Eight Queens solutions from Seven Languages in Seven Weeks, SWI-Prolog version

optimized_queens.pl

%% https://www.swi-prolog.org/man/clpfd.html
:- use_module(library(clpfd)).

%% ---
%%  Excerpted from "Seven Languages in Seven Weeks",
%%  published by The Pragmatic Bookshelf.
%%  Copyrights apply to this code. It may not be used to create training material,
%%  courses, books, articles, and the like. Contact us if you are in doubt.
%%  We make no guarantees that this code is fit for any purpose.
%%  Visit http://www.pragmaticprogrammer.com/titles/btlang for more book information.
%%---
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).
  all_distinct(Cols),
  all_distinct(Diags1),
  all_distinct(Diags2).

queens.pl

%% https://www.swi-prolog.org/man/clpfd.html
:- use_module(library(clpfd)).

%% ---
%%  Excerpted from "Seven Languages in Seven Weeks",
%%  published by The Pragmatic Bookshelf.
%%  Copyrights apply to this code. It may not be used to create training material,
%%  courses, books, articles, and the like. Contact us if you are in doubt.
%%  We make no guarantees that this code is fit for any purpose.
%%  Visit http://www.pragmaticprogrammer.com/titles/btlang for more book information.
%%---
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).
  all_distinct(Rows),
  all_distinct(Cols),
  all_distinct(Diags1),
  all_distinct(Diags2).