Recursive clause

• A rule defined in terms of itself.
  i.e. t :- t1,t2, ..., tn
• where t and atleast one ti have the same functor and arity

ancestor(Ancestor, Person) :-
  parent(Parent, Person),
  ancestor(Ancestor, Parent).
(tail recursion)
ancestor(Ancestor, Person) :-
 parent(Ancestor, Perso).
(base case)

Make head and tail of the following :

[X | [Y]]
[[ a,b,c | d],e | [ f,g | X]]
[ a | [ b | [ c | [d]]]]

Similarly with lists :

list_length([],0). 
list_length([H|T], N) :- 
	list_length(T,N1),
 	N is N1+1.

This can be explained as :

• Length of an empty list is zero (fact)
• Since each list is composed of a head and a tail, the length of a list is the length of the tail plus one element (the head)
  i.e. Length of list(L) = length of (H|T),
  L is divided into a head and tail,
  Length of (H|T) = Length of T + 1

The recursive use of the list_length relationship allows the length to be calculated. Each time, we are adding on one element (the head) to our calculation.