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PONew to gprolog - how can I make this short program reach the correct conclusion?
 

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  1. PONew to gprolog - how can I make this short program reach the correct conclusion?
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    <p>I have the following gprolog program:</p> <pre><code>father(charles, harry). daughter(elizabeth, george). son(william, charles). father(X, Y) :- son(Y, X). father(X, Y) :- daughter(Y, X). daughter(X, Y) :- \+son(X, Y), father(Y, X). son(X, Y) :- \+daughter(X, Y), father(Y, X). </code></pre> <p>Given what I have, I tried this query:</p> <pre><code>| ?- son(harry, charles). </code></pre> <p>...and got a <code>no</code> (a.k.a., from what I understand, "Prolog can't prove this."). When I ran a trace on it, it didn't seem too helpful:</p> <pre><code>{trace} | ?- son(harry, charles). 1 1 Call: son(harry,charles) ? 1 1 Fail: son(harry,charles) ? </code></pre> <p>Since <code>son(harry, charles)</code> should match up to the last line of my code, I figured the program would simply start by checking the first part: <code>\+daughter(X, Y)</code> ("Prolog can't prove that Harry is the daughter of Charles", right?). Indeed:</p> <pre><code>| ?- \+daughter(harry, charles). 1 1 Call: \+daughter(harry,charles) ? 2 2 Call: daughter(harry,charles) ? 2 2 Fail: daughter(harry,charles) ? 1 1 Exit: \+daughter(harry,charles) ? (1 ms) yes </code></pre> <p>So then it would proceed to the second part, right? It would check for <code>father(Y, X)</code>, i.e., <code>father(charles, harry)</code>, which was given as a fact! </p> <pre><code>| ?- father(charles, harry). 1 1 Call: father(charles,harry) ? 1 1 Exit: father(charles,harry) ? (1 ms) yes </code></pre> <p>With both of these true, why doesn't it give a <code>yes</code> for <code>son(harry, charles)</code>? As an important note, this did compile with discontiguous predicate warnings:</p> <pre><code>compiling /Users/nicolejulian/Dropbox/AI/test.pl for byte code... /Users/nicolejulian/Dropbox/AI/test.pl:4: warning: discontiguous predicate father/2 - clause ignored /Users/nicolejulian/Dropbox/AI/test.pl:5: warning: discontiguous predicate father/2 - clause ignored /Users/nicolejulian/Dropbox/AI/test.pl:6: warning: discontiguous predicate daughter/2 - clause ignored /Users/nicolejulian/Dropbox/AI/test.pl:7: warning: discontiguous predicate son/2 - clause ignored /Users/nicolejulian/Dropbox/AI/test.pl compiled, 8 lines read - 617 bytes written, 11 ms (1 ms) yes </code></pre> <p>A few earlier experiments made it seem like I basically had a choice between warnings and infinite loops, so I went along with this as-is. (for example, grouping all of the left-side labels removes the warnings but then causes the query <code>| ?- father(elizabeth, george).</code> to crash). </p> <p>At any rate, provided that I can't just 'cheat' and add <code>son(harry, charles)</code> in as a fact, what am I not understanding about Prolog's execution steps? </p>
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