jon-wei commented on a change in pull request #7588: multi-value string column 
support for expressions
URL: https://github.com/apache/incubator-druid/pull/7588#discussion_r290956997
 
 

 ##########
 File path: core/src/main/java/org/apache/druid/math/expr/Expr.java
 ##########
 @@ -56,23 +70,180 @@ default Object getLiteralValue()
     throw new ISE("Not a literal");
   }
 
+  /**
+   * Returns the string identifier of an {@link IdentifierExpr}, else null
+   */
+  @Nullable
+  default String getIdentifierIfIdentifier()
+  {
+    // overridden by things that are identifiers
+    return null;
+  }
+
+  /**
+   * Evaluate the {@link Expr} with the bindings which supply {@link 
IdentifierExpr} with their values, producing an
+   * {@link ExprEval} with the result.
+   */
   @Nonnull
   ExprEval eval(ObjectBinding bindings);
 
+  /**
+   * Programmatically inspect the {@link Expr} tree with a {@link Visitor}. 
Each {@link Expr} is responsible for
+   * ensuring the {@link Visitor} can visit all of it's {@link Expr} children 
before visiting itself.
 
 Review comment:
   it's -> its

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