diff --git a/src/main/scala/catalyst/expressions/Expression.scala b/src/main/scala/catalyst/expressions/Expression.scala
index 6a01833b6ca0c..97f9f0320dbac 100644
--- a/src/main/scala/catalyst/expressions/Expression.scala
+++ b/src/main/scala/catalyst/expressions/Expression.scala
@@ -12,11 +12,14 @@ abstract class Expression extends TreeNode[Expression] {
    * Returns true when an expression is a candidate for static evaluation before the query is
    * executed.
    * The following conditions are used to determine suitability for constant folding:
-   *  - A Coalesce is foldable if all of its children are foldable
-   *  - A BinaryExpression is foldable if its both left and right child are foldable.
-   *  - A Not, isNull, or isNotNull is foldable if its child is foldable.
-   *  - A Literal is foldable.
-   *  - A Cast or UnaryMinus is foldable if its child is foldable.
+   *  - A [[expressions.Coalesce Coalesce]] is foldable if all of its children are foldable
+   *  - A [[expressions.BinaryExpression BinaryExpression]] is foldable if its both left and right
+   *    child are foldable
+   *  - A [[expressions.Not Not]], [[expressions.IsNull IsNull]], or [[expressions.IsNotNull IsNotNull]]
+   *    is foldable if its child is foldable.
+   *  - A [[expressions.Literal]] is foldable.
+   *  - A [[expressions.Cast Cast]] or [[expressions.UnaryMinus UnaryMinus]] is foldable if its
+   *    child is foldable.
    */
   // TODO: Supporting more foldable expressions. For example, deterministic Hive UDFs.
   def foldable: Boolean = false