Recent questions tagged Scala

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scala - Advantages of F-bounded polymorphism over typeclass for return-current-type problem

Returning the current type questions are often asked on StackOverflow. Here is one such example. The usual answers seem ... : Bar = Bar(Scala) See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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scala - Akka-Stream implementation slower than single threaded implementation

UPDATE FROM 2015-10-30 based on Roland Kuhn Awnser: Akka Streams is using asynchronous message ... .apache.avro.specific.SpecificRecordBuilderBase<Event> implements org.apache...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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scala - How is this case class match pattern working?

I've just seen this case class in the Scala actors package: case class ! [a](ch: Channel[a], msg: a) And ... way to methods ending in a colon : See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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scala - Missing parameter type error by calling toSet?

Trying to generate, from a list of chars, a list of unique chars mapped to their frequency - e.g. ... why Scala complains about this syntax? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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scala - Spark 2.2 Illegal pattern component: XXX java.lang.IllegalArgumentException: Illegal pattern component: XXX

I'm trying to upgrade from Spark 2.1 to 2.2. When I try to read or write a dataframe to a location (CSV or ... |-- age: long (nullable = false) See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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scala - Spark: long delay between jobs

So we are running spark job that extract data and do some expansive data conversion and writes to several ... enable.summary-metadata to false See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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scala - Getting subclasses of a sealed trait

Is it possible (via macros, some form of Shapeless automagic or otherwise) to obtain a list of the subclasses ... At compile time? At runtime? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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scala - How to restrict actor messages to specific types?

In Akka, is there a way to restrict messages to actors to be of a specific static type other than using the " ... level messages (Exit, etc.). See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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scala - Limits of Nat type in Shapeless

In shapeless, the Nat type represents a way to encode natural numbers at a type level. This is used for example ... Scala compiler to give up? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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scala - "dynamically" creating case classes with macros

I would like to create a macro generated hierarchy of sealed abstract and case classes. There was an example ... to resolve all the classes. See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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639 views

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scala - Why isn't Validation a Monad?

an example use case: def div2(i: Int): Validation[String, Int] = if (i%2 == 0) Validation.success(i/2) else ... I'm using scalaz 7.0.0-M2 See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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scala - assembly-merge-strategy issues using sbt-assembly

I am trying to convert a scala project into a deployable fat jar using sbt-assembly. When I run my assembly ... be doing wrong here? Thanks See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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702 views

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scala - Converting Map[String,Any] to a case class using Shapeless

The question here asks about mapping a case class to a Map[String,Any]. I was wondering what would be the ... should I do that with Shapeless. See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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720 views

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scala - Map versus FlatMap on String

Listening to the Collections lecture from Functional Programming Principles in Scala, I saw this example: scala> val s ... map on the String? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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589 views

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scala - Validation versus disjunction

Suppose I want to write a method with the following signature: def parse(input: List[(String, String)]): ... prefer one over the other? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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750 views

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scala - How do I run an sbt main class from the shell as normal command-line program?

How can I run an sbt app from the shell, so that I can run my app as a normal command-line program (as if ... like there must be a better way. See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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645 views

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scala - How do I get an instance of the type class associated with a context bound?

Note: I'm posing this question to answer it myself, but other answers are welcome. Consider the following simple ... can invoke the plus method? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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694 views

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scala - Spark RDD default number of partitions

Version: Spark 1.6.2, Scala 2.10 I am executing below commands In the spark-shell. I am trying to see ... giving the result as 4 partitions ? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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725 views

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scala - How to reference external sbt project from another sbt project?

I have the following setup of a Scala application and a common core library: root -> /ApplicationA -> / ... of having this project dependency? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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623 views

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scala - Should I use the final modifier when declaring case classes?

According to scala-wartremover static analysis tool I have to put "final" in front of every case classes I create: ... Who is right, and why? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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678 views

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scala - How to split a sequence into two pieces by predicate?

How do I split a sequence into two lists by a predicate? Alternative: I can use filter and filterNot, or write ... general (built-in) method ? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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966 views

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scala - How to force IntelliJ IDEA to reload dependencies from build.sbt after they changed?

I'm using IntelliJ IDEA 13 (Community Edition) with the Scala plugin. My initial import of an existing ... the dependencies from build.sbt? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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717 views

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scala - Split list into multiple lists with fixed number of elements

How to split a List of elements into lists with at most N items? ex: Given a list with 7 elements, create groups of ... ,4), List(5,6,"seven")) See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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502 views

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scala - Why spark-shell fails with NullPointerException?

I try to execute spark-shell on Windows 10, but I keep getting this error every time I run it. I ... value sqlContext import sqlContext.sql ^ See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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586 views

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scala - Instantiating a case class from a list of parameters

Given: case class Foo(a: Int, b: String, c: Double) you can say: val params = Foo(1, "bar", ... .productIterator.toList) res11: Boolean = true See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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545 views

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scala - Package objects

What are package objects, not so much the concept but their usage? I've tried to get an example working and the ... common use? If so, how? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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656 views

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scala - How to wait for several Futures?

Suppose I have several futures and need to wait until either any of them fails or all of them succeed. For ... . How would you implement it? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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651 views

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scala - How to print the contents of RDD?

I'm attempting to print the contents of a collection to the Spark console. I have a type: linesWithSessionId: org. ... I can view its contents? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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