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Perfect numbers: Difference between revisions
→{{header|Prolog}}
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=={{header|Prolog}}==
===Classic approach===
Works with SWI-Prolog
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numlist(2, N, LN),
include(perfect, LN, L).
</lang>
===Functionnal approach===
Works with SWI-Prolog and module lambda, written by <b>Ulrich Neumerkel</b found there http://www.complang.tuwien.ac.at/ulrich/Prolog-inedit/lambda.pl
<lang Prolog>:- use_module(library(lambda)).
is_divisor(V, N) :-
0 =:= V mod N.
is_perfect(N) :-
N1 is floor(N/2),
numlist(1, N1, L),
f_compose_1(foldl((\X^Y^Z^(Z is X+Y)), 0), filter(is_divisor(N)), F),
call(F, L, N).
f_perfect_numbers(N, L) :-
numlist(2, N, LN),
filter(is_perfect, LN, L).
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% functionnal predicates
%% foldl(Pred, Init, List, R).
%
foldl(_Pred, Val, [], Val).
foldl(Pred, Val, [H | T], Res) :-
call(Pred, Val, H, Val1),
foldl(Pred, Val1, T, Res).
%% filter(Pred, LstIn, LstOut)
%
filter(_Pre, [], []).
filter(Pred, [H|T], L) :-
filter(Pred, T, L1),
( call(Pred,H) -> L = [H|L1]; L = L1).
%% f_compose_1(Pred1, Pred2, Pred1(Pred2)).
%
f_compose_1(F,G, \X^Z^(call(G,X,Y), call(F,Y,Z))).
</lang>
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