Merge pull request #877 from mP1/feature/ExpressionEvaluationContext-…

…REFERENCE_NOT_FOUND_VALUE-N-REFERENCE_NULL_VALUE-constants

ExpressionEvaluationContext.REFERENCE_NOT_FOUND_VALUE &  REFERENCE_NU…

src/main/java/walkingkooka/tree/expression/ExpressionEvaluationContext.java

@@ -136,6 +136,18 @@ Object evaluateFunction(final ExpressionFunction<?, ? extends ExpressionEvaluati
      */
     Object handleException(final RuntimeException exception);
 
+    /**
+     * A constant holding the value for a reference that was not found.
+     */
+    Optional<Optional<Object>> REFERENCE_NOT_FOUND_VALUE = Optional.empty();
+
+    /**
+     * A constant holding the value for a reference that has a null value.
+     */
+    Optional<Optional<Object>> REFERENCE_NULL_VALUE = Optional.of(
+        Optional.empty()
+    );
+
     /**
      * Locates the value or a {@link Expression} for the given {@link ExpressionReference}.
      * <pre>

src/test/java/walkingkooka/tree/expression/ExpressionEvaluationContextTest.java

@@ -330,6 +330,18 @@ public void testPrepareParameters() {
         );
     }
 
+    // reference........................................................................................................
+
+    @Test
+    public void testReferenceNotFoundValueDifferentReferenceNullValue() {
+        this.checkNotEquals(
+            ExpressionEvaluationContext.REFERENCE_NOT_FOUND_VALUE,
+            ExpressionEvaluationContext.REFERENCE_NULL_VALUE
+        );
+    }
+
+    // class............................................................................................................
+
     @Override
     public Class<ExpressionEvaluationContext> type() {
         return ExpressionEvaluationContext.class;