Github user mengxr commented on a diff in the pull request:
https://github.com/apache/spark/pull/3099#discussion_r19923238
--- Diff: mllib/src/main/scala/org/apache/spark/ml/parameters.scala ---
@@ -0,0 +1,267 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.ml
+
+import java.lang.reflect.Modifier
+
+import scala.language.implicitConversions
+import scala.collection.mutable
+
+/**
+ * A param with self-contained documentation and optionally default value.
+ *
+ * @param parent parent object
+ * @param name param name
+ * @param doc documentation
+ * @param default optional default value
+ * @tparam T param value type
+ */
+class Param[T] private (
+ val parent: Identifiable,
+ val name: String,
+ val doc: String,
+ val default: Option[T]) extends Serializable {
+
+ /**
+ * Creates a param without a default value.
+ *
+ * @param parent parent object
+ * @param name param name
+ * @param doc documentation
+ */
+ def this(parent: Identifiable, name: String, doc: String) = this(parent,
name, doc, None)
+
+ /**
+ * Creates a param with a default value.
+ *
+ * @param parent parent object
+ * @param name param name
+ * @param doc documentation
+ * @param default default value
+ */
+ def this(parent: Identifiable, name: String, doc: String, default: T) =
+ this(parent, name, doc, Some(default))
+
+ /**
+ * Creates a param pair with the given value (for Java).
+ */
+ def w(value: T): ParamPair[T] = ParamPair(this, value)
+
+ /**
+ * Creates a param pair with the given value (for Scala).
+ */
+ def ->(value: T): ParamPair[T] = ParamPair(this, value)
+
+ override def toString: String = {
+ if (default.isDefined) {
+ s"$name: $doc (default: ${default.get})"
+ } else {
+ s"$name: $doc"
+ }
+ }
+}
+
+/**
+ * A param amd its value.
+ */
+case class ParamPair[T](param: Param[T], value: T)
+
+/**
+ * Trait for components that take parameters.
+ */
+trait Params {
+
+ /** Returns all params. */
+ def params: Array[Param[_]] = {
+ val methods = this.getClass.getMethods
+ methods.filter { m =>
+ Modifier.isPublic(m.getModifiers) &&
+ classOf[Param[_]].isAssignableFrom(m.getReturnType) &&
+ m.getParameterTypes.isEmpty
+ }.map(m => m.invoke(this).asInstanceOf[Param[_]])
+ }
+
+ /** Gets a param by its name. */
+ def getParam(paramName: String): Param[Any] = {
+ val m = this.getClass.getMethod(paramName)
+ assert(Modifier.isPublic(m.getModifiers) &&
+ classOf[Param[_]].isAssignableFrom(m.getReturnType))
+ m.invoke(this).asInstanceOf[Param[Any]]
+ }
+
+ /**
+ * Validates parameters specified by the input parameter map.
+ * Raises an exception if any parameter belongs to this object is
invalid.
+ */
+ def validateParams(paramMap: ParamMap): Unit = {}
+
+ /**
+ * Returns the documentation of all params.
+ */
+ def explainParams(): String = params.mkString("\n")
+}
+
+/**
+ * Trait for instances that hold their own param maps.
+ */
+trait OwnParamMap {
+
+ /**
+ * Internal param map.
+ */
+ val paramMap: ParamMap = ParamMap.empty
+
+ /**
+ * Sets a parameter in the own parameter map.
+ */
+ def set[T](param: Param[T], value: T): this.type = {
+ paramMap.put(param.asInstanceOf[Param[Any]], value)
+ this
+ }
+}
+
+private[ml] object Params {
+
+ /**
+ * Returns a Params implementation without any
+ */
+ val empty: Params = new Params {
+ override def params: Array[Param[_]] = Array.empty
+ }
+}
+
+/**
+ * A param to value map.
+ */
+class ParamMap private[ml] (
+ private val params: mutable.Map[Param[Any], Any]) extends Serializable
{
+
+ /**
+ * Creates an empty param map.
+ */
+ def this() = this(mutable.Map.empty[Param[Any], Any])
+
+ /**
+ * Puts a (param, value) pair (overwrites if the input param exists).
+ */
+ def put[T](param: Param[T], value: T): this.type = {
+ params(param.asInstanceOf[Param[Any]]) = value
+ this
+ }
+
+ /**
+ * Puts a param pair (overwrites if the input param exists).
+ */
+ def put(firstParamPair: ParamPair[_], otherParamPairs: ParamPair[_]*):
this.type = {
+ put(firstParamPair.param.asInstanceOf[Param[Any]],
firstParamPair.value)
+ otherParamPairs.foreach { p =>
+ put(p.param.asInstanceOf[Param[Any]], p.value)
+ }
+ this
+ }
+
+ /**
+ * Gets the value of the input param or the default value if it does not
exist.
+ * Raises a NoSuchElementException if there is no value associated with
the input param.
+ */
+ def apply[T](param: Param[T]): T = {
+ params.getOrElse(param.asInstanceOf[Param[Any]],
param.default.get).asInstanceOf[T]
+ }
+
+ /**
+ * Checks whether a parameter is specified.
+ */
+ def contains(param: Param[_]): Boolean = {
+ params.contains(param.asInstanceOf[Param[Any]])
+ }
+
+ /**
+ * Filter this param map for the given parent.
+ */
+ def filter(parent: Identifiable): ParamMap = {
--- End diff --
This is useful when we try to store training parameters in trained models.
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