Index: TODO
===================================================================
diff -u -r06f89c385bcc658773eb4dcda50e56abba2eb656 -r3cb6a6a8f1e33e63abeec25b3c36231702af6fe2
--- TODO	(.../TODO)	(revision 06f89c385bcc658773eb4dcda50e56abba2eb656)
+++ TODO	(.../TODO)	(revision 3cb6a6a8f1e33e63abeec25b3c36231702af6fe2)
@@ -1021,6 +1021,8 @@
   
 - replaced several occurrences of "eval" in nx.tcl and xotcl2.tcl
 
+- implemented parameter option "allowempty"
+- extended regression test
 
 TODO: 
 - nameing
@@ -1053,6 +1055,15 @@
         package require nx::test
         namespace import ::nx::*
 
+    * parameter option allowempty is a little longish; we should
+      choose probably a shorter name
+
+      Object create o {
+        :method foo {x:integer,allowempty y:integer os:object,multivalued,allowempty} {
+          return $x
+        }
+      }
+
 - documentation
    - documentationssytem
    - langref (xotcl 2.0 + next-scripting)
@@ -1192,7 +1203,6 @@
 - info valuecheck?
 - turn off optionally value checking
 - get rid of "::xotcl::is"?
-- value checker (allowempty)
 
 # TODO (optimization): optimizer can improve parameter checking:
 #       (a) simple approach: make scripted setter methods on domain
Index: generic/xotcl.c
===================================================================
diff -u -r033c63d771af5253b0e94c2a9c1c6a94df40242e -r3cb6a6a8f1e33e63abeec25b3c36231702af6fe2
--- generic/xotcl.c	(.../xotcl.c)	(revision 033c63d771af5253b0e94c2a9c1c6a94df40242e)
+++ generic/xotcl.c	(.../xotcl.c)	(revision 3cb6a6a8f1e33e63abeec25b3c36231702af6fe2)
@@ -5387,6 +5387,9 @@
       if ((pPtr->flags & XOTCL_ARG_SUBST_DEFAULT)) {
         ParamDefsFormatOption(interp, nameStringObj, "substdefault", &colonWritten, &first);
       }
+      if ((pPtr->flags & XOTCL_ARG_ALLOW_EMPTY)) {
+        ParamDefsFormatOption(interp, nameStringObj, "allowempty", &colonWritten, &first);
+      }
       if ((pPtr->flags & XOTCL_ARG_INITCMD)) {
         ParamDefsFormatOption(interp, nameStringObj, "initcmd", &colonWritten, &first);
       } else if ((pPtr->flags & XOTCL_ARG_METHOD)) {
@@ -6424,6 +6427,8 @@
     paramPtr->flags &= ~XOTCL_ARG_REQUIRED;
   } else if (strncmp(option, "substdefault", 12) == 0) {
     paramPtr->flags |= XOTCL_ARG_SUBST_DEFAULT;
+  } else if (strncmp(option, "allowempty", 10) == 0) {
+    paramPtr->flags |= XOTCL_ARG_ALLOW_EMPTY;
   } else if (strncmp(option, "initcmd", 7) == 0) {
     paramPtr->flags |= XOTCL_ARG_INITCMD;
   } else if (strncmp(option, "method", 6) == 0) {
@@ -9450,7 +9455,14 @@
   
   for (i=0; i<objc; i++) {
     Tcl_Obj *elementObjPtr;
-    result = (*pPtr->converter)(interp, ov[i], pPtr, clientData, &elementObjPtr);
+    const char *valueString = ObjStr(ov[i]);
+
+    if (pPtr->flags & XOTCL_ARG_ALLOW_EMPTY && *valueString == '\0') {
+      result = convertToString(interp, ov[i], pPtr, clientData, &elementObjPtr);
+    } else {
+      result = (*pPtr->converter)(interp, ov[i], pPtr, clientData, &elementObjPtr);
+    }
+
     if (result == TCL_OK) {
       Tcl_ListObjAppendElement(interp, *outObjPtr, elementObjPtr);
     } else {
@@ -9477,20 +9489,32 @@
     int objc, i;
     Tcl_Obj **ov;
     
+    /*
+     * In the multivalued case, we have either to check a list of
+     * values or to build a new list of values (in case, the converter
+     * normalizes the values).
+     */
     result = Tcl_ListObjGetElements(interp, objPtr, &objc, &ov);
     if (result != TCL_OK) {
       return result;
     }
 
     /* 
-       Default assumption: outObjPtr is not modified, in cases where
-       necessary, we switch to the helper function 
-    */
+     * Default assumption: outObjPtr is not modified, in cases where
+     * necessary, we switch to the helper function
+     */
     *outObjPtr = objPtr;
 
     for (i=0; i<objc; i++) {
       Tcl_Obj *elementObjPtr;
-      result = (*pPtr->converter)(interp, ov[i], pPtr, clientData, &elementObjPtr);
+      const char *valueString = ObjStr(ov[i]);
+
+      if (pPtr->flags & XOTCL_ARG_ALLOW_EMPTY && *valueString == '\0') {
+	result = convertToString(interp, ov[i], pPtr, clientData, &elementObjPtr);
+      } else {
+	result = (*pPtr->converter)(interp, ov[i], pPtr, clientData, &elementObjPtr);
+      }
+
       if (result == TCL_OK) {
         if (ov[i] != elementObjPtr) {
           /* 
@@ -9514,7 +9538,12 @@
       }
     }
   } else {
-    result = (*pPtr->converter)(interp, objPtr, pPtr, clientData, outObjPtr);
+    const char *valueString = ObjStr(objPtr);
+    if (pPtr->flags & XOTCL_ARG_ALLOW_EMPTY && *valueString == '\0') {
+      result = convertToString(interp, objPtr, pPtr, clientData, outObjPtr);
+    } else {
+      result = (*pPtr->converter)(interp, objPtr, pPtr, clientData, outObjPtr);
+    }
   }
   return result;
 }
Index: generic/xotclInt.h
===================================================================
diff -u -rf3cb5afe6aa1b6761b4a9909058f64ff7d64ab92 -r3cb6a6a8f1e33e63abeec25b3c36231702af6fe2
--- generic/xotclInt.h	(.../xotclInt.h)	(revision f3cb5afe6aa1b6761b4a9909058f64ff7d64ab92)
+++ generic/xotclInt.h	(.../xotclInt.h)	(revision 3cb6a6a8f1e33e63abeec25b3c36231702af6fe2)
@@ -366,8 +366,9 @@
 #define XOTCL_ARG_NOARG 		     0x0004
 #define XOTCL_ARG_CURRENTLY_UNKNOWN	     0x0008
 #define XOTCL_ARG_SUBST_DEFAULT		     0x0010
-#define XOTCL_ARG_INITCMD		     0x0020
-#define XOTCL_ARG_METHOD		     0x0040
+#define XOTCL_ARG_ALLOW_EMPTY		     0x0020
+#define XOTCL_ARG_INITCMD		     0x0040
+#define XOTCL_ARG_METHOD		     0x0080
 #define XOTCL_ARG_RELATION		     0x0100
 #define XOTCL_ARG_SWITCH		     0x0200
 #define XOTCL_ARG_HAS_DEFAULT		     0x1000
Index: tests/parameters.tcl
===================================================================
diff -u -r033c63d771af5253b0e94c2a9c1c6a94df40242e -r3cb6a6a8f1e33e63abeec25b3c36231702af6fe2
--- tests/parameters.tcl	(.../parameters.tcl)	(revision 033c63d771af5253b0e94c2a9c1c6a94df40242e)
+++ tests/parameters.tcl	(.../parameters.tcl)	(revision 3cb6a6a8f1e33e63abeec25b3c36231702af6fe2)
@@ -901,23 +901,40 @@
   ? {o foo 3} 4
 }
 
+
 #######################################################
+# allow empty values
+#######################################################
+Test case allow-empty {
+
+  Object create o1
+  Object create o2
+  Object create o3
+
+  Object create o {
+    :method foo {x:integer,allowempty y:integer os:object,multivalued,allowempty} {
+      return $x
+    }
+  }
+  
+  ? {o foo 1 2 {o1 o2}} 1 "all values specified"
+  ? {o foo "" 2 {o1 o2}} "" "first is empty"
+  ? {o foo 1 "" {o1 o2}} {expected integer but got "" for parameter y} "second is empty"
+  ? {o foo 1 2 {}} 1 "empty list, does not require allowempty"
+  ? {o foo 1 2 {o1 "" o2}} 1 "list contains empty value"
+
+  ? {o info method parameter foo} "x:integer,allowempty y:integer os:object,multivalued,allowempty"
+
+  o method foo {x:integer,allowempty y:integer os:object,multivalued} {return $x}
+  ? {o foo 1 2 {o1 "" o2}} {invalid value in "o1 "" o2": expected object but got "" for parameter os} \
+      "list contains empty value"
+
+}
+#######################################################
 # slot specific converter
 #######################################################
 Test case slot-specfic-converter {
-  #  Class create Person
-  # Person slots {
-  #   ::nx::Attribute create sex -type "sex" {
-  #     :method type=sex {name value} {
-  # 	#puts stderr "[current] slot specific converter"
-  # 	switch -glob $value {
-  # 	  m* {return m}
-  # 	  f* {return f}
-  # 	  default {error "expected sex but got $value"}
-  # 	}
-  #     }
-  #   }
-  # }
+
   Class create Person {
     :attribute sex {
       :type "sex"
@@ -931,6 +948,7 @@
       }
     }
   }
+
   Person create p1 -sex male
   ? {p1 sex} m
   Person method foo {s:sex,slot=::Person::slot::sex} {return $s}