string <span class="arrow">&#45;&gt;</span></span> string</span></code></div></div><div class="odoc-spec"><div class="spec value anchored" id="val-mapi"><a href="#val-mapi" class="anchor"></a><code><span><span class="keyword">val</span> mapi : <span><span class="label">f</span>:<span>(<span>int <span class="arrow">&#45;&gt;</span></span> <span>char <span class="arrow">&#45;&gt;</span></span> char)</span> <span class="arrow">&#45;&gt;</span></span> <span>string <span class="arrow">&#45;&gt;</span></span> str