[r2003-09-22 16:18:19 by cvscast] Empty log message

Original author: cvscast
Date: 2003-09-22 16:21:12+00:00

typing/typer.ml

@@ -772,7 +772,7 @@ let rec expr glb loc = function
 	    let x = U.to_string (Id.value x) in
 	    warning br_loc 
 	      ("The capture variable " ^ x ^ 
-	       " is declared in the pattern but not used in the body of this branch. It might be a misspelled type or name."));
+	       " is declared in the pattern but not used in the body of this branch. It might be a misspelled type or name (if not use _ instead)."));
       let fv2 = Fv.diff fv2 (Patterns.fv p) in
       fv := Fv.cup !fv fv2;
       accept := Types.cup !accept (Types.descr (Patterns.accept p));

web/tutorial/patterns.xml

@@ -13,6 +13,70 @@
 <b style="color:#FF0080">TO BE DONE</b>
 
 </box>
+<box title="Handling optional attributes" link="pr">
+<p>
+The blend of type type constructors and boolean combinator can be used to 
+reduce verbosity in writing pattern matching. As an exmple we show how to handle 
+tags with several optional attibutes.
+</p>
+<p>
+Consider the following fragment of code from site.cd from the CDuce
+distribution that we have changed a bit so that it stands alone:
+</p>
+<sample><![CDATA[
+type Sample = <sample highlight=?"true"|"false">String
+
+let content (Sample -> String)
+  | <sample highlight="false">_ -> "non-higlighted code"
+  | <sample>_ -> "highlighted code"
+
+]]></sample> 
+<p>
+The idea here is to use the highlight attribute to specify that
+certain pieces of <code>&lt;sample></code> should be emphasized. When the higlight
+attribute is missing, the default value of "true" is presumed.
+</p>
+<p>
+But what if we have two optional attributes? The naive solution would be
+to write
+<i>three</i> cases in the above function:
+</p>
+<sample><![CDATA[
+type Sample = <sample lineno=?"true"|"false" highlight=?"true"|"false">String
+
+let content (Sample -> String)
+  | <sample highlight="false" lineno="false">_ -> "no lineno, no highlight"
+  | <sample lineno="false">_ -> "no lineno, highlight"
+  | <sample highlight="false">_ -> "lineno, no highlight"
+  | <sample>_ -> "lineno, highlight"
+
+]]></sample> 
+<p>
+The intended use for the <code>lineno</code> attribute is to tell us whether line
+numbers should be displayed alongside the sample code.)
+<br/>
+While this situation is still bearable it soon become unfeasible with more
+than two optional attributes. A much better way of handling this situation
+is to resort to intersection  and default patterns as follows:
+</p>
+<sample><![CDATA[
+let content (Sample -> String)
+  | <sample ( ({ highlight = h } | (h := "true"))
+             &({ lineno = l } | (l := "true")) )>_
+        -> (match l with "true" -> "lineno, " | _ -> " no lineno, ")@ 
+           (match h with "true" -> "highlight" | _ -> "no highlight") 
+
+]]></sample> 
+<p>
+The intersection pattern <code>&amp;</code> makes both patterns to be matched
+against the record of attributes: each pattern check the presence of an specific
+attributes, if it present it captures the attribute value in a given variables
+while if the attribute is absent the default subpattern to assign to the
+variable a default value (disclaimer, for the moment defaut patterns accepts
+only constants; this limitation will soon desappear).
+</p>
+</box>
+
 
 
 <box title="Recursive patterns" link="pr">