%  Rotate a list N places to the left

% rotate(L1,N,L2) :- the list L2 is obtained from the list L1 by
%    rotating the elements of L1 N places to the left.
%    Examples:
%    rotate([a,b,c,d,e,f,g,h],3,[d,e,f,g,h,a,b,c])
%    rotate([a,b,c,d,e,f,g,h],-2,[g,h,a,b,c,d,e,f])
%    (list,integer,list) (+,+,?)

% Split a list into two parts

% split(L,N,L1,L2) :- the list L1 contains the first N elements
%    of the list L, the list L2 contains the remaining elements.
%    (list,integer,list,list) (?,+,?,?)

domains
 list=symbol*
predicates
 rotate(list,integer,list).
 length(list,integer).
 rotate_left(list,integer,list).
 append(list,list,list).
 split(list,integer,list,list).
clauses

rotate(L1,N,L2) :- N >= 0,
   length(L1,NL1), N1 = N mod NL1, rotate_left(L1,N1,L2).
rotate(L1,N,L2) :- N < 0,
   length(L1,NL1), N1 = NL1 + (N mod NL1), rotate_left(L1,N1,L2).

rotate_left(L,0,L).
rotate_left(L1,N,L2) :- N > 0, split(L1,N,S1,S2), append(S2,S1,L2).

append([],Listb,Listb).
 
 append([X|List1],List2,[X|List3]):-

 append(List1,List2,List3).

split(L,0,[],L).
split([X|Xs],N,[X|Ys],Zs) :- N > 0, N1 = N - 1, split(Xs,N1,Ys,Zs).

length([],0).
length([_|L],N) :- length(L,N1), N = N1 + 1.

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Goal: rotate([a,b,c,d,e,f,g,h],3,X)

X=(["d","e","f","g","h","a","b","c"])

1 solution