[SPARK-7979] Enforce structural type checker.

Author: Reynold Xin <rxin@databricks.com>

Closes #6536 from rxin/structural-type-checker and squashes the following commits:

f833151 [Reynold Xin] Fixed compilation.
633f9a1 [Reynold Xin] Fixed typo.
d1fa804 [Reynold Xin] [SPARK-7979] Enforce structural type checker.

1 files changed, 5 insertions, 1 deletions

diff --git a/examples/src/main/scala/org/apache/spark/examples/mllib/DecisionTreeRunner.scala b/examples/src/main/scala/org/apache/spark/examples/mllib/DecisionTreeRunner.scala
index b0613632c9..3381941673 100644
--- a/examples/src/main/scala/org/apache/spark/examples/mllib/DecisionTreeRunner.scala
+++ b/examples/src/main/scala/org/apache/spark/examples/mllib/DecisionTreeRunner.scala
@@ -22,7 +22,6 @@ import scala.language.reflectiveCalls
 import scopt.OptionParser
 
 import org.apache.spark.{SparkConf, SparkContext}
-import org.apache.spark.SparkContext._
 import org.apache.spark.mllib.evaluation.MulticlassMetrics
 import org.apache.spark.mllib.linalg.Vector
 import org.apache.spark.mllib.regression.LabeledPoint
@@ -354,7 +353,11 @@ object DecisionTreeRunner {
 
   /**
    * Calculates the mean squared error for regression.
+   *
+   * This is just for demo purpose. In general, don't copy this code because it is NOT efficient
+   * due to the use of structural types, which leads to one reflection call per record.
    */
+  // scalastyle:off structural.type
   private[mllib] def meanSquaredError(
       model: { def predict(features: Vector): Double },
       data: RDD[LabeledPoint]): Double = {
@@ -363,4 +366,5 @@ object DecisionTreeRunner {
       err * err
     }.mean()
   }
+  // scalastyle:on structural.type
 }