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Prolog: recursive relations

We will again (in class) use swi-prolog.org's online interpreter. If you're interested in a different explanation/view, here is another prolog tutorial: www.doc.gold.ac.uk/~mas02gw/prolog_tutorial/prologpages/

You can also run /usr/local/bin/xsb from rucs.
In XSB, you can load a file family.pl from the xsb prompt, by typing: [family].

warning: Don't include spaces or hyphens in your filename. xsb interprets hyphens as subtraction, spaces as a list with two objects, and gives a syntax error.

1. Review: facts vs rules
2. a standard family-tree example, with rules like “father(X,Y) :- male(X), parent(X,Y).”.
3. Self-exercise: grandfather, w/ a partner.
   (What is the def'n in English, for "Old is a grandparent of Yng"?)
4. talk/reflect about how this is declarative programming (just saying 'prolog searches for all solutions', without saying more);
5. write a recursive rule, “ancestor/2”. (note that the “/2” is not part of the name per se; it indicates that ancestor is a binary predicate — so you'll say make queries like ancestor(bart,lisaJr).
6. have them take a crack at writing with a partner:
   1. nth-father (generalized grandfather; call 'ancestor')
   2. sameLastName (or, “sameYChromosone”): check for male-only lineage
7. Write cousin.
8. Digress, in English:
   cousin, 2nd cousin, 3rd cousin;
   nth cousin m times removed.
9. Write nthCousin (which detects exact nth-cousins, 0 times removed)
10. Remember facts just rules w/o variables; no intrinsic difference inside prolog.
11. Helpful tips: trace, with a few further tips here.

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