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    <p>The syntax error results from the fact that the first two clauses have no body.</p> <p>To answer your question, observe that the maximum of a list can be defined inductively as follows:</p> <ul> <li>The maximum of a list with one element is that element.</li> <li>The maximum of a list with multiple elements is the largest of the head and the maximum of the tail.</li> </ul> <p>Thus,</p> <pre><code>max_list([H], H). max_list([H|T], M2) :- max_list(T, M), M2 is max(H, M). </code></pre> <p>This code uses <code>max/2</code> (<a href="http://www.swi-prolog.org/pldoc/doc_for?object=max%2f2" rel="nofollow noreferrer">SWI-Prolog</a>, <a href="http://www.gprolog.org/manual/html_node/gprolog029.html" rel="nofollow noreferrer">GNU-Prolog</a>). Note that most or all Prolog implementations will have a built-in function <code>max_list/2</code> (<a href="http://www.swi-prolog.org/pldoc/doc_for?object=max_list%2f2" rel="nofollow noreferrer">S</a>, <a href="http://www.gprolog.org/manual/gprolog.html#htoc211" rel="nofollow noreferrer">G</a>), so there is actually no need to define it yourself.</p> <p><strong>Edit:</strong> <a href="https://stackoverflow.com/questions/1816057/two-clause-definition-to-find-the-maximum-number-on-a-list/1816797#1816797">Bakore notes</a> that a tail recursive implementation may be more efficient. You can do this by defining a predicate <code>max_list/3</code> which takes an additional argument <code>C</code>, namely the largest value seen so far.</p> <pre><code>max_list([H|T], M) :- max_list(T, H, M). max_list([], C, C). max_list([H|T], C, M) :- C2 is max(C, H), max_list(T, C2, M). </code></pre>
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