57, jeff) ? creep Fail: (10) mother(_G57, jeff) ? creep Fail: (9) offspring(jeff, _G58) ? creep Fail: (8) uncle(mark, jeff) ? creep Redo: (9) father(_G57, _G0) ? creep Exit: (9) father(mark, jeff) ? creep Exit: (8) offspring(jeff, mark) ? creep Call: (8) uncle(mark, jeff) ? creep Call: (9) offspring(jeff, _G58) ? creep Call: (10) father(_G57, jeff) ? creep Exit: (10) father(mark, jeff) ? creep Exit: (9) offspring(jeff, mark) ? creep Call: (9) offspring(mark, _G58) ? creep Call: (10) father(_G57, mark) ? creep Fail: (10) father(_G57, mark) ? creep Redo: (9) offspring(mark, _G58) ? creep Call: (10) mother(_G57, mark) ? creep Fail: (10) mother(_G57, mark) ? creep Fail: (9) offspring(mark, _G58) ? creep Redo: (10) father(_G57, jeff) ? creep Fail: (10) father(_G57, jeff) ? creep Redo: (9) offspring(jeff, _G58) ? creep Call: (10) mother(_G57, jeff) ? creep Exit: (10) mother(margaret, jeff) ? creep Exit: (9) offspring(jeff, margaret) ? creep Call: (9) offspring(margaret, _G58) ? creep Call: (10) mother(jacob, mark) ? creep Fail: (10) mother(jacob, mark) ? creep Fail: (9) offspring(mark, jacob) ? creep Redo: (10) father(_G57, margaret) ? creep Fail: (10) father(_G57, margaret) ? creep Redo: (9) offspring(margaret, _G58) ? creep Call: (10) mother(_G57, margaret) ? creep Exit: (10) mother(diana, margaret) ? creep Exit: (9) offspring(margaret, diana) ? creep Call: (9) offspring(mark, diana) ? creep Call: (10) father(diana, mark) ? creep Fail: (10) father(diana, mark) ? creep Redo: (9) offspring(mark, diana) ? creep Call: (10) mother(diana, mark) ? creep Fail: (10) mother(diana, mark) ? creep Fail: (9) offspring(mark, diana) ? creep Redo: (10) mother(_G57, margaret) ? creep Fail: (10) mother(_G57, margaret) ? creep Fail: (9) offspring(margaret, _G58) ? creep Redo: (10) mother(_G57, jeff) ? creep Fail: (10) mother(_G57, jeff) ? creep Fail: (9) offspring(jeff, _G58) ? creep Fail: (8) uncle(mark, jeff) ? creep Redo: (9) father(_G57, _G0) ? creep Exit: (9) father(bob, tim) ? creep Exit: (8) offspring(tim, bob) ? creep Call: (8) uncle(bob, jeff) ? creep Call: (9) offspring(jeff, _G58) ? creep Call: (10) father(_G57, jeff) ? creep Exit: (10) father(mark, jeff) ? creep Exit: (9) offspring(jeff, mark) ? creep Call: (9) offspring(mark, _G58) ? creep Call: (10) father(_G57, mark) ? creep Fail: (10) father(_G57, mark) ? creep Redo: (9) offspring(mark, _G58) ? creep Call: (10) mother(_G57, mark) ? creep Fail: (10) mother(_G57, mark) ? creep Fail: (9) offspring(mark, _G58) ? creep Redo: (10) father(_G57, jeff) ? creep Fail: (10) father(_G57, jeff) ? creep Redo: (9) offspring(jeff, _G58) ? creep Call: (10) mother(_G57, jeff) ? creep Exit: (10) mother(margaret, jeff) ? creep Exit: (9) offspring(jeff, margaret) ? creep Call: (9) offspring(margaret, _G58) ? creep Call: (10) father(_G57, margaret) ? creep Exit: (10) father(jacob, margaret) ? creep Exit: (9) offspring(margaret, jacob) ? creep Call: (9) offspring(bob, jacob) ? creep Call: (10) father(jacob, bob) ? creep Exit: (10) father(jacob, bob) ? creep Exit: (9) offspring(bob, jacob) ? creep Call: (9) male(bob) ? creep Exit: (9) male(bob) ? creep Exit: (8) uncle(bob, jeff) ? creep Exit: (7) cousin(jeff, tim) ? creep Who = tim .
The output above is quite considerable. It represents each action that occurs by which the program determines “who is the cousin of jeff?”.
The answer to that is Tim. Enchanting isn’t it?
