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https://issues.apache.org/jira/browse/SPARK-2331?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14048041#comment-14048041
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Sean Owen edited comment on SPARK-2331 at 6/30/14 7:48 PM:
-----------------------------------------------------------
My 2 cents.
{code}
val rdds = Seq("a","b","c").foldLeft(sc.emptyRDD){ (rdd,path) =>
rdd.union(sc.textFile(path)) }
{code}
yields
{code}
<console>:12: error: type mismatch;
found : org.apache.spark.rdd.RDD[String]
required: org.apache.spark.rdd.RDD[Nothing]
Note: String >: Nothing, but class RDD is invariant in type T.
You may wish to define T as -T instead. (SLS 4.5)
val rdds = Seq("a","b","c").foldLeft(sc.emptyRDD){ (rdd,path) =>
rdd.union(sc.textFile(path)) }
{code}
but even
{code}
val rdds = Seq("a","b","c").foldLeft(sc.emptyRDD[String]){ (rdd,path) =>
rdd.union(sc.textFile(path)) }
{code}
yields
{code}
<console>:12: error: type mismatch;
found : org.apache.spark.rdd.RDD[String]
required: org.apache.spark.rdd.EmptyRDD[String]
val rdds = Seq("a","b","c").foldLeft(sc.emptyRDD[String]){ (rdd,path) =>
rdd.union(sc.textFile(path)) }
{code}
So I think this is really about RDDs being invariant, rather than the return
type here, and that seems to be how it's going to be:
https://issues.apache.org/jira/browse/SPARK-1296
I think there's an argument for hiding EmptyRDD although that would be a little
API change at this point.
was (Author: srowen):
My 2 cents. You mean the type is EmptyRDD[Nothing] right?
{code}
val rdds = Seq("a","b","c").foldLeft(sc.emptyRDD){ (rdd,path) =>
rdd.union(sc.textFile(path)) }
{code}
yields
{code}
<console>:12: error: type mismatch;
found : org.apache.spark.rdd.RDD[String]
required: org.apache.spark.rdd.RDD[Nothing]
Note: String >: Nothing, but class RDD is invariant in type T.
You may wish to define T as -T instead. (SLS 4.5)
val rdds = Seq("a","b","c").foldLeft(sc.emptyRDD){ (rdd,path) =>
rdd.union(sc.textFile(path)) }
{code}
but even
{code}
val rdds = Seq("a","b","c").foldLeft(sc.emptyRDD[String]){ (rdd,path) =>
rdd.union(sc.textFile(path)) }
{code}
yields
{code}
<console>:12: error: type mismatch;
found : org.apache.spark.rdd.RDD[String]
required: org.apache.spark.rdd.EmptyRDD[String]
val rdds = Seq("a","b","c").foldLeft(sc.emptyRDD[String]){ (rdd,path) =>
rdd.union(sc.textFile(path)) }
{code}
So I think this is really about RDDs being invariant, rather than the return
type here, and that seems to be how it's going to be:
https://issues.apache.org/jira/browse/SPARK-1296
I think there's an argument for hiding EmptyRDD although that would be a little
API change at this point.
> SparkContext.emptyRDD has wrong return type
> -------------------------------------------
>
> Key: SPARK-2331
> URL: https://issues.apache.org/jira/browse/SPARK-2331
> Project: Spark
> Issue Type: Bug
> Affects Versions: 1.0.0
> Reporter: Ian Hummel
>
> The return type for SparkContext.emptyRDD is EmptyRDD[T].
> It should be RDD[T]. That means you have to add extra type annotations on
> code like the below (which creates a union of RDDs over some subset of paths
> in a folder)
> val rdds = Seq("a", "b", "c").foldLeft[RDD[String]](sc.emptyRDD[String]) {
> (rdd, path) ⇒
> rdd.union(sc.textFile(path))
> }
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