[r2005-06-24 11:22:54 by beppe] Empty log message

Original author: beppe
Date: 2005-06-24 11:22:54+00:00

web/tutorial/getting_started.xml

@@ -270,7 +270,21 @@ variable <code>parents</code> it matches it against the type
 type <code>ParentBook</code>) then it performs the assignment (the variable
 <code>x</code> is bound to the result of the load_xml expression by the pattern
 <code>x&amp;ParentBook</code>) otherwise it raises an exception.
-
+</p>
+<p>
+Of course an exception such as "parents.xml is not a document of type ParentBook" it is not
+very informative about why the document failed the match an where the error might be. In CDuce it is 
+possible to ask the program to perform this check and raise an informative exception (a string
+that describes and localize the problem) by using the dynamic type check construction <code>(%%e%%:?%%t%%)</code> which 
+checks whether the expression  <code>%%exp%%</code> has type  <code>%%t%%</code> and it either returns the result of
+ <code>%%exp%%</code> or raise an informative exception. The dynamic type check can be also used in a let construction as follows
+</p>
+<sample><![CDATA[
+let parents :? ParentBook = load_xml "parents.xml"
+]]></sample>
+<p>
+which does the same test as the previous program but in case of failure give
+information to the programmer on the reasons why the type check failed.
 </p>
 <p>
 The command <code>load_xml "parents.xml"</code> is just an abbreviated form for