[r2005-06-07 21:57:41 by beppe] added "else"

Original author: beppe
Date: 2005-06-07 21:57:41+00:00

web/manual/types_patterns.xml

@@ -459,16 +459,16 @@ enumeration of the possible fields. The semi-colon between fields is optional.
 Additionally, both for open and close record types,
 it is possible to specify optional fields by using <code>=?</code>
 instead of <code>=</code> between a label and a type.
-For instance, <code>{ x = Int; y =? Bool }</code>
-represents records with an <code>x</code> field of type
-<code>Int</code>, an optional field <code>y</code> (when it is
-present, it has type <code>Bool</code>), and no other field.
+For instance, <code>{ x =? Int; y = Bool }</code>
+represents records with a <code>y</code> field of type
+<code>Bool</code>, and an optional field <code>y</code> (that when it is
+present,  has type <code>Int</code>), and no other field.
 </p>
 
 <p>
 The syntax is the same for patterns. Note that capture variables
 cannot appear in an optional field. A common idiom is to bind
-default values to replace missing optinal fields:<code>
+default values to replace missing optinal fields: <code>
 ({ x = a } | (a := 1)) &amp; { y = b }</code>. A special syntax
 makes this idiom more convenient:
 <code>{ x = a else (a:=1); y = b }</code>.

web/tutorial/patterns.xml

@@ -235,6 +235,13 @@ attribute (the other is ignored by matching it with <code>..</code>), if it is p
 while if the attribute is absent the default sub-pattern is used to assign the
 variable a default value.
 </p>
+<p>
+The use of pattern of the form <code>
+({ label1= x } | (x := %%v%%)) &amp; { label2 = y }</code> is so common in handling 
+optional fields (hence, XML attributes) that  CDuce has a special syntax for this 
+pattern:  <code>
+{ label1 = x else (x := %%v%%) ; label2 = y }</code>
+</p>
 </box>