A Prolog solver for the Skyline puzzle

skyline.pl

% A Prolog solver for the Skyline puzzle
% http://www.constantin-jean-clau.de/

print_solution(X,Y) :- solve(X,Y,Sol), print_board(Sol).

pos(X,Y,_) :- member(X,[1,2,3,4,5,6,7]), member(Y,[1,2,3,4,5,6,7]).

board(Board) :- findall(pos(X,Y,_),pos(X,Y,_),Board).

solve(X,Y,Board) :- 
    board(Board), 
    member(pos(X,Y,' '),Board),
    solve(1,1,Board,[]).

solve(7,7,_,_) :- !.
solve(8,Y,Board,Placed) :-
    Yn is Y + 1,
    solve(1,Yn,Board,Placed), !.
solve(X,Y,Board,Placed) :-
    member(pos(X,Y,V),Board),
    nonvar(V),
    Xn is X + 1,
    solve(Xn,Y,Board,Placed).
solve(X,Y,Board,Placed) :-
    member(pos(X,Y,V),Board),
    var(V),
    member(Piece,[i,g,h,f,e,d,a,c,b]),
    not(member(Piece,Placed)),
    piece(Piece,Locs),
    place_piece(Piece,X,Y,Locs,Board),
    Xn is X + 1,
    solve(Xn,Y,Board,[Piece|Placed]).

print_board(Board) :- 
    write('+-------+'), nl,
    findall(_,(member(Y,[1,2,3,4,5,6,7]),print_line(Y,Board)),_),
    write('+-------+'), nl.

print_line(Y,Board) :- 
    write('|'), 
    findall(_,(member(X,[1,2,3,4,5,6,7]),print_piece(X,Y,Board)),_), 
    write('|'),
    nl.

print_piece(X,Y,Board) :- 
    member(pos(X,Y,P),Board), 
    not(var(P)),
    write(P), !.
print_piece(_,_,_) :- 
    write('_').

place_piece(_,_,_,[],_).
place_piece(Piece,X0,Y0,[(Xd,Yd)|Locs],Board) :-
    X is X0 + Xd, X > 0, X =< 7, 
    Y is Y0 + Yd, Y > 0, Y =< 7,
    member(pos(X,Y,Piece),Board),
    place_piece(Piece,X0,Y0,Locs,Board).

piece(a,[(0,0),(0,1),(0,2),(0,3),(0,4)]).
piece(a,[(0,0),(1,0),(2,0),(3,0),(4,0)]).

piece(b,[(0,0),(1,0)]).
piece(b,[(0,0),(0,1)]).

piece(c,[(0,0),(-1,1),(0,1),(1,1)]).
piece(c,[(0,0),(1,0),(1,1),(2,0)]).
piece(c,[(0,0),(0,1),(-1,1),(0,2)]).
piece(c,[(0,0),(0,1),(1,1),(0,2)]).

piece(d,[(0,0),(0,1),(1,1),(1,2),(2,2)]).
piece(d,[(0,0),(1,0),(-1,1),(0,1),(-1,2)]).
piece(d,[(0,0),(1,0),(1,1),(2,1),(2,2)]). 
piece(d,[(0,0),(-1,1),(0,1),(-2,2),(-1,2)]).

piece(e,[(0,0),(0,1),(0,2),(0,3),(1,1)]).
piece(e,[(0,0),(1,0),(2,0),(3,0),(2,1)]).
piece(e,[(0,0),(0,1),(0,2),(0,3),(-1,2)]).
piece(e,[(0,0),(-1,1),(0,1),(1,1),(2,1)]).

piece(f,[(0,0),(-1,1),(0,1),(-2,2),(-1,2),(0,2)]).
piece(f,[(0,0),(0,1),(1,1),(0,2),(1,2),(2,2)]).
piece(f,[(0,0),(1,0),(2,0),(0,1),(1,1),(0,2)]).
piece(f,[(0,0),(1,0),(2,0),(1,1),(2,1),(2,2)]).

piece(g,[(0,0),(1,0),(0,1),(1,1),(2,1),(0,2),(1,2)]).
piece(g,[(0,0),(1,0),(2,0),(0,1),(1,1),(2,1),(1,2)]).
piece(g,[(0,0),(1,0),(-1,1),(0,1),(1,1),(0,2),(1,2)]).
piece(g,[(0,0),(-1,1),(0,1),(1,1),(-1,2),(0,2),(1,2)]).

piece(h,[(0,0),(0,1),(0,2),(0,3),(1,1),(1,2)]).
piece(h,[(0,0),(1,0),(2,0),(3,0),(1,1),(2,1)]).
piece(h,[(0,0),(-1,1),(0,1),(-1,2),(0,2),(0,3)]).
piece(h,[(0,0),(1,0),(-1,1),(0,1),(1,1),(2,1)]).

piece(i,[(0,0),(-2,1),(-1,1),(0,1),(-2,2),(-1,2),(0,2),(-1,3)]).
piece(i,[(0,0),(1,0),(-1,1),(0,1),(1,1),(0,2),(1,2),(2,2)]).
piece(i,[(0,0),(-1,1),(0,1),(1,1),(-1,2),(0,2),(1,2),(-1,3)]).
piece(i,[(0,0),(1,0),(2,0),(1,1),(2,1),(3,1),(1,2),(2,2)]).