-module(queens).
-export([queens/1]).

queens(0) ->
    [[]];
queens(N) ->
    [ [Row | Columns] || Columns <- queens(N-1),
                         Row <- [1,2,3,4,5,6,7,8] -- Columns, % -- returns the list difference
                         safe(Row, Columns, 1) ].

safe(_Row, [], _N) ->
    true;
safe(Row, [Column|Columns], N) ->
    (Row /= Column + N) andalso (Row /= Column - N) andalso
        safe(Row, Columns, (N+1)).