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From "Michal (JIRA)" <j...@apache.org>
Subject [jira] [Created] (KAFKA-7301) KTable to KTable join invocation does not resolve in Scala DSL
Date Thu, 16 Aug 2018 07:22:00 GMT
Michal created KAFKA-7301:

             Summary: KTable to KTable join invocation does not resolve in Scala DSL
                 Key: KAFKA-7301
                 URL: https://issues.apache.org/jira/browse/KAFKA-7301
             Project: Kafka
          Issue Type: Bug
          Components: streams
            Reporter: Michal

I found a peculiar problem while doing KTable to KTable join using Scala DSL. The following

val t1: KTable[String, Int] = ...
val t2: KTable[String, Int] = ...
val result = t1.join(t2)((x: Int, y: Int) => x + y) 

does not compile with "ambiguous reference to overloaded function". 

A quick look at the code shows the join functions are defined as follows:

def join[VO, VR](other: KTable[K, VO])(
 joiner: (V, VO) => VR,
 materialized: Materialized[K, VR, ByteArrayKeyValueStore]
def join[VO, VR](other: KTable[K, VO])(joiner: (V, VO) => VR)

the reason it does not compile is the fact that the first parameter list is identical. For
some peculiar reason it actually compiles until you try to use it:)

The same problem exists for KTable to KTable leftJoin. Other joins (stream-stream, stream-table)
do not seem to be affected as there are no overloaded versions of the functions.

This can be reproduced in smaller scale by some simple scala code:

object F {
 //def x(a: Int): Int = 5
 //def x(a: Int): Int = 6 //obviously does not compile

 def f(x: Int)(y: Int): Int = x

 def f(x: Int)(y: Int, z: Int): Int = x
val r = F.f(5)(4) //Cannot resolve
val r2 = F.f(5)(4, 6) //cannot resolve
val partial = F.f(5) _ //cannot resolve

/* you get following error:
Error: ambiguous reference to overloaded definition,
both method f in object F of type (x: Int)(y: Int, z: Int)Int
and method f in object F of type (x: Int)(y: Int)Int
match argument types (Int)

The solution: get rid of the multiple parameter lists. I fail to see what practical purpose
they serve anyways. I am happy to supply appropriate PR if there is agreement.


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