 <pre><code>let exists k l = List.fold_left (fun b x -&gt; b || x = k) false l </code></pre> <p>Two comments on @tonio's answer:</p> <ul> <li>Use <code>||</code> instead of superfluous <code>if ... then true else ...</code>.</li> <li>Use structural equality (<code>=</code>) instead of reference equality (<code>==</code>) for comparing values.</li> </ul> <p>Moreover, <code>exists</code> is available in <a href="http://caml.inria.fr/pub/docs/manual-ocaml/libref/List.html" rel="nofollow">List module</a>. The built-in function is more efficient since it doesn't have to go through the whole list every time.</p>