This commit contains three changes:

* Expose `DataType`s in the sql package (internal details are private to sql).
* Introduce `createSchemaRDD` to create a `SchemaRDD` from an `RDD` with a provided schema (represented by a `StructType`) and a provided function to construct `Row`,
* Add a function `simpleString` to every `DataType`. Also, the schema represented by a `StructType` can be visualized by `printSchema`.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Row.scala

@@ -32,6 +32,11 @@ object Row {
    * }}}
    */
   def unapplySeq(row: Row): Some[Seq[Any]] = Some(row)
+
+  /**
+   * Construct a [[Row]] with the given values.
+   */
+  def apply(values: Any*): Row = new GenericRow(values.toArray)
 }
 
 /**

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/WrapDynamic.scala

@@ -21,7 +21,9 @@ import scala.language.dynamics
 
 import org.apache.spark.sql.catalyst.types.DataType
 
-case object DynamicType extends DataType
+case object DynamicType extends DataType {
+  def simpleString: String = "dynamic"
+}
 
 case class WrapDynamic(children: Seq[Attribute]) extends Expression {
   type EvaluatedType = DynamicRow

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/QueryPlan.scala

@@ -125,52 +125,11 @@ abstract class QueryPlan[PlanType <: TreeNode[PlanType]] extends TreeNode[PlanTy
     }.toSeq
   }
 
-  protected def generateSchemaString(schema: Seq[Attribute]): String = {
-    val builder = new StringBuilder
-    builder.append("root\n")
-    val prefix = " |"
-    schema.foreach { attribute =>
-      val name = attribute.name
-      val dataType = attribute.dataType
-      dataType match {
-        case fields: StructType =>
-          builder.append(s"$prefix-- $name: $StructType\n")
-          generateSchemaString(fields, s"$prefix    |", builder)
-        case ArrayType(fields: StructType) =>
-          builder.append(s"$prefix-- $name: $ArrayType[$StructType]\n")
-          generateSchemaString(fields, s"$prefix    |", builder)
-        case ArrayType(elementType: DataType) =>
-          builder.append(s"$prefix-- $name: $ArrayType[$elementType]\n")
-        case _ => builder.append(s"$prefix-- $name: $dataType\n")
-      }
-    }
-
-    builder.toString()
-  }
-
-  protected def generateSchemaString(
-      schema: StructType,
-      prefix: String,
-      builder: StringBuilder): StringBuilder = {
-    schema.fields.foreach {
-      case StructField(name, fields: StructType, _) =>
-        builder.append(s"$prefix-- $name: $StructType\n")
-        generateSchemaString(fields, s"$prefix    |", builder)
-      case StructField(name, ArrayType(fields: StructType), _) =>
-        builder.append(s"$prefix-- $name: $ArrayType[$StructType]\n")
-        generateSchemaString(fields, s"$prefix    |", builder)
-      case StructField(name, ArrayType(elementType: DataType), _) =>
-        builder.append(s"$prefix-- $name: $ArrayType[$elementType]\n")
-      case StructField(name, fieldType: DataType, _) =>
-        builder.append(s"$prefix-- $name: $fieldType\n")
-    }
-
-    builder
-  }
+  def schema: StructType = StructType.fromAttributes(output)
 
   /** Returns the output schema in the tree format. */
-  def schemaString: String = generateSchemaString(output)
+  def formattedSchemaString: String = schema.formattedSchemaString
 
   /** Prints out the schema in the tree format */
-  def printSchema(): Unit = println(schemaString)
+  def printSchema(): Unit = println(formattedSchemaString)
 }

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/types/dataTypes.scala

@@ -62,7 +62,6 @@ object DataType extends RegexParsers {
     "true" ^^^ true |
     "false" ^^^ false
 
-
   protected lazy val structType: Parser[DataType] =
     "StructType\\([A-zA-z]*\\(".r ~> repsep(structField, ",") <~ "))" ^^ {
       case fields => new StructType(fields)
@@ -93,47 +92,56 @@ abstract class DataType {
   }
 
   def isPrimitive: Boolean = false
+
+  def simpleString: String
 }
 
-case object NullType extends DataType
+case object NullType extends DataType {
+  def simpleString: String = "null"
+}
 
 trait PrimitiveType extends DataType {
   override def isPrimitive = true
 }
 
 abstract class NativeType extends DataType {
-  type JvmType
-  @transient val tag: TypeTag[JvmType]
-  val ordering: Ordering[JvmType]
+  private[sql] type JvmType
+  @transient private[sql] val tag: TypeTag[JvmType]
+  private[sql] val ordering: Ordering[JvmType]
 
-  @transient val classTag = {
+  @transient private[sql] val classTag = {
     val mirror = runtimeMirror(Utils.getSparkClassLoader)
     ClassTag[JvmType](mirror.runtimeClass(tag.tpe))
   }
 }
 
 case object StringType extends NativeType with PrimitiveType {
-  type JvmType = String
-  @transient lazy val tag = typeTag[JvmType]
-  val ordering = implicitly[Ordering[JvmType]]
+  private[sql] type JvmType = String
+  @transient private[sql] lazy val tag = typeTag[JvmType]
+  private[sql] val ordering = implicitly[Ordering[JvmType]]
+  def simpleString: String = "string"
 }
 case object BinaryType extends DataType with PrimitiveType {
-  type JvmType = Array[Byte]
+  private[sql] type JvmType = Array[Byte]
+  def simpleString: String = "binary"
 }
 case object BooleanType extends NativeType with PrimitiveType {
-  type JvmType = Boolean
-  @transient lazy val tag = typeTag[JvmType]
-  val ordering = implicitly[Ordering[JvmType]]
+  private[sql] type JvmType = Boolean
+  @transient private[sql] lazy val tag = typeTag[JvmType]
+  private[sql] val ordering = implicitly[Ordering[JvmType]]
+  def simpleString: String = "boolean"
 }
 
 case object TimestampType extends NativeType {
-  type JvmType = Timestamp
+  private[sql] type JvmType = Timestamp
 
-  @transient lazy val tag = typeTag[JvmType]
+  @transient private[sql] lazy val tag = typeTag[JvmType]
 
-  val ordering = new Ordering[JvmType] {
+  private[sql] val ordering = new Ordering[JvmType] {
     def compare(x: Timestamp, y: Timestamp) = x.compareTo(y)
   }
+
+  def simpleString: String = "timestamp"
 }
 
 abstract class NumericType extends NativeType with PrimitiveType {
@@ -142,7 +150,7 @@ abstract class NumericType extends NativeType with PrimitiveType {
   // type parameter and and add a numeric annotation (i.e., [JvmType : Numeric]). This gets
   // desugared by the compiler into an argument to the objects constructor. This means there is no
   // longer an no argument constructor and thus the JVM cannot serialize the object anymore.
-  val numeric: Numeric[JvmType]
+  private[sql] val numeric: Numeric[JvmType]
 }
 
 /** Matcher for any expressions that evaluate to [[IntegralType]]s */
@@ -154,39 +162,43 @@ object IntegralType {
 }
 
 abstract class IntegralType extends NumericType {
-  val integral: Integral[JvmType]
+  private[sql] val integral: Integral[JvmType]
 }
 
 case object LongType extends IntegralType {
-  type JvmType = Long
-  @transient lazy val tag = typeTag[JvmType]
-  val numeric = implicitly[Numeric[Long]]
-  val integral = implicitly[Integral[Long]]
-  val ordering = implicitly[Ordering[JvmType]]
+  private[sql] type JvmType = Long
+  @transient private[sql] lazy val tag = typeTag[JvmType]
+  private[sql] val numeric = implicitly[Numeric[Long]]
+  private[sql] val integral = implicitly[Integral[Long]]
+  private[sql] val ordering = implicitly[Ordering[JvmType]]
+  def simpleString: String = "long"
 }
 
 case object IntegerType extends IntegralType {
-  type JvmType = Int
-  @transient lazy val tag = typeTag[JvmType]
-  val numeric = implicitly[Numeric[Int]]
-  val integral = implicitly[Integral[Int]]
-  val ordering = implicitly[Ordering[JvmType]]
+  private[sql] type JvmType = Int
+  @transient private[sql] lazy val tag = typeTag[JvmType]
+  private[sql] val numeric = implicitly[Numeric[Int]]
+  private[sql] val integral = implicitly[Integral[Int]]
+  private[sql] val ordering = implicitly[Ordering[JvmType]]
+  def simpleString: String = "integer"
 }
 
 case object ShortType extends IntegralType {
-  type JvmType = Short
-  @transient lazy val tag = typeTag[JvmType]
-  val numeric = implicitly[Numeric[Short]]
-  val integral = implicitly[Integral[Short]]
-  val ordering = implicitly[Ordering[JvmType]]
+  private[sql] type JvmType = Short
+  @transient private[sql] lazy val tag = typeTag[JvmType]
+  private[sql] val numeric = implicitly[Numeric[Short]]
+  private[sql] val integral = implicitly[Integral[Short]]
+  private[sql] val ordering = implicitly[Ordering[JvmType]]
+  def simpleString: String = "short"
 }
 
 case object ByteType extends IntegralType {
-  type JvmType = Byte
-  @transient lazy val tag = typeTag[JvmType]
-  val numeric = implicitly[Numeric[Byte]]
-  val integral = implicitly[Integral[Byte]]
-  val ordering = implicitly[Ordering[JvmType]]
+  private[sql] type JvmType = Byte
+  @transient private[sql] lazy val tag = typeTag[JvmType]
+  private[sql] val numeric = implicitly[Numeric[Byte]]
+  private[sql] val integral = implicitly[Integral[Byte]]
+  private[sql] val ordering = implicitly[Ordering[JvmType]]
+  def simpleString: String = "byte"
 }
 
 /** Matcher for any expressions that evaluate to [[FractionalType]]s */
@@ -197,47 +209,127 @@ object FractionalType {
   }
 }
 abstract class FractionalType extends NumericType {
-  val fractional: Fractional[JvmType]
+  private[sql] val fractional: Fractional[JvmType]
 }
 
 case object DecimalType extends FractionalType {
-  type JvmType = BigDecimal
-  @transient lazy val tag = typeTag[JvmType]
-  val numeric = implicitly[Numeric[BigDecimal]]
-  val fractional = implicitly[Fractional[BigDecimal]]
-  val ordering = implicitly[Ordering[JvmType]]
+  private[sql] type JvmType = BigDecimal
+  @transient private[sql] lazy val tag = typeTag[JvmType]
+  private[sql] val numeric = implicitly[Numeric[BigDecimal]]
+  private[sql] val fractional = implicitly[Fractional[BigDecimal]]
+  private[sql] val ordering = implicitly[Ordering[JvmType]]
+  def simpleString: String = "decimal"
 }
 
 case object DoubleType extends FractionalType {
-  type JvmType = Double
-  @transient lazy val tag = typeTag[JvmType]
-  val numeric = implicitly[Numeric[Double]]
-  val fractional = implicitly[Fractional[Double]]
-  val ordering = implicitly[Ordering[JvmType]]
+  private[sql] type JvmType = Double
+  @transient private[sql] lazy val tag = typeTag[JvmType]
+  private[sql] val numeric = implicitly[Numeric[Double]]
+  private[sql] val fractional = implicitly[Fractional[Double]]
+  private[sql] val ordering = implicitly[Ordering[JvmType]]
+  def simpleString: String = "double"
 }
 
 case object FloatType extends FractionalType {
-  type JvmType = Float
-  @transient lazy val tag = typeTag[JvmType]
-  val numeric = implicitly[Numeric[Float]]
-  val fractional = implicitly[Fractional[Float]]
-  val ordering = implicitly[Ordering[JvmType]]
+  private[sql] type JvmType = Float
+  @transient private[sql] lazy val tag = typeTag[JvmType]
+  private[sql] val numeric = implicitly[Numeric[Float]]
+  private[sql] val fractional = implicitly[Fractional[Float]]
+  private[sql] val ordering = implicitly[Ordering[JvmType]]
+  def simpleString: String = "float"
 }
 
-case class ArrayType(elementType: DataType) extends DataType
+case class ArrayType(elementType: DataType) extends DataType {
+  private[sql] def buildFormattedString(prefix: String, builder: StringBuilder): Unit = {
+    builder.append(s"${prefix}-- element: ${elementType.simpleString}\n")
+    elementType match {
+      case array: ArrayType =>
+        array.buildFormattedString(s"$prefix    |", builder)
+      case struct: StructType =>
+        struct.buildFormattedString(s"$prefix    |", builder)
+      case map: MapType =>
+        map.buildFormattedString(s"$prefix    |", builder)
+      case _ =>
+    }
+  }
 
-case class StructField(name: String, dataType: DataType, nullable: Boolean)
+  def simpleString: String = "array"
+}
+
+case class StructField(name: String, dataType: DataType, nullable: Boolean) {
+
+  private[sql] def buildFormattedString(prefix: String, builder: StringBuilder): Unit = {
+    builder.append(s"${prefix}-- ${name}: ${dataType.simpleString} (nullable = ${nullable})\n")
+    dataType match {
+      case array: ArrayType =>
+        array.buildFormattedString(s"$prefix    |", builder)
+      case struct: StructType =>
+        struct.buildFormattedString(s"$prefix    |", builder)
+      case map: MapType =>
+        map.buildFormattedString(s"$prefix    |", builder)
+      case _ =>
+    }
+  }
+}
 
 object StructType {
   def fromAttributes(attributes: Seq[Attribute]): StructType = {
     StructType(attributes.map(a => StructField(a.name, a.dataType, a.nullable)))
   }
 
+  private def validateFields(fields: Seq[StructField]): Boolean =
+    fields.map(field => field.name).distinct.size == fields.size
+
   // def apply(fields: Seq[StructField]) = new StructType(fields.toIndexedSeq)
 }
 
 case class StructType(fields: Seq[StructField]) extends DataType {
+  require(StructType.validateFields(fields), "Found fields with the same name.")
+
   def toAttributes = fields.map(f => AttributeReference(f.name, f.dataType, f.nullable)())
+
+  def formattedSchemaString: String = {
+    val builder = new StringBuilder
+    builder.append("root\n")
+    val prefix = " |"
+    fields.foreach(field => field.buildFormattedString(prefix, builder))
+
+    builder.toString()
+  }
+
+  def printSchema(): Unit = println(formattedSchemaString)
+
+  private[sql] def buildFormattedString(prefix: String, builder: StringBuilder): Unit = {
+    fields.foreach(field => field.buildFormattedString(prefix, builder))
+  }
+
+  def simpleString: String = "struct"
 }
 
-case class MapType(keyType: DataType, valueType: DataType) extends DataType
+case class MapType(keyType: DataType, valueType: DataType) extends DataType {
+  private[sql] def buildFormattedString(prefix: String, builder: StringBuilder): Unit = {
+    builder.append(s"${prefix}-- key: ${keyType.simpleString}\n")
+    keyType match {
+      case array: ArrayType =>
+        array.buildFormattedString(s"$prefix    |", builder)
+      case struct: StructType =>
+        struct.buildFormattedString(s"$prefix    |", builder)
+      case map: MapType =>
+        map.buildFormattedString(s"$prefix    |", builder)
+      case _ =>
+    }
+
+    builder.append(s"${prefix}-- value: ${valueType.simpleString}\n")
+    valueType match {
+      case array: ArrayType =>
+        array.buildFormattedString(s"$prefix    |", builder)
+      case struct: StructType =>
+        struct.buildFormattedString(s"$prefix    |", builder)
+      case map: MapType =>
+        map.buildFormattedString(s"$prefix    |", builder)
+      case _ =>
+    }
+  }
+
+  def simpleString: String = "map"
+}