scala,enums,metaprogramming,value-type,scala-macros , How to use macros for concise, type-safe, unboxed enum?


How to use macros for concise, type-safe, unboxed enum?

Question:

Tag: scala,enums,metaprogramming,value-type,scala-macros

I am learning Scala macros and thinking of this as an exercise.

Is it possible to use Scala macros to write down something like this (maybe not exactly this concrete syntax, but something without boilerplate)

enum DayOfWeek = Monday | Tuesday | Wednesday | Thursday | Friday | Saturday | Sunday

And have it macro-expand to something like this?

class DayOfWeek private (val ord: Int) extends AnyVal {

  def next: DayOfWeek = ord match {
    case 0 => Tuesday
    case 1 => Wednesday
    case 2 => Thursday
    case 3 => Friday
    case 4 => Saturday
    case 5 => Sunday
    case _ => throw Error("Sunday does not have next")
  }

  override def toString: String = ord match {
    case 0 => "Monday"
    case 1 => "Tuesday"
    case 2 => "Wednesday"
    case 3 => "Thursday"
    case 4 => "Friday"
    case 5 => "Saturday"
    case _ => "Sunday"
  }
}

object DayOfWeek {
  def count = 7

  val Monday    = new DayOfWeek(0)
  val Tuesday   = new DayOfWeek(1)
  val Wednesday = new DayOfWeek(2)
  val Thursday  = new DayOfWeek(3)
  val Friday    = new DayOfWeek(4)
  val Saturday  = new DayOfWeek(5)
  val Sunday    = new DayOfWeek(6)
}

And maybe not neccessarily using consecutive integers to represent enumerations. For example, with some flag, we can have enums with no more than 32 or 64 alternatives be represented as bits, and have an efficient unboxed implementation of its set (as Int or Long).

Another compelling reason for having something like this is we can also customize different aspects for slightly different flavors of enum, for example, above, we could have provided some parameter to enum so that next cycles around instead of erroring on Sunday.


Answer:

So from Rex Kerr's answer, I figured out I needed to look at StaticAnnotation, then made a simplistic implementation here. For my personal usage, I don't see the need to maintain nice interoperability with Java, but there I go.

(Allow me to be shameless and accept my own answer ^^)


Related:


Type to impose required constrains on a double


scala,implicit-conversion
I would like to have a run time check on a Double, without having to scatter the check all over my code. I thought that defining an implicit class would do the job, something on the line: implicit class Probability(val x: Double) { require(x >= 0.0 && x <= 1.0,...

Preventing a class instantiation in Scala using Factory Pattern [duplicate]


scala,factory-pattern
This question already has an answer here: How to check constructor arguments and throw an exception or make an assertion in a default constructor in Scala? 2 answers Suppose that I have the following class defined in Scala: class ClassA(val n: Int) { ... } I want to limit...

Scala string replacement of entire words that comply with a pattern


string,scala,scala-collections,scala-string
In a string, how to replace words that start with a given pattern ? For instance replace each word that starts with "th", with "123", val in = "this is the example, that we think of" val out = "123 is 123 example, 123 we 123 of" Namely how to...

Collapse similar case statements in Scala


scala,functional-programming,pattern-matching
Is there an elegant way to do something like the following example using just one case statement? foobar match { case Node(Leaf(key, value), parent, qux) => { // Do something with parent & qux } case Node(parent, Leaf(key, value), qux) => { // Do something with parent & qux (code...

refer to scala function by name?


scala
Here is another stupid scala question regarding functions as first class objects in Scala. I'm very sorry if this is a repeat, as it probably is. In Python, Lisp, Perl, Scheme, etcetera I'm used to creating function values and assigning them names and passing them around to other functions, like...

Same enum values for multiple columns


ruby-on-rails,ruby,enums
I need to do something like this: class PlanetEdge < ActiveRecord::Base enum :first_planet [ :earth, :mars, :jupiter] enum :second_planet [ :earth, :mars, :jupiter] end Where my table is a table of edges but each vertex is an integer. However, it seems the abvove is not possible in rails. What might...

SCALA: change the separator in Array


arrays,string,scala,delimiter
I have an Array like this. scala> var x=Array("a","x,y","b") x: Array[String] = Array(a, x,y, b) How do I change the separator comma in array to a :. And finally convert it to string like this. String = "a:x,y:b" My aim is to change the comma(separators only) to other separator(say,:), so...

Enums in C++ - how does a function assign a value


c++,enums
I'm trying to understand a C++ code given here. Apologies if the question is too basic, but I am not a regular C++ coder. Here are two Enums defined in the code and a a few functions that operate on the Enums: ... enum Value { VALUE_ZERO = 0, VALUE_DRAW...

How to use the Akka ask pattern without blocking


scala,asynchronous,akka,future
Hi I have a actor which is responsible for fetching data from a database, turning it into a list and sending it back to the sender. I am using the ask pattern to receive response from my actor, because I don't want to use await.result because this approach will block...

Scala rep separator for specific area of text


scala,parser-combinators
Imaging i've got following: --open Client: enter Nick Age 28 Rosewell, USA Client: enter Maria Age 19 Cleveland, USA --open-- I need a result close to the following: List(List(Nick, Age 28, Rosewell), List(Maria, Age19, Cleveland)) It can be as many clients inside open body as you can imagine, so the...

Spray route get response from child actor


scala,akka,spray
I am trying to figure out how I can setup a Master Actor that calls the appropriate children, in support of some spray routes where I am trying to emulate db calls. I am new to akka / spray, so just trying to gain a better understanding of how you...

Scala, how to set up a node class?


scala
Is there a resource for setting up a generic class in Scala? I am trying to make a segment tree where the node class contains a pointer to a left node, a pointer to a right node, and the value of the sum (and some other stuff but I think...

Scala running issue on eclipse


eclipse,scala
I configured everthing within eclipse for scala. I create a snippet to show you the issue, i can't see in run options run as scala application, i also tried to find my main class under build configuration option but i can't find it. How i can solve it?...

PlayFramework: value as is not a member of Array[Byte]


scala,playframework
I want to make file download from a database using Play framework. But when I use this code I get this message: value as is not a member of Array[Byte] And if I change Ok(bytOfImage.as("image/jpg")) to Ok(bytOfImage) it works good but I get a file with a name: secondindex without...

Scala first program issue


scala,recursion,case,frequency
I have just started to learn Scala after some experience with functional programming in other languages. def freq(c:Char, y:String, list:List[(Char,Int)]): List[(Char,Int)] = list match{ case _ => freq(c, y.filter(_ == c), list :: List((count(c,y),c))) case nil => list } In the above code I am getting an error when trying...

implicit resolution for a function argument


scala,implicit,context-bound
I tried to implement mergesort in Scala. I got to the following: def mergeSort[A: Ordering](as: List[A]): List[A] = as match { case Nil => as case head :: Nil => as case _ => { val (l, r) = split(as) merge(mergeSort(l), mergeSort(r)) } } def split[A](as: List[A]): (List[A], List[A]) =...

How to define a Regex in StandardTokenParsers to identify path?


regex,scala,parsing,lexical-analysis
I am writing a parser in which I want to parse arithmetic expressions like: /hdfs://xxx.xx.xx.x:xxxx/path1/file1.jpg+1 I want to parse it change the infix to postfix and do the calculation. I used helps from a part of code in another discussion as well. class InfixToPostfix extends StandardTokenParsers { import lexical._ def...

Access key from mapValues or flatMapValues?


scala,apache-spark
In Spark 1.3, is there a way to access the key from mapValues? Specifically, if I have val y = x.groupBy(someKey) val z = y.mapValues(someFun) can someFun know which key of y it is currently operating on? Or do I have to do val y = x.map(r => (someKey(r), r)).groupBy(_._1)...

ZipList with Scalaz


list,scala,scalaz,applicative
Suppose I have a list of numbers and list of functions to apply to numbers: val xs: List[Int] = List(1, 2, 3) val fs: List[Int => Int] = List(f1, f2, f3) Now I would like to use an Applicative to apply f1 to 1, f2 to 2, etc. val ys:...

IntelliJ - use imported modules as dependencies like maven projects in Eclipse


eclipse,scala,maven,intellij-idea,sbt
I've been asking myself this question for a couple of years but never really found the solution. I used to work with eclipse (on maven java projects). I could import a project -let's call it 'proj-A'- and if one of proj-A's dependencies was found in the workspace with the same...

Scala unapplySeq extractor syntax


scala,pattern-matching,scala-2.11
I (inadvertently) came across a bit of pattern matching syntax I did not expect to compile and now cannot figure out. It appears related to unapplySeq. Note the case x List(_,_) part in this simple example: val xs = List(1, 2, 3) //> xs : List[Int] = List(1, 2, 3)...

Is this definition of a tail recursive fibonacci function tail-recursive?


scala,f#,functional-programming,tail-recursion,continuation-passing
I've seen around the following F# definition of a continuation-passing-style fibonacci function, that I always assumed to be tail recursive: let fib k = let rec fib' k cont = match k with | 0 | 1 -> cont 1 | k -> fib' (k-1) (fun a -> fib' (k-2)...

Convert RDD[Map[String,Double]] to RDD[(String,Double)]


scala,apache-spark,rdd
I did some calculation and returned my values in a RDD containing scala map and now I want to remove this map and want to collect all keys values in a RDD. Any help will be appreciated....

How to effectively get indices of 1s for given binary string using Scala?


scala,functional-programming,higher-order-functions
Suppose we have a binary string such as 10010010. All I want is a function returning indices of 1s for that string: indicesOfOnes("10010010") -> List(0, 3, 6) indicesOfOnes("0") -> List() And what I implemented is: def indicesOfOnes(bs: String): List[Int] = { val lb = ListBuffer[Int]() bs.zipWithIndex.foreach { case (v, i)...

Passing a function foreach key of an Array


scala,apache-spark,scala-collections,spark-graphx
I have an array like that : val pairs: Array[(Int, ((VertexId, Seq[Int]), Int))] which generates this output : (11,((11,ArraySeq(2, 5, 4, 5)),1)) (11,((12,ArraySeq(7, 7, 8, 2)),1)) (11,((13,ArraySeq(5, 9, 8, 7)),1)) (1,((1,ArraySeq(1, 2, 3, 4)),1)) (1,((4,ArraySeq(1, 5, 1, 1)),1)) I want to build a Graph for each pairs._1. That means for...

Future yielding with flatMap


scala
Given Futures fa, fb, fc, I can use f: Function1[(A,B,C), Future[D]], to return a Future[D] either by: (for { a <- fa b <- fb c <- fc } yield (a,b,c)).flatMap(f) which has the unenviable property of declaring the variables a,b,c twice. or a.zip(b).zip(c).flatMap{ case (a, (b, c)) => f(a,...

Implementing map on a tree using fold


scala,haskell
I am trying to implement a map using fold. I could do so in Haskell data Tree a = EmptyTree | Node a (Tree a) (Tree a) deriving (Show) foldTree :: Tree a -> b -> (b -> a -> b -> b) -> b foldTree EmptyTree d _ =...

Scodec: Coproducts could not find implicit value for parameter auto: scodec.codecs.CoproductBuilderAuto


scala,scodec
I am trying to define an Scodec coproduct codec for communicating with an EELink GPS. Here is the code: import scodec.Codec import scodec.bits.ByteVector import scodec.codecs._ trait Message object Message { implicit val discriminated: Discriminated[ Message, Int ] = Discriminated(uint8) val codec: Codec[ Message ] = Codec.coproduct[ Message ].discriminatedByIndex(uint8) } case...

Scala - Option Type Var Manipulation


scala,scala-option
I am working on an online exercise practicing Options and threads, both of which I have very little experience. The online exercise comes with a test suite, so right now I am trying to get my Option test cases to pass before I move on to the thread test cases....

How to unmarshall akka http request entity as string?


json,scala,akka-http
I'm trying to unmarshall request payload as string, but for some reason it's failing. My code: path("mypath") { post { decodeRequest { entity(as[String]) {jsonStr => //could not find implicit value for...FromRequestUnmarshaller[String] complete { val json: JsObject = Json.parse(jsonStr).as[JsObject] val jsObjectFuture: Future[JsObject] = MyDatabase.addListItem(json) jsObjectFuture.map(_.as[String]) } } } } } In...

Solving maze with Backtracking


scala,backtracking,maze
I am trying to solve a maze with scala using backtracking. My problem is that I keep on getting StackOverflow errors. I have tried qite alot of things but I always end up with a StackOverflow. findStart() and the getWay() show two approaches I have used. I know that it...

How to get Display Text from EnumDropDownListFor on post?


c#,asp.net-mvc,enums,html-helper
Is there a way to get DisplayName (Text) from EnumDropDownListFor helper for enum? Enum: public enum PartnersGroup { [Display(Name="Partner_SystemsGroup",ResourceType=typeof(Global) )] SystemsGroup, [Display(Name="Partner_SoftwarePartners",ResourceType=typeof(Global))] SoftwarePartners, [Display(Name="Partner_IntegrationPartners",ResourceType=typeof(Global))] IntegrationPartners, } Model public class Partner { public PartnersGroup PartnersGroup { get; set; } } Controller // GET: Partners/Create public ActionResult Create() { ---- return...

Scala (Slick) HList splitting to case classes


scala,slick
currently I have a HList with more than 22 fields and now I want to split it to 2-3 case classes, is there an easy functional way to do it? Currently I use the following syntax: CaseClass1(c.head, c.tail.head, c.tail.tail.head, etc...) However that doesn't seem to be right since I have...

Implementing enum-based singleton


java,enums,singleton
J. Bloch in his Effective Java suggests we use an enum-based singleton implementation. For instance: public enum Application { INSTANCE; //methods, fields } This implementation is nice in the case of serialization because enums provide us with the capability of serialization by default (and we don't have to be afraid...

Scala slf4j dynamic file name


scala,logging,slf4j
I just successfully added Grizzled-SLF4J logger to my project using this link http://alvinalexander.com/scala/how-to-log-output-file-grizzled-slf4j-scala-simplelogger.properties But using this properties, there is no option to create dynamic file name: org.slf4j.simpleLogger.logFile = /tmp/myapp.log org.slf4j.simpleLogger.defaultLogLevel = info org.slf4j.simpleLogger.showDateTime = true org.slf4j.simpleLogger.dateTimeFormat = yyyy'/'MM'/'dd' 'HH':'mm':'ss'-'S org.slf4j.simpleLogger.showThreadName = true...

My Scala program won't print anything


scala
Basically I'm trying to write a program that has a list of books and authors and then prints 2 things out, res1, which should print the book titles of the authors that have the name "Andrei" in it and res2 that should print the book titles that have the string...

Spray microservice assembly deduplicate


scala,sbt,akka,spray,microservices
I'm using this template to develop a microservice: http://www.typesafe.com/activator/template/activator-service-container-tutorial My sbt file is like this: import sbt._ import Keys._ name := "activator-service-container-tutorial" version := "1.0.1" scalaVersion := "2.11.6" crossScalaVersions := Seq("2.10.5", "2.11.6") resolvers += "Scalaz Bintray Repo" at "https://dl.bintray.com/scalaz/releases" libraryDependencies ++= { val containerVersion = "1.0.1" val configVersion = "1.2.1"...

How to reuse MappedColumnType in Table classes?


scala,playframework,slick
The use of MappedColumnType is demonstrated in this example: https://github.com/playframework/play-slick/blob/master/samples/computer-database/app/dao/ComputersDAO.scala#L21 How can I reuse dateColumnType in another table class?...

How to get notified when unfiltered Netty server actually gets shutdown?


scala,testing,netty,unfiltered
I have an Unfiltered Netty server that I need to shutdown and restart after every test. val mockService = unfiltered.netty.Server.http(mockServicePort).handler(mockServicePlan) before { proxyServer.start() } after { proxyServer.stop() } Currently, this is not working, and I am fairly certain that is because the stop() function is non-blocking and so the following...

Zipping two arrays together with index in Scala?


arrays,scala,zip
I have two arrays populated with integers. They are the same size (val array1 and val array2). I want to fuse them together into tuples with their index as the third element. For example if we have val array1 = Array(5,2,6,2) and val array2 = Array(9,8,3,4) then I want to...

Difficulty with SBT


scala,sbt
I have started using scala sbt pretty recently. One thing which I find very difficult with SBT is to find the right dependency signature. for example, I am reading a blog which was written in 2014 and it says add the following libraryDependencies "net.liftweb" %% "lift-son" % "2.5+" "org.apache.httpcomponents" %%...

How to generalize the round methods


scala
I have the following four methods, using BigDecimal to round a number: private def round(input: Byte, scale: Int): Byte = { BigDecimal(input).setScale(scale, RoundingMode.HALF_UP).byteValue() } private def round(input: Short, scale: Int): Short = { BigDecimal(input).setScale(scale, RoundingMode.HALF_UP).shortValue() } private def round(input: Int, scale: Int): Int = { BigDecimal(input).setScale(scale, RoundingMode.HALF_UP).intValue() } private def...

Play Framework Form Error Handling


scala,playframework,playframework-2.3,playframework-2.4
This is my view file containing the form that has to filled in by the user: @helper.form(call) { @helper.input(resumeForm("surname"), '_label -> "Surname") { (id, name, value, args) => <input name="@name" type="text" value="@value" placeholder="Enter your surname"> } } This is my custom field constructor: @(elements: helper.FieldElements) @if(!elements.args.isDefinedAt('showLabel) || elements.args('showLabel) == true)...

Implicit Generic.Aux missing on conversion from Shapeless HList to case class


scala,shapeless,type-level-computation
I just recently started learning scala and today I decided I wanted to write a CSV parser that would load nicely into case classes but store the data in rows (lists) of Shapeless's HList object so that I could get some exposure to type-level programming. Here's what I have so...

Operand order in Scala List.prepend (::)


list,scala,operators
Odersky has brilliantly optimized Java syntax, enabling object calls without dots and parenthesis. I.e. instead of list.prepend(item), you now simply write list :: item, which also turns language operators into simple object methods. Here, List defines :: (prepend) operator. However, you normally write it vice-verse in Scala, using item ::...

Is there any scala library that treat tuples as monads


scala,tuples,monads
Is there any scala library that enriches basic scala tuples with monad syntax. Something similar to the Writer monad but adjusted for usage with tuples. What I look for: val pair = (2, "as") pair >>= (a => point(a+1)) should equal to (3, "as"). As well as for (p <-...

Stackoverflow exception when using many enumarations inside my class


c#,enums,stack-overflow
Hey guys I have some enum declarations that for unknown reason to me are causing StackOverflowException. I have the following: public enum PrimaryAttribute { Strength, Agility, Intelligence } public enum Class { Tank, Fighter, Sorcerer } public class Hero { public PrimaryAttribute PrimaryAttribute { get; private set; } public Class...

Retrieving TriangleCount


scala,apache-spark,spark-graphx
I'm trying to retrieve the amount of triangles from a graph using graphX. As I'm new to both Scala and graphX, I'm currently quite stuck. I'm creating a graph from an edgefile: 1 2 1 3 2 3 This should be 1 triangle. Next I'm using the build in function...

How to instantiate lexical.Scanner in a JavaTokenParsers class?


scala,parsing,lexical-scanner
I am writing a parser which inherits from JavaTokenParsers in that I have a function as follow: import scala.util.parsing.combinator.lexical._ import scala.util.parsing._ import scala.util.parsing.combinator.RegexParsers; import scala.util.parsing.combinator.syntactical.StdTokenParsers import scala.util.parsing.combinator.token.StdTokens import scala.util.parsing.combinator.lexical.StdLexical import scala.util.parsing.combinator.lexical.Scanners import scala.util.parsing.combinator.lexical.Lexical import...

Like clause not working with int column in slick


scala,slick,slick-2.0
Slick doesn't seem to support like clauses on int columns. The following code where Status.code is of int type doesn't seem to work. Would there be a workaround for this? val query = for { s <- Status if s.code like "1%" } yield (s) ...