_cons-until-empty_: Add @only@

* vector/list.xml (_cons-until-empty_): Add @only@, which has the opposite
    behavior to @glance@.
  (_merge-until-empty_): Proxy @only@.

1 files changed, 29 insertions, 7 deletions

diff --git a/core/vector/list.xml b/core/vector/list.xml
index 8d3e9a6..eafbbdc 100644
--- a/core/vector/list.xml
+++ b/core/vector/list.xml
@@ -26,6 +26,7 @@
   <import package="../base" />
 
   <import package="../numeric/common" export="true" />
+  <import package="../when"           export="true" />
 
 
 
@@ -49,6 +50,7 @@
 
     <param name="@base@" desc="Base set to return (otherwise an empty set)" />
     <param name="@glance@" desc="Glance at (but do nothing with) this value; recurse without action" />
+    <param name="@only@" desc="Process only this value; otherwise recurse without action" />
 
     <!-- intended for use by merge-until-empty to reduce duplicate code; not to
          be set via direct template applications -->
@@ -102,17 +104,35 @@
             </c:values>
 
             <!-- this case statement will be optimized away if we have no
-                 @glance@ value -->
+                 @glance@ or @only@ value -->
             <c:cases>
               <!-- if we have a glancing value, then immediately recurse
                    without processing if we have a match -->
               <if name="@glance@">
                 <c:case>
-                  <c:when name="@car@">
-                    <c:eq>
-                      <c:value-of name="@glance@" />
-                    </c:eq>
-                  </c:when>
+                  <t:when-eq name="@car@" value="@glance@" />
+
+                  <c:recurse>
+                    <c:arg name="@set@">
+                      <c:value-of name="@cdr@" />
+                    </c:arg>
+
+                    <if name="@index@">
+                      <c:arg name="@index@">
+                        <t:inc>
+                          <c:value-of name="@index@" />
+                        </t:inc>
+                      </c:arg>
+                    </if>
+                  </c:recurse>
+                </c:case>
+              </if>
+
+              <!-- if we should only recurse when a value matches, ignore
+                   non-match -->
+              <if name="@only@">
+                <c:case>
+                  <t:when-ne name="@car@" value="@only@" />
 
                   <c:recurse>
                     <c:arg name="@set@">
@@ -206,9 +226,11 @@
     </param>
 
     <param name="@glance@" desc="Glance at (but do nothing with) this value; recurse without action" />
+    <param name="@only@"   desc="Process only this value; otherwise recurse without action" />
 
     <!-- to reduce duplicate template code, we simply set a merge flag on cons-until-empty -->
-    <t:cons-until-empty set="@set@" car="@car@" cdr="@cdr@" glance="@glance@" merge="true">
+    <t:cons-until-empty set="@set@" car="@car@" cdr="@cdr@"
+                        glance="@glance@" only="@only@" merge="true">
       <param-copy name="@values@" />
     </t:cons-until-empty>
   </template>