Dear caml-list,<br><br>I try to define the signature of three interdependant classes that should then be derived and completed for various implementation. The code below causes the strange error messsage:<br><br>         File &quot;<a href="http://tmp.ml">tmp.ml</a>&quot;, line 4, characters 7-92:<br>
         In the definition of job, type (&#39;a, &#39;b) batch should be (&#39;a, &#39;c) batch<br><br>Can you explain what is wrong with this signature?<br>I was unable to find an answer to a similar problem in the archives. <br>
<br>Thank you very much.<br>Jean<br><br>----------------------------------------------------------------------------<br> <br>module type JOB =<br>sig<br><br>  type (&#39;a, &#39;b) batch =  private &lt; build : &#39;a job -&gt; (&#39;a priority * &#39;a) list -&gt; &#39;b job; .. &gt;<br>
  and &#39;b priority = private &lt; next : &#39;b priority;  ..  &gt;<br>  and &#39;a job =  private &lt; create_batch : &#39;c.(&#39;a -&gt; &#39;c) -&gt; (&#39;a, &#39;c) batch; ..  &gt;<br><br>end<br><br><br><br>