scala - Scala : compilation error: not found type

Question

我已经开始学习一些Spark \ Scala \ GraphX以便与Pregel一起使用，我在这里找到了一些简单的代码:
http://www.cakesolutions.net/teamblogs/graphx-pregel-api-an-example
想尝试一下。
因此，我尝试按照我认为的那样编译此代码(这是我第一次使用Scala):

/* SimpleApp.scala */
import org.apache.spark.SparkContext
import org.apache.spark.SparkContext._
import org.apache.spark.SparkConf
import org.apache.spark._
import org.apache.spark.graphx._
import org.apache.spark.rdd.RDD

class Graph[VD, ED] (
        val vertices: VertexRDD[VD],
        val edges: EdgeRDD[ED]) {
}

object SimpleApp {
    val initialMsg = 9999

def vprog(vertexId: VertexId, value: (Int, Int), message: Int): (Int, Int) = {
    if (message == initialMsg)
        value
    else
        (message min value._1, value._1)
}

def sendMsg(triplet: EdgeTriplet[(Int, Int), Boolean]): Iterator[(VertexId, Int)] = {
    val sourceVertex = triplet.srcAttr

    if (sourceVertex._1 == sourceVertex._2)
        Iterator.empty
    else 
        Iterator((triplet.dstId, sourceVertex._1))
}

def mergeMsg(msg1: Int, msg2: Int): Int = msg1 min msg2

def pregel[A]
  (initialMsg: A,
   maxIter: Int = Int.MaxValue,
   activeDir: EdgeDirection = EdgeDirection.Out)
  (vprog: (VertexId, VD, A) => VD,
   sendMsg: EdgeTriplet[VD, ED] => Iterator[(VertexId, A)],
   mergeMsg: (A, A) => A)
: Graph[VD, ED]

def main(args: Array[String]) {
    val conf = new SparkConf().setAppName("Simple Application")
    val sc = new SparkContext(conf)

    val minGraph = graph.pregel(initialMsg, 
                            Int.MaxValue, 
                            EdgeDirection.Out)(
                            vprog,
                            sendMsg,
                            mergeMsg)

    minGraph.vertices.collect.foreach{
        case (vertexId, (value, original_value)) => println(value)
    }

    sc.stop()
    }
}

但是我得到这个错误:

$ C:\"Program Files (x86)"\sbt\bin\sbt package
Java HotSpot(TM) 64-Bit Server VM warning: ignoring option MaxPermSize=256m; support was removed in 8.0
[info] Set current project to Simple Project (in build file:/C:/spark/simple/)
[info] Compiling 1 Scala source to C:\spark\simple\target\scala-2.10\classes...
[error] C:\spark\simple\src\main\scala\SimpleApp.scala:42: not found: type VD
[error]         : Graph[VD, ED]
[error]                 ^
[error] C:\spark\simple\src\main\scala\SimpleApp.scala:42: not found: type ED
[error]         : Graph[VD, ED]
[error]                     ^
[error] C:\spark\simple\src\main\scala\SimpleApp.scala:39: not found: type VD
[error]           (vprog: (VertexId, VD, A) => VD,
[error]                              ^
[error] C:\spark\simple\src\main\scala\SimpleApp.scala:39: not found: type VD
[error]           (vprog: (VertexId, VD, A) => VD,
[error]                                        ^
[error] C:\spark\simple\src\main\scala\SimpleApp.scala:40: not found: type VD
[error]            sendMsg: EdgeTriplet[VD, ED] => Iterator[(VertexId, A)],
[error]                                 ^
[error] C:\spark\simple\src\main\scala\SimpleApp.scala:40: not found: type ED
[error]            sendMsg: EdgeTriplet[VD, ED] => Iterator[(VertexId, A)],
[error]                                     ^
[error] C:\spark\simple\src\main\scala\SimpleApp.scala:35: only classes can have declared but undefined members
[error]         def pregel[A]
[error]             ^
[error] C:\spark\simple\src\main\scala\SimpleApp.scala:48: not found: value graph
[error]                 val minGraph = graph.pregel(initialMsg,
[error]                                ^
[error] 8 errors found
[error] (compile:compileIncremental) Compilation failed
[error] Total time: 5 s, completed Jan 17, 2017 12:35:00 AM

我对Scala还是很陌生，所以我不完全了解这些定义中的VD \ ED角色。

Answer 1

问题是您无法在方法定义中使用尚未在作用域中某个地方声明为类型参数的类型(例如，该方法或包含类等)。

查看您的方法def pregel[A]。它返回类型为Graph[VD, ED]的值。但是编译器如何知道VD指的是什么？如果您不添加代码功能，则只需添加VD作为方法类型参数即可轻松解决此问题:

def pregel[A, VD]

请注意，如果pregel是Graph类中的方法，则可以，因为Graph定义了该类型:class Graph[VD, ED]。根据您发布的代码，您的方法似乎徘徊在无处可寻的地方，这是不允许的-也许您可能要考虑将它们移到Graph类中？