当我编写PairDStreamFunctions.reduceByKey时，为什么scala编译器无法推断类型(why scala compiler failed to infer type when i write PairDStreamFunctions.reduceByKey)

 我想知道为什么当我使用PairDStreamFunctions.reduceByKey时scala编译器无法推断出我的函数参数的类型，这里是代码： 
 
val ssc = new StreamingContext(conf, Seconds(10))
ssc.checkpoint(".checkpoint")
val lines = ssc.socketTextStream("localhost", 9999)
val words = lines.flatMap(_.split(" "))
val wordCounts = words
  .map((_, 1))
  .reduceByKey((x: Int, y: Int) => x + y, 4)  //here i must specify the type Int,and this format can't work : reduceByKey((x, y) => x + y, 4)
 
 这里我必须指定我的函数参数的类型Int ，如reduceByKey（（x：Int，y：Int）=> x + y，4）当我使用PairDStreamFunctions .reduceByKey时，这种格式无法工作 ： reduceByKey（（x ，y）=> x + y，4） 。 
 
 另一方面，当我使用PairRDDFunctions.reduceByKey api时，它可以推断出类型，这里是代码： 
 
val conf = new SparkConf()
val sc = new SparkContext(conf)
val rdd = sc.parallelize(List(
  "hi what"
  , "show you"
  , "matter how"
))
rdd.flatMap(_.split(" "))
  .map((_, 1))
  .reduceByKey((x, y) => x + y, 4)//in this code,scala compiler could infer the type of my function parameter (x,y) => x+y
 
 当我使用PairRDDFunctions .reduceByKey时， reduceByKey（（x，y）=> x + y，4）可以工作。 我真的不明白是什么让它与众不同？ 

I wonder why scala compiler couldn't infer a type of my function parameter when i using PairDStreamFunctions.reduceByKey,here is code:
 
val ssc = new StreamingContext(conf, Seconds(10))
ssc.checkpoint(".checkpoint")
val lines = ssc.socketTextStream("localhost", 9999)
val words = lines.flatMap(_.split(" "))
val wordCounts = words
  .map((_, 1))
  .reduceByKey((x: Int, y: Int) => x + y, 4)  //here i must specify the type Int,and this format can't work : reduceByKey((x, y) => x + y, 4)
 
here i must specify the type Int of my function parameter like reduceByKey((x: Int, y: Int) => x + y, 4) when i use PairDStreamFunctions.reduceByKey ,and this format couldn't work : reduceByKey((x, y) => x + y, 4).
 
On the other hand,when i use PairRDDFunctions.reduceByKey api,it can infer the type,here is the code:
 
val conf = new SparkConf()
val sc = new SparkContext(conf)
val rdd = sc.parallelize(List(
  "hi what"
  , "show you"
  , "matter how"
))
rdd.flatMap(_.split(" "))
  .map((_, 1))
  .reduceByKey((x, y) => x + y, 4)//in this code,scala compiler could infer the type of my function parameter (x,y) => x+y
 
When i use PairRDDFunctions.reduceByKey, reduceByKey((x, y) => x + y, 4) could work. I really don't understand what makes it different?

原文：https://stackoverflow.com/questions/49103223