Scala: compilation error: not found type

Question

我已经开始学习一些 Spark\Scala\GraphX 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 并想尝试一下。所以我尝试按我认为应该的方式编译这段代码（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

我是 Scala 的新手，所以我不完全理解 VD\ED 这些定义中的角色。

Answer 1

问题是您不能在方法定义中使用尚未在范围内某处声明为类型参数的类型（例如该方法，或包含 class 等）。

看看你的方法def pregel[A]。它 returns 类型 Graph[VD, ED] 的值。但是编译器怎么知道 VD 指的是什么？无需深入了解代码的作用，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 class Graph 中的一个方法，那就没问题，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不知名的地方，这是不允许的 - 也许您可能需要考虑将它们移动到 Graph class?

Scala: compilation error: not found type

Scala: compilation error: not found type

scala

apache-spark

compiler-errors

spark-graphx