Prolog - recursing down family tree

Question

I am trying to write a Prolog program that will print out the male successors of British Royalty in order. My attempt so far:

son(elizabeth, charles).
son(charle         


        

Answer 1

As rati noted above, Prolog queries are resolved by choosing a rule, recursively evaluating it using depth-first search, then choosing the next rule and repeating the process. However, the particular rules you're starting with actually result in a breadth-first search of the family tree, which, as you noted, does not give output that matches the actual line of succession. Instead, you want to do a depth-first traversal of the royal family tree. This version gives the result you're looking for:

successor(X, Y) :- son(X, Z), (Y = Z; successor(Z, Y)).

Using this rule, Prolog resolves the query successor(X, Y) roughly as follows:

• For each Z who is a son of X:
  • Bind Y to Z, giving Z as a solution.
  • The ; operator functions as a logical OR, so now Y is unbound and successor/2 is called recursively to get the successors who are sons of Z.

And yes, please do try to get a copy of the Art of Prolog. It's not the easiest programming book to read, but I found it extremely helpful in my (ongoing) attempt to understand logic programming. There seem to have been some cheap hardcover copies of the 1994 edition floating around eBay lately.