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Learn Prolog in x minutes

Learn Prolog in x minutes

Prolog is a general purpose logic programming language associated with artificial intelligence and computational linguistics.

Prolog has its roots in first-order logic, a formal logic, and unlike many other programming languages, Prolog is declarative: the program logic is expressed in terms of relations, represented as facts and rules. A computation is initiated by running a query over these relations.

The language was first conceived by a group around Alain Colmerauer in Marseille, France, in the early 1970s and the first Prolog system was developed in 1972 by Colmerauer with Philippe Roussel.

Installation

Here uses GNU Prolog as the compiler. It can be downloaded from gprolog.

First Friend

friends.pl

likes(cat, python).
likes(pig, python).
likes(mouse, ruby).

friend(X, Y) :- \+(X = Y), likes(X, Z), likes(Y, Z).

$ gprolog
| ?- ['friends.pl'].
compiling /Users/Jeoygin/Temp/trying/prolog/friends.pl for byte code...
/Users/Jeoygin/Temp/trying/prolog/friends.pl compiled, 5 lines read - 933 bytes written, 5 ms

(1 ms) yes
| ?- likes(cat, python).

yes
| ?- likes(pig, ruby).

no
| ?-

Inferences and Variables

| ?- friend(pig, pig).

no
| ?- friend(cat, pig).

yes
| ?- friend(pig, cat).

yes
| ?- friend(cat, mouse).

no
| ?-

Filling in the Blanks

food.pl

food_type(velveeta, cheese).
food_type(ritz, cracker).
food_type(spam, meat).
food_type(sausage, meat).
food_type(jolt, soda).
food_type(twinkie, dessert).

flavor(sweet, dessert).
flavor(savory, meat).
flavor(savory, cheese).
flavor(sweet, soda).

food_flavor(X, Y) :- food_type(X, Z), flavor(Y, Z).

Map Coloring

map.pl

different(red, green). different(red, blue).
different(green, red). different(green, blue).
different(blue, red). different(blue, green).

coloring(Alabama, Mississippi, Georgia, Tennessee, Florida) :-
  different(Mississippi, Tennessee),
  different(Mississippi, Alabama),
  different(Alabama, Tennessee),
  different(Alabama, Mississippi),
  different(Alabama, Georgia),
  different(Alabama, Florida),
  different(Georgia, Florida),
  different(Georgia, Tennessee).

| ?- ['map.pl'].
compiling /Users/Jeoygin/Temp/trying/prolog/map.pl for byte code...
/Users/Jeoygin/Temp/trying/prolog/map.pl compiled, 13 lines read - 1732 bytes written, 7 ms

(1 ms) yes
| ?- coloring(Alabama, Mississippi, Georgia, Tennessee, Florida).

Alabama = blue
Florida = green
Georgia = red
Mississippi = red
Tennessee = green ?

Unification, Part 1

ohmy.pl

| ?- dorothy(lion, tiger, bear).

yes
| ?- dorothy(One, Two, Three).

One = lion
Three = bear
Two = tiger

yes
| ?- twin_cats(One, Two).

One = lion
Two = lion ? a

One = lion
Two = tiger

One = tiger
Two = lion

One = tiger
Two = tiger

yes
| ?-

Recursion

family.pl

| ?- ancestor(john, mike).

true ? ;

no
| ?- ancestor(tom, john).

true ?

yes
| ?- ancestor(tom, Who).

Who = john ? a

Who = mike

(1 ms) no
| ?- ancestor(Who, mike).

Who = john ? a

Who = tom

no
| ?-

Lists and Tuples

Unification, Part 2

| ?- (1, 2, 3) = (1, 2, 3).

yes
| ?- (1, 2, 3) = (1, 2, 3, 4).

no
| ?- (1, 2, 3) = (3, 2, 1).

no
| ?- (A, B, C) = (1, 2, 3).

A = 1
B = 2
C = 3

(1 ms) yes
| ?- (1, 2, 3) = (A, B, C).

A = 1
B = 2
C = 3

yes
| ?- (A, 2, C) = (1, B, 3).

A = 1
B = 2
C = 3

yes
| ?- [1, 2, 3] = [1, 2, 3].

yes
| ?- [1, 2, 3] = [X, Y, Z].

X = 1
Y = 2
Z = 3

yes
| ?- [2, 2, 3] = [X, X, Z].

X = 2
Z = 3

yes
| ?- [1, 2, 3] = [X, X, Z].

no
| ?- [] = [].

yes
| ?- [a, b, c] = [Head|Tail].

Head = a
Tail = [b,c]

(1 ms) yes
| ?- [] = [Head|Tail].

no
| ?- [a] = [Head|Tail].

Head = a
Tail = []

yes
| ?- [a, b, c] = [a|Tail].

Tail = [b,c]

yes
| ?- [a, b, c] = [a|[Head|Tail]].

Head = b
Tail = [c]

yes
| ?- [a, b, c, d, e] = [_, _|[Head|_]].

Head = c

yes

Lists and Math

list_math.pl

count(0, []).
count(Count, [Head|Tail]) :- count(TailCount, Tail), Count is TailCount + 1.

sum(0, []).
sum(Total, [Head|Tail]) :- sum(Sum, Tail), Total is Head + Sum.

average(Average, List) :- sum(Sum, List), count(Count, List), Average is Sum/Count.

| ?- count(What, [1]).

What = 1 ? ;

no
| ?- sum(What, [1, 2, 3]).

What = 6 ? ;

no
| ?- average(What, [1, 2, 3]).

What = 2.0 ? ;

no

Using Rules in Both Directions

| ?- append([1], [2], [1, 2]).

yes
| ?- append([1], [2], [1, 3]).

no
| ?- append([1], [2], What).

What = [1,2]

yes
| ?- append([0], Who , [0, 1]).

Who = [1]

yes
| ?- append(One, Two, [1, 2]).

One = []
Two = [1,2] ? a

One = [1]
Two = [2]

One = [1,2]
Two = []

yes

concat.pl

concatenate([], List, List).
concatenate([Head|Tail1], List, [Head|Tail2]) :-
  concatenate(Tail1, List, Tail2).

| ?- concatenate([1, 2], [3], What).

What = [1,2,3]

yes

Sudoku

sudoku4.pl

valid([]).
valid([Head|Tail]) :-
    fd_all_different(Head),
    valid(Tail).
sudoku(Puzzle, Solution) :-
        Solution = Puzzle,
        Puzzle = [S11, S12, S13, S14,
                  S21, S22, S23, S24,
                  S31, S32, S33, S34,
                  S41, S42, S43, S44],
        fd_domain(Solution, 1, 4),
        Row1 = [S11, S12, S13, S14],
        Row2 = [S21, S22, S23, S24],
        Row3 = [S31, S32, S33, S34],
        Row4 = [S41, S42, S43, S44],
        Col1 = [S11, S21, S31, S41],
        Col2 = [S12, S22, S32, S42],
        Col3 = [S13, S23, S33, S43],
        Col4 = [S14, S24, S34, S44],
        Square1 = [S11, S12, S21, S22],
        Square2 = [S13, S14, S23, S24],
        Square3 = [S31, S32, S41, S42],
        Square4 = [S33, S34, S43, S44],
        valid([Row1, Row2, Row3, Row4,
               Col1, Col2, Col3, Col4,
               Square1, Square2, Square3, Square4]).

| ?- sudoku([_, _, 2, 3, 
             _, _, _, _, 
             _, _, _, _, 
             3, 4, _, _], 
             Solution).

Solution = [4,1,2,3,2,3,4,1,1,2,3,4,3,4,1,2]

yes

Eight Queens

queens.pl

valid_queen((Row, Col)) :-
    Range = [1,2,3,4,5,6,7,8],
    member(Row, Range), member(Col, Range).

valid_board([]).
valid_board([Head|Tail]) :- valid_queen(Head), valid_board(Tail).

rows([], []).
rows([(Row, _)|QueensTail], [Row|RowsTail]) :-
  rows(QueensTail, RowsTail).

cols([], []).
cols([(_, Col)|QueensTail], [Col|ColsTail]) :-
  cols(QueensTail, ColsTail).

diags1([], []).
diags1([(Row, Col)|QueensTail], [Diagonal|DiagonalsTail]) :-
  Diagonal is Col - Row,
  diags1(QueensTail, DiagonalsTail).

diags2([], []).
diags2([(Row, Col)|QueensTail], [Diagonal|DiagonalsTail]) :-
  Diagonal is Col + Row,
  diags2(QueensTail, DiagonalsTail).

eight_queens(Board) :- length(Board, 8), valid_board(Board),
  rows(Board, Rows),
  cols(Board, Cols),
  diags1(Board, Diags1),
  diags2(Board, Diags2),
  fd_all_different(Rows),
  fd_all_different(Cols),
  fd_all_different(Diags1),
  fd_all_different(Diags2).

| ?- eight_queens([(1, A), (2, B), (3, C), (4, D), (5, E), (6, F), (7, G), (8, H)]).

A = 1
B = 5
C = 8
D = 6
E = 3
F = 7
G = 2
H = 4 ?

optimized_queens.pl

valid_queen((Row, Col)) :- member(Col, [1,2,3,4,5,6,7,8]).

valid_board([]).
valid_board([Head|Tail]) :- valid_queen(Head), valid_board(Tail).

cols([], []).
cols([(_, Col)|QueensTail], [Col|ColsTail]) :-
  cols(QueensTail, ColsTail).

diags1([], []).
diags1([(Row, Col)|QueensTail], [Diagonal|DiagonalsTail]) :-
  Diagonal is Col - Row,
  diags1(QueensTail, DiagonalsTail).

diags2([], []).
diags2([(Row, Col)|QueensTail], [Diagonal|DiagonalsTail]) :-
  Diagonal is Col + Row,
  diags2(QueensTail, DiagonalsTail).

eight_queens(Board) :-
  Board = [(1, _), (2, _), (3, _), (4, _), (5, _), (6, _), (7, _), (8, _)],
  valid_board(Board),
  cols(Board, Cols),
  diags1(Board, Diags1),
  diags2(Board, Diags2),
  fd_all_different(Cols),
  fd_all_different(Diags1),
  fd_all_different(Diags2).

Reference

Wikipedia: Prolog