Improve AssociateTo error message

Author: axkr

symja_android_library/matheclipse-core/src/main/java/org/matheclipse/core/builtin/AssociationFunctions.java

@@ -140,7 +140,9 @@ public IExpr apply(final IExpr symbolValue) {
             return IOFunctions.printMessage(S.AssociateTo, "invdt", F.List(), EvalEngine.get());
           }
         }
-        return F.NIL;
+        // The argument `1` is not a valid Association.
+        return IOFunctions.printMessage(
+            S.AssociateTo, "invak", F.List(symbolValue), EvalEngine.get());
       }
     }
 
@@ -185,7 +187,8 @@ private static IExpr assignPartTo(
             return symbol.assignedValue();
           }
           // The argument `1` is not a valid Association.
-          return IOFunctions.printMessage(ast.topHead(), "invak", F.List(oldValue), EvalEngine.get());
+          return IOFunctions.printMessage(
+              ast.topHead(), "invak", F.List(oldValue), EvalEngine.get());
         }
         // The argument is not a rule or a list of rules.
         return IOFunctions.printMessage(ast.topHead(), "invdt", F.List(), EvalEngine.get());

symja_android_library/matheclipse-io/src/test/java/org/matheclipse/io/system/AssociationTest.java

@@ -17,6 +17,14 @@ public AssociationTest(String name) {
   }
 
   public void testAssociateTo() {
+    check(
+        "aa=42", //
+        "42");
+    // AssociateTo: aa is not a variable with a value, so its value cannot be changed.
+    check(
+        "AssociateTo(aa , c->17)", //
+        "AssociateTo(aa,c->17)");
+
     check(
         "assoc = <|\"A\" -> <|\"a\" -> 1, \"b\" -> 2, \"c\" -> 3|>|>", //
         "<|A-><|a->1,b->2,c->3|>|>");