of
   <code class="code">s</code> to pass it to <code class="code">f</code>, and could not call <code class="code">unsafe_to_string</code>
   safely.</p>

<p>We have provided the <a href="String.html#VALinit"><code class="code">String.init</code></a>, <a href="String.html#VALmap"><code class="code">String.map</code></a> and
   <a href="String.html#VALmapi"><code class="code">String.mapi</code></a> functions to cover most cases of building
   new strings. You should prefer those over <code class="code">to_st