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Prolog: recursive relations
tutorials: www.doc.gold.ac.uk/~mas02gw/prolog_tutorial/prologpages/
You can also run jlog, a web interpreter: see hints on Running JLog
We saw already:
- run /usr/local/bin/xsb from rucs
- facts vs rules
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To load a file family.pl from the xsb prompt, type:
[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.
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a standard family-tree example, with rules like “father(X,Y) :- male(X), parent(X,Y).”.
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Self-exercise: grandfather, w/ a partner.
(What is the def'n in English, for "Old is a grandparent of Yng"?)
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talk/reflect about how this is declarative programming
(just saying 'prolog searches for all solutions', without saying more);
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write a recursive rule, “ancestor/2”.
(note that the “/2” is not part of the name by itself;
it indicates that ancestor is a binary predicate —
so you'll say make queries like ancestor(bart,lisaJr).
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have them take a crack at writing with a partner:
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nth-father (generalized grandfather; call 'ancestor')
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sameLastName (or, “sameYChromosone”): check for male-only lineage
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Write cousin.
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Digress, in English:
cousin, 2nd cousin, 3rd cousin;
nth cousin m times removed.
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Write nthCousin (which detects exact nth-cousins, 0 times removed)
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Remember facts just rules w/o variables;
no intrinsic difference inside prolog.
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Helpful tips:
trace,
and see http://www.cs.toronto.edu/~joanis/324/w01/xsbprolog.txt.
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