Spray route get response from child actor
The first problem with your code is that you need to forward from the master actor to the child so that the sender is properly propagated and available for the child to respond to. So change this (in RedisActor): summaryActor ! msg To: summaryActor forward msg That's the primary issue....
Convert RDD[Map[String,Double]] to RDD[(String,Double)]
You can call flatMap with the identity function to 'flatten' the structure of your RDD. rdd.flatMap(identity) ...
Is complexity of scala.xml.RuleTransformer really exponential?
This will be not a very rigours analysis, but the problem seems to be with the BasicTransformer's transform(Seq[Node]) method[1]. The child transform method will be called twice for a node which is changed. Specifically in your example all the nodes will be called twice for this reason. And you are...
Scala (Slick) HList splitting to case classes
Using the tuple functionality in shapeless you could do: import shapeless._ import syntax.std.tuple._ case class Foo(a: Int, b: String) val hlist = 1 :: "a" :: 2 :: "b" :: HNil Foo.tupled(hlist.take(2).tupled) ...
Zipping two arrays together with index in Scala?
Simply do: array1.zip(array2).zipWithIndex.map { case ((a, b), i) => (a, b, i) } ...
Passing a function foreach key of an Array
scala,apache-spark,scala-collections,spark-graphx
You're looking for the groupBy function followed by mapValues to process each group. pairs groupBy {_._1} mapValues { groupOfPairs => doSomething(groupOfPairs) } ...
IntelliJ - use imported modules as dependencies like maven projects in Eclipse
eclipse,scala,maven,intellij-idea,sbt
It should work out of box for dependencies, which are imported to the project as modules, no additional settings needed. At least for Java. Just do not run a Maven goal, that would use dependencies from the repository. ...
Instantiate type-parametrized class at runtime with default constructor
The T: ClassTag syntax is called a context bound. It is syntactic sugar for an implicit parameter of type ClassTag[T]. In other words, the following class signatures are equivalent: class Test[T : ClassTag](val value : T) class Test[T](val value: T)(implicit val ev: ClassTag[T]) So the constructor you're looking for expects...
SBT won't pick the specified mainClass [duplicate]
See the answer to How to set main class in build? You'll need something like: mainClass in (Compile, run) := Some("com.abhi.Main1") ...
Specify plugins in Build.scala
No, you can't. You can put it in project/project/Build.scala, which would let you avoid .sbt files, but probably isn't what you want. The thing is, project directory is itself an SBT project, and SBT handles project/*.sbt files and project/project/*.scala files in it just the same as it does on the...
Regex Extraction in Scala
scala> val RecipeItem = """(\S+)\s+(\S+)\s+(.*)""".r RecipeItem: scala.util.matching.Regex = (\S+)\s+(\S+)\s+(.*) scala> val RecipeItem(qty, unit, ingredient) = "1/4 cup chopped green onion" qty: String = 1/4 unit: String = cup ingredient: String = chopped green onion ...
Specs2 with Futures and Case Classes
It looks like the json parsing itself doesn't work. The No usable value for $outer message indicates that reflection might not work with Direction as an inner class of the Specification. So you should move it outside as a top-level class.
Scala which data structure is most efficient for my intended operations?
A simple singly linked list will provide O(1) for the append/discard portions of what you describe. Worst case, if you need to reverse the list at the end before processing, that would be an O(n) operation paid once. Note that if you go down this road, during the accumulation phase...
Scala official documentation Variances example code
A is covariant so a Stack[C] is a subtype of Stack[B] since C is a subtype of B. The signature of push means that if you push a supertype of A, you will receive a Stack[Supertype]. This means pushing a B onto a Stack[C] will return a Stack[B]: val s...
ScalaJson: Traversing JSValue structure (JSONPath syntax) where key might be one of two different strings
json,scala,playframework,playframework-2.0,jsonpath
You can do: val text = (json \\ "name").asOpt[String] .getOrElse((json \\ "title").as[String]) ...
Error while Parsing json into scala case class
The problem is the Java type erasure. classOf[List[MyClass]] at runtime is the same as classOf[List[_]]. That is why Jackson cannot know, which types of the elements to create. Luckily Jackson does support parsing with the JavaType, which describes the types themselves. Here a simple sample in Java: JavaType type =...
Scala wrong forward reference
You have 2 problems in this code: Not explicitly specifying that the empty and cons methods are located in the companion object Stream To fix this you need to either import Stream._ into your class: sealed trait Stream[+A] { import Stream._ def take(n: Int): Stream[A] = this match { case...
How to get notified when unfiltered Netty server actually gets shutdown?
scala,testing,netty,unfiltered
Easy answer: replace your Unfiltered Netty server with a HTTP4S Blaze server. var server: org.http4s.server.Server = null val go: Task[Server] = org.http4s.server.blaze.BlazeBuilder .bindHttp(mockServicePort) .mountService(mockService) .start before { server = go.run } after { server.shutdown.run } There's also an awaitShutdown that blocks until the server shuts down. But since shutdown is...
Expression of type Pointer[Float] doesn't conform to expected type Pointer[Float] in Scala
One of them is probably a java.lang.Float. You probably would need to val aPtr: Pointer[java.lang.Float] = allocateFloats(n) If it is not obvious from the allocateFloats documentation, you can find out what type it is by doing something like allocateFloats(0).getClass.getName which will give you the full name with a [L before...
How to effectively get indices of 1s for given binary string using Scala?
scala,functional-programming,higher-order-functions
You can use a filter and then map to get the index : scala> val s = "10010010" s: String = 10010010 scala> s.zipWithIndex.withFilter(_._1 == '1').map(_._2) res0: scala.collection.immutable.IndexedSeq[Int] = Vector(0, 3, 6) Note: I'm using withFilter and not filter to avoid creating a temporary collection. Or you can use collect,...
My Scala program won't print anything
So, lots of problems. All that stuff you are doing? It's getting done in the constructor of Book, and redone for every instance. Your main method? That's gets compiled to instance method of Book, not a static method, so it does not serve an an entry point for an executable...
How to apply ordering to a Scala Priority Queue?
algorithm,scala,priority-queue
If your elements are Ints, the easiest way to define such an Ordering is by taking a negative of the elements which should be ordered in reverse. You can create the Ordering, using methods provided by the object Ordering, and pass it to the PriotityQueue either explicitly: // I'm using...
Providing implicit value for singletons in Play Json library
json,scala,playframework,scala-macros
For doing that I should define an implicit object like this: implicit object StatusFormat extends Format[Status] { def reads(json: JsValue) = json match { case JsString("Edited") => JsSuccess(Edited) case JsString("NotEdited") => JsSuccess(NotEdited) case _ => JsError("cannot parse it") } def writes(stat: Status) = JsString(stat.toString) } ...
Intellij isn't catching non-exhaustive matches that use functions as guards
This has nothing to do with IntelliJ, but is simply a limitation of the Scala compiler. The warning does not appear whenever a nontrivial if clause is present. You can even try this in the REPL: Option(1) match { case Some(x) => ??? } //warning Option(1) match { case Some(x)...
Scala slf4j dynamic file name
The slf4j library is really an interface to some underlying logging implementation. You would have log4j, logback or some other logging implementation do the heavy lifting, with an adapter jar, as explained in the slf4j documentation. You would then provide the details in the properties file for log4j for instance,...
The right encoding for `URLEncoder.encode`
java,scala,urlencode,url-encoding
You must encode each parameter value. Not the whole URL. val query = "http://localhost:8080/ask?text=" + URLEncoder.encode(text, "UTF-8") ...
Slick column mapper java.sql.Time to Joda LocalTime
The issue is that you are missing the LocalTime => Time conversion for ResultSet -> Scala Collection conversion: MappedColumnType.base[java.sql.Time, org.joda.time.LocalTime] ( time => new LocalTime(time.getTime), localTime => new Time(localTime.toDateTimeToday().getMillis())) ...
Scala try-catch not catching exception
The problem is delayed evaluation from Stream/Iterable. This is very easy to see if you break it up into smaller pieces. scala> Stream("A", "").toIterable.map(_.head) res7: Iterable[Char] = Stream(A, ?) Mapping the Stream to _.head means we're going to treat the strings as collections, and the empty string as a collection...
Play Framework: How to get the current port number
Here below is my solution: def localHost()(implicit request: RequestHeader = null): Host = { def loadFromConfig = { val ssl = config.getBoolean("ssl").getOrElse(false) val host = config.getString("host").getOrElse(EndPoint.DefaultHostName) val port = Play.isTest match { case false => System.getProperty("http.port", null) match { case port if port != null => parse[Int](port).getOrElse(EndPoint.DefaultPort) case _ =>...
How to import directory of .scala files in sbt (unmanaged)?
as written in the manual, you can customize the sources (or source directories) pretty freely. by default, sbt will expect to have scala and java sources under a source directory. you can customize that too. depending on your exact use case, maybe you want these sources under a different configuration?...
Scala running issue on eclipse
to run as scala application, you need to create Scala App and not class In eclipse, package explorer select project/src/package right click new>scala app inform Name e.g. Test and click "finish" select Test.scala right click "run as Scala Application" see results in console window....
Profiling a scala spark application
I would recommend you to use directly the UI that spark provides. It provides a lot of information and metrics regarding time, steps, network usage, etc... You can check more about it here: https://spark.apache.org/docs/latest/monitoring.html Also, in the new Spark version (1.4.0) there is a nice visualizer to understand the steps...
How to search for an Exchange appointment for an iCalUid using Java EWS API?
java,scala,ews,exchange-server-2010,ewsjavaapi
Apparently there is a difference between a meeting and an appointment. Replacing this line: new ExtendedPropertyDefinition(DefaultExtendedPropertySet.Appointment, 0x03, MapiPropertyType.Binary), with new ExtendedPropertyDefinition(DefaultExtendedPropertySet.Meeting, 0x03, MapiPropertyType.Binary) works....
convert scala object to json using json4s
You can make a custom serialiser for your class as it is described in here: https://github.com/json4s/json4s#serializing-non-supported-types Unless you really need it I wouldn't advice to make it part of your toString though; I'd rather advice to have some other object perform the transformation as it will make things a bit...
Difficulty with SBT
The %% in the dependency automatically appends a _2.XX scala version to your artifact id. It makes scala dependencies easier to manage, but you can't use it with java dependencies like apache httpcomponents. Instead just use %: "org.apache.httpcomponents" % "httpclient" % "4.5" ...
implicit resolution for a function argument
You can overcome this by passing a function that calls mergesort to generalizedMergeSort. This call will capture the implicit Ordering: def mergesort[A: Ordering](as: List[A]): List[A] = { generalizedMergeSort(as, mergesort(_: List[A])) } mergesort(_: List[A]) is a closure function of type List[A] => List[A], which calls mergesort with its argument, and the...
How to un-nest a spark rdd that has the following type ((String, scala.collection.immutable.Map[String,scala.collection.immutable.Map[String,Int]]))
Fairly straightforward using a for-comprehension and some pattern matching to destructure things: val in = List((5, Map ( "ABCD" -> Map("3200" -> 3, "3350.800" -> 4, "200.300" -> 3))), (1, Map ("DEF" -> Map("1200" -> 32, "1320.800" -> 4, "2100" -> 3)))) case class Thing(a:Int, b:String, c:String, d:Int) for {...
Scala unapplySeq extractor syntax
scala,pattern-matching,scala-2.11
The equivalent non-infix version is: xs match { case List(x, _, _) => "yes" case _ => "no" } Scala specification says: An infix operation pattern p;op;q is a shorthand for the constructor or extractor pattern op(p,q). The precedence and associativity of operators in patterns is the same as in...
Scala: Convert a csv string to Array
arrays,string,scala,split,scala-collections
Use split with -1 argument. string.split(",",-1) This behavior comes from Java (since Scala uses Java Strings). Here is the documentation directly out of Javadoc. ...
Dynamic growth of `Actor`s vs Primal creation
If you'll crate one actor per job, you'll get parallel processing of messages, while if you'll create actor on initialization, you messages of the same type will be processed one by one. Usually, you shouldn't use actors for parallel execution of your programm, their task is to process common resources,...
Play Framework Form Error Handling
scala,playframework,playframework-2.3,playframework-2.4
Have a look at play documentation: Writing your own field constructor. You can check on errors with @if(elements.hasErrors) within the template of your custom field constructor. <div class="input-with-label text-left @if(elements.hasErrors){field-error}"> ... Edit: You can pass the error state of your field via the args parameter to your input. From the...
Is there any scala library that treat tuples as monads
Yep, Scalaz provides monad instances for tuples (up to Tuple8): import scalaz.std.anyVal._, scalaz.std.tuple._, scalaz.syntax.monad._ scala> type IntTuple[A] = (Int, A) defined type alias IntTuple scala> pair >>= (a => (a+1).point[IntTuple]) res0: (Int, String) = (2,as1) scala> for (p <- pair) yield (p + 1) res1: (Int, String) = (2,as1) (Note...
refer to scala function by name?
yyy is not a function, it's a method. You have to either convert it to a function using η-expansion yyy _ or use a function in the first place val yyy = (c: Char) => c.toUpper // or val yyy: Char => Char = c => c.toUpper // or val...
Scala string replacement of entire words that comply with a pattern
string,scala,scala-collections,scala-string
You can use the \bth\w* pattern to look for words that begin with th followed by other word characters, and then replace all matches with "123" scala> "this is the example, that we think of, anne hathaway".replaceAll("\\bth\\w*", "123") res0: String = 123 is 123 example, 123 we 123 of, anne...
How to define a Regex in StandardTokenParsers to identify path?
regex,scala,parsing,lexical-analysis
In a double quoted string backslash is an escape character. If you mean to use the literal backslash in a double quotes string you must escape it, thus "\d" should be "\\d". Furthermore you do not need to escape the regex dot within a character class, since dot has no...
Convert existing SBT Scala application to Play
UPDATE Answer: Start in your Project File the activator Then start the web App with run and open the browser with http://localhost:9000 This loads all the dependencies and compiles the Scala Play Application. This should correct your IDEA Ide Problems about missing Dependencies. In Scala Play 2.4 you can choose...
Wait for a list of futures with composing Option in Scala
Several points: The first problem at (1) is that you don't handle the case where findLookupId returns None. You need to decide what to do in this case. Fail the whole process? Exclude that file from the list? The second problem at (1) is that findIssues will itself return a...
Pattern Matching chooses the wrong case if used with ClassTag
I definitely think it is a compiler bug. def test1[T:ClassTag](v:In[T]) = { val t1 = classTag[T] == classTag[T1] val t2 = classTag[T] == classTag[T2] println(v match { case x : VIn[[email protected]] if t1 => "T1" case y : VIn[[email protected]] if t2 => "T2" }) v match { case x:In[T1] if...
Scala rep separator for specific area of text
I guess you are using the RegexParsers (just note that it skips white spaces by default). I'm assuming that it ends with "\n\n--open--" instead (if you can change that otherwise I'll show you how to modify the repsep parser). With this change we see that the text has the following...
How does Scala specs2 sequential keyword work
sequential is not variable; It is a method call. Take a look at the specs2 code base: def sequential: Boolean = execute.sequential And why wouldn't it compile? specs2 is mature testing framework....
scala minecraft forgemod 'gradle runClient' gives runtime exception
scala,gradle,akka,minecraft,minecraft-forge
I figured it out. application.conf had: provider = "akka.cluster.ClusterActorRefProvider" cluster isn't part of akka-actor its part of akka-cluster. I've switched to provider = "akka.actor.LocalActorRefProvider" That works now. The other option is to add akka-cluster to the dependancies list. If your actually trying to use ClusterActorRefProvider...
PlayFramework: value as is not a member of Array[Byte]
You are calling the as method on the wrong object. It should look as follows: Ok(bytOfImage).as("image/jpg") ...
Calling the default constructor in scala
The code you have provided does not compile. this() is not valid because there is no default constructor for the class A. this.p1 = somevalue is wrong, because there is not member p1. Correctly it might look like this: class A(p1: Type1, p2: Type2) { def this(p1: Type1) { this(p1,...
Scala child class fails to recognize a generic map in the parent?
Scala maps are Invariant in the key type, thus Map[String, _] has no type relation to Map[Any, _] Map documentation trait Map[A, +B] Note that there is no variance marker on A, therefore it's invariant. You could parameterize it: abstract class MultipleOutputWriter[K, V <: OutputStream] { protected def writers: collection.mutable.Map[K,...
Spark-submit class not found exception
The thing is that we have to submit the class file to the spark cluster whom we want to execute or will take use as a supporting file, so follow these steps - Create a jar file of this class -> In eclipse you can export this class as a...
Scala: Better way for String Formatting to PhoneNumber and using Java's MessageFormat
scala,formatting,messageformat
Since we don't like nulls I wrapped your possibly-null value into an Option. If the option have a value I create the array from it and then pass it into the formatter. If it didn't have a value I give it the value "". val szPhoneFrmt = new MessageFormat("1 {0}{1}-{2}")...
Scala - increasing prefix of a sequence
You can take your solution, @Samlik, and effectively zip in the currentElement variable, but then map it out when you're done with it. sequence.take(1) ++ sequence.zip(sequence.drop(1)). takeWhile({case (a, b) => a < b}).map({case (a, b) => b}) Also works with infinite sequences: val sequence = Seq(1, 2, 3).toStream ++ Stream.from(1)...
Java encode raw bytes into image simple image fomat/ file
java,image,scala,image-processing
Before you write it out with ImageIO, create a BufferedImage first. It can be as simple as using the setRGB methods, and has the added benefit of allowing you to observe the image before writing it out.
SCALA: change the separator in Array
Your question is unclear, but I'll take a shot. To go from: val x = Array("a","x,y","b") to "a:x,y:b" You can use mkString: x.mkString(":") ...
How to convert Map values to a set in scala
You're probably looking for the toSet method on TraversableOnce: def toSet[B >: A]: immutable.Set[B] Converts this traversable or iterator to a set. Note: will not terminate for infinite-sized collections. returns a set containing all elements of this traversable or iterator. You can use that together with the values or valuesIterator...
Scala, how to set up a node class?
Algebraic data types break encapsulation by exposing the internal representation of the type publicly. When you take a functional programming point of view with regards to your design, then mutable state is not something that is a concern normally. Therefore, exposing the internal representation is not really a big deal...
Retrieving TriangleCount
scala,apache-spark,spark-graphx
triangleCount counts number of triangles per vertex and returns Graph[Int,Int], so you have to extract vertices: scala> graph.triangleCount().vertices.collect() res0: Array[(org.apache.spark.graphx.VertexId, Int)] = Array((1,1), (3,1), (2,1)) ...
Shuffled vs non-shuffled coalesce in Apache Spark
scala,apache-spark,bigdata,distributed-computing
shuffle=true and shuffle=false aren't going to have any practical differences in the resulting output since they are both going down to a single partition. However, when you set it to true you will do a shuffle which isn't of any use. With shuffle=true the output is evenly distributed amongst the...
How to reuse MappedColumnType in Table classes?
You could, for example, move it to a trait like this: trait DateColumnMapper extends HasDatabaseConfig[JdbcProfile] { protected val dbConfig: DatabaseConfig[JdbcProfile] import driver.api._ implicit val dateColumnType = MappedColumnType.base[Date, Long]( d => d.getTime, d => new Date(d) ) } Then you can include this trait in whatever DAO or db component you...
Is this definition of a tail recursive fibonacci function tail-recursive?
scala,f#,functional-programming,tail-recursion,continuation-passing
The second call to go on line 4 is not in tail position, it is wrapped inside an anonymous function. (It is in tail position for that function, but not for go itself.) For continuation passing style you need Proper Tail Calls, which Scala unfortunately doesn't have. (In order to...
Listing Scala nested objects methods through Reflection
When you use typeOf[m.type] in the foreach, you are getting the type of the instance of m, which is of type Symbol, so that isn't what you want. You can use the info method on Symbol to get the Type instance you are looking for: typeOf[BusinessFacade.type].members.filter(_.isModule).foreach { m => println(m.name...
Access key from mapValues or flatMapValues?
In this case you can use mapPartitions with the preservesPartitioning attribute. x.map((it => it.map { case (k,rr) => (k, someFun(rr, k)) }), preservesPartitioning = true) You just have to make sure you are not changing the partitioning, i.e. don't change the key....
Scala factory method with generics
0__'s answer is almost there. If you make the Factory[A[_,_]] a typeclass you are all set. Here is an example with the names standardised: // enable higher kinded types to prevent warnings import scala.language.higherKinds // our case classes case class A[A1,A2](j:A1, k:A2) case class B[B1,B2](j:B1, k:B2) // Define our factory...
Solving maze with Backtracking
I'm only going to comment on findStart for now. There are two things wrong with findStart: findStart is recursively called on every adjacent cell. Unfortunately, the neighbouring cell of any neighbour is the cell itself. The function never checks if you can actually walk on a given cell (I assume...
Scala first program issue
scala,recursion,case,frequency
Cons operator (::) is an infix operator so if you want to get a type of List[T] and not List[List[T]] then you should write freq(c, y.filter(_ == c),(count(c,y),c)) :: list) ...
Scodec: Coproducts could not find implicit value for parameter auto: scodec.codecs.CoproductBuilderAuto
The code that is there now needs two minor changes: The Message trait must be sealed, or otherwise, Shapeless will not provide a Generic.Aux[Message, SomeCoproduct] instance. The call to Codec.coproduct[Message] must be after all the subtypes are defined. Moving the companion to the end of the file is sufficient. With...
Type to impose required constrains on a double
This enabled basic run time checks: trait RangeBound type Probability = Double with RangeBound implicit def makeProb(p: Double): Probability = { assert (p >= 0.0 && p <= 1.0) p.asInstanceOf[Probability] } implicit val probabilityOrdering = Ordering.Double.asInstanceOf[Ordering[Probability]] ...
How to change HTTP status code of AsyncResult using Scalatra
After some digging in the Scalatra FutureSupport mixin, I found: implicit val response: HttpServletResponse = scalatraContext.response defined as a member of AsyncResult which allows me change the status code of the HTTP request inside the onComplete callback....
Scala - Having trouble with filter & vals
The issue is simply that isAllergicTo is implemented incorrectly. If you fix it, you won't need to change score at all. As a hint: think about binary representation of score.
ZipList with Scalaz
pure for zip lists repeats the value forever, so it's not possible to define a zippy applicative instance for Scala's List (or for anything like lists). Scalaz does provide a Zip tag for Stream and the appropriate zippy applicative instance, but as far as I know it's still pretty broken....
IntelliJ Plugin written in Scala - doesn't work
java,scala,intellij-idea,sbt,intellij-plugin
Okay, it seems that this question has been answered here: https://devnet.jetbrains.com/message/5546566#5546566...
Scala - Option Type Var Manipulation
var balance = Some(0) is inferred to be of type Some[Int], when you need to tell this explicitly that it's of type Option[Int]: var balance: Option[Int] = Some(0) Then balance will be able to take in either Some(0) or None. By the way, it's sometimes a good practice to always...
Limit An Iteration By A Predicate
I'm not sure if such a thing is possible directly with lists, possibly because you could create a potential infinite number of values. That's why you need to provide a len parameter. But yes, you can create a Stream that generates values followed by a takeWhile. If you know that...
How to instantiate lexical.Scanner in a JavaTokenParsers class?
The JavaTokenParsers does not implement the Scanners trait. So you would need to extends also from this trait (or a trait that extends it) in order to have access to this class. Unless your expr parser accepts the Reader as a parameter (not from its apply method), you'd need to...
How to unmarshall akka http request entity as string?
Your code should be okay provided you have the right implicits in scope. If you have an implicit FlowMaterializer in scope then things should work as expected as this code that compiles shows: import akka.http.scaladsl.server.Route import akka.actor.ActorSystem import akka.stream.ActorFlowMaterializer import akka.http.scaladsl.model.StatusCodes._ import akka.http.scaladsl.server.Directives._ import akka.stream.FlowMaterializer implicit val system = ActorSystem("test")...
Creating a new File in Scala with PrintWriter
There is no need to store the file in an Option. Using Option.get is usually a sign that you're doing something wrong, because you assume the option is set. Then if you already have a string, there is no reason why you would want to scan it, write into another...
How to generalize the round methods
You could use the Numeric type class def round[T](input: T, scale: Int, f: BigDecimal => T)(implicit n: Numeric[T]): T = { f(BigDecimal(n.toDouble(input)).setScale(scale, RoundingMode.HALF_UP)) } Which can be used as: round(5.525, 2, _.doubleValue) res0: Double = 5.53 round(123456789L, -5, _.longValue) res1: Long = 123500000 Another way might be to create a...
Like clause not working with int column in slick
Can you post your Status class definitation .If code is type column[Int] your code should be giving error as like works on column[string]. The below snippet works for doing a like on integer field. class Coffees(tag: Tag) extends Table[(String, Int)](tag, "COFFEES") { def name = column[String]("NAME") def status = column[Int]("STATUS")...
Combining Spray Routing + Actor Pattern Matching
You can compose PartialFunctions like Receive with PartialFunction.orElse: class TransformationFrontend extends Actor { // ... def myReceive: Receive = { case job: TransformationJob => // ... // ... } def defaultRoute: Route = get { // ... } override def receive: Receive = runRoute(defaultRoute) orElse myReceive } That said, it...
Spray microservice assembly deduplicate
scala,sbt,akka,spray,microservices
The issue as it seems transitive dependency of the dependency is resulting with two different versions of metrics-core. The best thing to do would be to used the right library dependency so that you end up with a single version of this library. Please use https://github.com/jrudolph/sbt-dependency-graph , if it is...
Akka actorSelection vs actorOf Difference
actorOf is used to create new actors by supplying their Props objects. actorSelection is a "pointer" to a path in actor tree. By using resolveOne you will get actorRef of already existing actor under that path - but that actorRef takes time to resolve, hence the Future. Here's more detailed...
Implementing map on a tree using fold
Try to write your last line as def map(tree:Tree[Int])(f:Int=>Int) : Tree[Int] = fold(tree , EmptyTree:Tree[Int])((l,x,r) => Node(f(x),l,r)) Scala's type inference is very limited compared to haskell, in this case it tries to infere type of fold from it's arguments left to right, and incorectly decides that result type of fold...
Returning NonEmptyList `sealed trait`'s w/ scalaz.Validation
You don't need to describe how to combine errors—that's taken care of by ValidationNel's applicative functor via the Semigroup instance for NonEmptyList. The argument to the applicative builder should instead indicate how to create people: def makePerson(name: String, age: Int, gender: Gender): Validation[NonEmptyList[InvalidPersonError], Person] = (validateName(name) |@| validateAge(age))(Person(_, _, gender))...
Scala function partial application
All you need to do is change myCustomConcat(_) to myCustomConcat _, or indeed just myCustomConcat What you are doing isn't exactly partial application - it's just using a method as a function value. In some cases (where a function value is expected) the compiler will work out what you mean,...
How to use the Akka ask pattern without blocking
scala,asynchronous,akka,future
You don't want to block, by waiting on the response of the actor, so you are using Future correctly. The code in the onComplete function is executed, when your actor responds with the list. And since you don't want to block and handle it asynchronously, your last println statement is...
How to make an sbt task run on under multiple scopes
This is a topic discussed in Plugins Best Practices, specifically in the Configuration advices section. Provide raw settings and configured settings If your plugin is ObfuscatePlugin, provide baseObfuscateSettings that's not scoped in any configuration: lazy val baseObfuscateSettings: Seq[Def.Setting[_]] = Seq( obfuscate := Obfuscate((sources in obfuscate).value), sources in obfuscate := sources.value...
Akka consolidate concurrent database requests
Following the reading of Effective Akka by Jamie Allen, I am going to attempt to apply his "Cameo" pattern suggestion. Slideshare: http://www.slideshare.net/shinolajla/effective-akka-scalaio Github: https://github.com/jamie-allen/effective_akka I think what I created will work, but doesn't sound like the best approach based on Jamie's comments in his talks. I will update / edit...
Collapse similar case statements in Scala
scala,functional-programming,pattern-matching
You can use a custom extractor to abstract the matching part away from the logic part: object Leafed { def unapply(tree: Tree) = tree match { case Node(Leaf(_, _), parent, qux) => Some((parent, qux)) case Node(parent, Leaf(_, _), qux) => Some((parent, qux)) case _ => None } } And then...
Implicit Generic.Aux missing on conversion from Shapeless HList to case class
scala,shapeless,type-level-computation
You're very close. The problem is that Scala isn't going to propagate implicit requirements up the call chain automatically for you. If you need a Generic[A, T] instance to call convert, then you'll have to make sure that one's in scope every time you call convert convert. If A and...
Future yielding with flatMap
There's no reason to flatMap in the yield. It should be another line in the for-comprehension. for { a <- fa b <- fb c <- fc d <- f(a, b, c) } yield d I don't think it can get more concise than that....
Do scala classes have “self” like Python?
Yes, and it is implemented as this, like in Java or C++. You don't have to pass it explicitly at the method call, like in python. Then what you're creating in your example are instance variables, also called members. They belong to instances of the class, i.e. they are not...
For-Comprehension Example with \/ and IO
You can't combine String \/ A and IO[A] computations like this—if you desugared your for-comprehension you'd see that the types just don't work out. If you really wanted to, you could use a monad transformer to compose the two monads. Suppose we have the following setup, for example (which is...
Preventing a class instantiation in Scala using Factory Pattern [duplicate]
The conventional way to write a factory in Scala is to define an apply method on the companion object. Here's an example using Either (because null is never/rarely used in Scala, and exceptions are ugly): class A private (n: Int) { override def toString = s"A($n)" } object A {...