递归定义都包括两个部分：边界条件与递归部分。
边界条件定义最简单的情况。而递归部分，则首先解决一部分问题，然后再调用其自身来解决剩下的部分，
每一次都将进行边界检测，如果剩下的部分已经是边界条件中所定义的情况时，那么递归就圆满成功了。
递归解决阶乘问题
%swipl -s factorial.pl
%Hansen
fun(N):- N>0,fact(N,1).
fun(N):- write(N), write(' is smaller than 1. '),fail.
fact(1,M):- write(M),!.
fact(N,M):- N1 is N-1, M1 is M*N, fact(N1,M1).
1 ?- fun(0). 0 is smaller than 1. false. 2 ?- fun(1). 1 true . 3 ?- fun(2). 2 true . 4 ?- fun(3). 6 true . 5 ?- fun(4). 24 true . 6 ?- fun(5). 120 true . 7 ?- fun(6). 720 true .
递归解决汉诺塔问题
%swipl -s hanoi.pl
%Hansen
%N个盘子从A到C的问题，用递归解决的思路：
%N-1个盘子，从A到B
%1个盘子，从A到C
%N-1个盘子，从B到C
hanoi(N):-move(N,a,b,c).
move(1,A,_,C):-fromto(A,C),!.
move(N,A,B,C):-N1 is N-1,move(N1,A,C,B),fromto(A,C),move(N1,B,A,C).
fromto(Loc1,Loc2):-nl,write('move one disk from '),write(Loc1),write(' to '),write(Loc2).
执行查询：
1 ?- hanoi(1). move one disk from a to c true. 2 ?- hanoi(2). move one disk from a to b move one disk from a to c move one disk from b to c true. 3 ?- hanoi(3). move one disk from a to c move one disk from a to b move one disk from c to b move one disk from a to c move one disk from b to a move one disk from b to c move one disk from a to c true. %这里就会出错啦 4 ?- hanoi(0). ERROR: Out of local stack