Can I have a higher order function that returns a func(varargs*), for example, (s: String*) => String?
I am trying to do the following:
def concatKeys(delimiter: String) = {
def concat(k1: String, k2: String): String = if (k1.isEmpty) k2 else k1 + delimiter + k2
(keys: String*) => keys.foldLeft("")(concat)
}
But when I use it as expected, the code does not compile:
val key: String = concatKeys(delimiter)(keyAcc, kv._1)
If I change it to a Traversable:
def concatKeys(delimiter: String) = {
def concat(k1: String, k2: String): String = if (k1.isEmpty) k2 else k1 + delimiter + k2
(keys: Traversable[String]) => keys.foldLeft("")(concat)
}
It naturally compiles:
val key: String = concatKeys(delimiter)(Set(keyAcc, kv._1))
So, is not possible to return a HOF with varargs? If not, why not?
Thank you all!
Varargs are not a valid type, they are just sugar syntax that is only available on methods, not in functions.
Remember that inside the method body, a vararg parameter is actually just a Seq
So no, you can't.
Related
Is there a nice way I can convert a Scala case class instance, e.g.
case class MyClass(param1: String, param2: String)
val x = MyClass("hello", "world")
into a mapping of some kind, e.g.
getCCParams(x) returns "param1" -> "hello", "param2" -> "world"
Which works for any case class, not just predefined ones. I've found you can pull the case class name out by writing a method that interrogates the underlying Product class, e.g.
def getCCName(caseobj: Product) = caseobj.productPrefix
getCCName(x) returns "MyClass"
So I'm looking for a similar solution but for the case class fields. I'd imagine a solution might have to use Java reflection, but I'd hate to write something that might break in a future release of Scala if the underlying implementation of case classes changes.
Currently I'm working on a Scala server and defining the protocol and all its messages and exceptions using case classes, as they are such a beautiful, concise construct for this. But I then need to translate them into a Java map to send over the messaging layer for any client implementation to use. My current implementation just defines a translation for each case class separately, but it would be nice to find a generalised solution.
This should work:
def getCCParams(cc: AnyRef) =
cc.getClass.getDeclaredFields.foldLeft(Map.empty[String, Any]) { (a, f) =>
f.setAccessible(true)
a + (f.getName -> f.get(cc))
}
Because case classes extend Product one can simply use .productIterator to get field values:
def getCCParams(cc: Product) = cc.getClass.getDeclaredFields.map( _.getName ) // all field names
.zip( cc.productIterator.to ).toMap // zipped with all values
Or alternatively:
def getCCParams(cc: Product) = {
val values = cc.productIterator
cc.getClass.getDeclaredFields.map( _.getName -> values.next ).toMap
}
One advantage of Product is that you don't need to call setAccessible on the field to read its value. Another is that productIterator doesn't use reflection.
Note that this example works with simple case classes that don't extend other classes and don't declare fields outside the constructor.
Starting Scala 2.13, case classes (as implementations of Product) are provided with a productElementNames method which returns an iterator over their field's names.
By zipping field names with field values obtained with productIterator we can generically obtain the associated Map:
// case class MyClass(param1: String, param2: String)
// val x = MyClass("hello", "world")
(x.productElementNames zip x.productIterator).toMap
// Map[String,Any] = Map("param1" -> "hello", "param2" -> "world")
If anybody looks for a recursive version, here is the modification of #Andrejs's solution:
def getCCParams(cc: Product): Map[String, Any] = {
val values = cc.productIterator
cc.getClass.getDeclaredFields.map {
_.getName -> (values.next() match {
case p: Product if p.productArity > 0 => getCCParams(p)
case x => x
})
}.toMap
}
It also expands the nested case-classes into maps at any level of nesting.
Here's a simple variation if you don't care about making it a generic function:
case class Person(name:String, age:Int)
def personToMap(person: Person): Map[String, Any] = {
val fieldNames = person.getClass.getDeclaredFields.map(_.getName)
val vals = Person.unapply(person).get.productIterator.toSeq
fieldNames.zip(vals).toMap
}
scala> println(personToMap(Person("Tom", 50)))
res02: scala.collection.immutable.Map[String,Any] = Map(name -> Tom, age -> 50)
If you happen to be using Json4s, you could do the following:
import org.json4s.{Extraction, _}
case class MyClass(param1: String, param2: String)
val x = MyClass("hello", "world")
Extraction.decompose(x)(DefaultFormats).values.asInstanceOf[Map[String,String]]
Solution with ProductCompletion from interpreter package:
import tools.nsc.interpreter.ProductCompletion
def getCCParams(cc: Product) = {
val pc = new ProductCompletion(cc)
pc.caseNames.zip(pc.caseFields).toMap
}
You could use shapeless.
Let
case class X(a: Boolean, b: String,c:Int)
case class Y(a: String, b: String)
Define a LabelledGeneric representation
import shapeless._
import shapeless.ops.product._
import shapeless.syntax.std.product._
object X {
implicit val lgenX = LabelledGeneric[X]
}
object Y {
implicit val lgenY = LabelledGeneric[Y]
}
Define two typeclasses to provide the toMap methods
object ToMapImplicits {
implicit class ToMapOps[A <: Product](val a: A)
extends AnyVal {
def mkMapAny(implicit toMap: ToMap.Aux[A, Symbol, Any]): Map[String, Any] =
a.toMap[Symbol, Any]
.map { case (k: Symbol, v) => k.name -> v }
}
implicit class ToMapOps2[A <: Product](val a: A)
extends AnyVal {
def mkMapString(implicit toMap: ToMap.Aux[A, Symbol, Any]): Map[String, String] =
a.toMap[Symbol, Any]
.map { case (k: Symbol, v) => k.name -> v.toString }
}
}
Then you can use it like this.
object Run extends App {
import ToMapImplicits._
val x: X = X(true, "bike",26)
val y: Y = Y("first", "second")
val anyMapX: Map[String, Any] = x.mkMapAny
val anyMapY: Map[String, Any] = y.mkMapAny
println("anyMapX = " + anyMapX)
println("anyMapY = " + anyMapY)
val stringMapX: Map[String, String] = x.mkMapString
val stringMapY: Map[String, String] = y.mkMapString
println("anyMapX = " + anyMapX)
println("anyMapY = " + anyMapY)
}
which prints
anyMapX = Map(c -> 26, b -> bike, a -> true)
anyMapY = Map(b -> second, a -> first)
stringMapX = Map(c -> 26, b -> bike, a -> true)
stringMapY = Map(b -> second, a -> first)
For nested case classes, (thus nested maps)
check another answer
I don't know about nice... but this seems to work, at least for this very very basic example. It probably needs some work but might be enough to get you started? Basically it filters out all "known" methods from a case class (or any other class :/ )
object CaseMappingTest {
case class MyCase(a: String, b: Int)
def caseClassToMap(obj: AnyRef) = {
val c = obj.getClass
val predefined = List("$tag", "productArity", "productPrefix", "hashCode",
"toString")
val casemethods = c.getMethods.toList.filter{
n =>
(n.getParameterTypes.size == 0) &&
(n.getDeclaringClass == c) &&
(! predefined.exists(_ == n.getName))
}
val values = casemethods.map(_.invoke(obj, null))
casemethods.map(_.getName).zip(values).foldLeft(Map[String, Any]())(_+_)
}
def main(args: Array[String]) {
println(caseClassToMap(MyCase("foo", 1)))
// prints: Map(a -> foo, b -> 1)
}
}
commons.mapper.Mappers.Mappers.beanToMap(caseClassBean)
Details: https://github.com/hank-whu/common4s
With the use of Java reflection, but no change of access level. Converts Product and case class to Map[String, String]:
def productToMap[T <: Product](obj: T, prefix: String): Map[String, String] = {
val clazz = obj.getClass
val fields = clazz.getDeclaredFields.map(_.getName).toSet
val methods = clazz.getDeclaredMethods.filter(method => fields.contains(method.getName))
methods.foldLeft(Map[String, String]()) { case (acc, method) =>
val value = method.invoke(obj).toString
val key = if (prefix.isEmpty) method.getName else s"${prefix}_${method.getName}"
acc + (key -> value)
}
}
Modern variation with Scala 3 might also be a bit simplified as with the following example that is similar to the answer posted by Walter Chang above.
def getCCParams(cc: AnyRef): Map[String, Any] =
cc.getClass.getDeclaredFields
.tapEach(_.setAccessible(true))
.foldLeft(Map.empty)((a, f) => a + (f.getName -> f.get(cc)))
IM trying to understand the below high order functions in Scala but need some clarifications on the parameters of the functions.
Questions:-
What does the Int => String in the apply function mean?
v: Int indicates that the parameter v is of type Int.
What does the [A](x: A) mean in layout function?
object Demo {
def main(args: Array[String]) {
println( apply( layout, 10) )
}
def apply(f: Int => String, v: Int) = f(v)
def layout[A](x: A) = "[" + x.toString() + "]"
}
f: Int => String means that f is a function with one argument of type Int and with a return type String
def layout[A](x: A) means that parameter x is of type A, which can be of any type. Here are a couple of examples on how to invoke layout:
layout[String]("abc") //returns "[abc]"
layout[Int](123) //returns "[123]"
When main runs it invokes apply with the layout function and the argument 10. This will output "[10]"
The syntax of Int => String means passing a function that accepts Int and returns String.
Here is a useful example for passing function:
case class Person(name: String, lastName: String)
val person = Person("Johnny", "Cage")
def updateName(name: String) = {
updatePerson(_.copy(name = name))
}
def updateLastName(lastName: String) {
updatePerson(_.copy(lastName = lastName))
}
private def updatePerson(transformer: Person => Person): Unit = {
transformer(person)
}
Note how each update function passes the copy constructor.
It's been while Since I've started working on scala and I am wondering what kind of variable type is the best when I create a method which requires to return multiple data.
let's say If I have to make a method to get user info and it'll be called from many places.
def getUserParam(userId: String):Map[String,Any] = {
//do something
Map(
"isExist" -> true,
"userDataA" -> "String",
"userDataB" -> 1 // int
)
}
in this case, the result type is Map[String,Any] and since each param would be recognized as Any, You cannot pass the value to some other method requiring something spesifically.
def doSomething(foo: String){}
val foo = getUserParam("bar")
doSomething(foo("userDataA")) // type mismatch error
If I use Tuple, I can avoid that error, but I don't think it is easy to guess what each indexed number contains.
and of course there is a choice to use Case Class but once I use case class as a return type, I need to import the case class where ever I call the method.
What I want to ask is what is the best way to make a method returning more than 2 different variable type values.
Here are three options. Even though you might like the third option (using anonymous class) it's actually my least favorite. As you can see, it requires you to enable reflective calls (otherwise it throws a compilation warning). Scala will use reflection to achieve this which is not that great.
Personally, if there are only 2 values I use tuple. If there are more than two I will use a case class since it greatly improves code readability. The anonymous class option I knew it existed for a while, but I never used that it my code.
import java.util.Date
def returnTwoUsingTuple: (Date, String) = {
val date = new Date()
val str = "Hello world"
(date,str)
}
val tupleVer = returnTwoUsingTuple
println(tupleVer._1)
println(tupleVer._2)
case class Reply(date: Date, str: String)
def returnTwoUsingCaseClass: Reply = {
val date = new Date()
val str = "Hello world"
Reply(date,str)
}
val caseClassVer = returnTwoUsingCaseClass
println(caseClassVer.date)
println(caseClassVer.str)
import scala.language.reflectiveCalls
def returnTwoUsingAnonymousClass = {
val date = new Date()
val str = "Hello world"
new {
val getDate = date
val getStr = str
}
}
val anonClassVer = returnTwoUsingAnonymousClass
println(anonClassVer.getDate)
println(anonClassVer.getStr)
Sinse your logic with Map[String,Any] is more like for each key I have one of .. not for each key I have both ... more effective use in this case would be Either or even more effectively - scalaz.\/
scalaz.\/
import scalaz._
import scalaz.syntax.either._
def getUserParam(userId: String): Map[String, String \/ Int \/ Boolean] = {
//do something
Map(
"isExist" -> true.right,
"userDataA" -> "String".left.left,
"userDataB" -> 1.right.left
)
}
String \/ Int \/ Boolean is left-associatited to (String \/ Int) \/ Boolean
now you have
def doSomething(foo: String){}
unluckily it's the most complex case, if for example you had
def doSomethingB(foo: Boolean){}
you could've just
foo("userDataA").foreach(doSomethingB)
since the right value considered as correct so for String which is left to the left you could write
foo("userdata").swap.foreach(_.swap.foreach(doSomething))
Closed Family
Or you could craft you own simple type for large number of alternatives like
sealed trait Either3[+A, +B, +C] {
def ifFirst[T](action: A => T): Option[T] = None
def ifSecond[T](action: B => T): Option[T] = None
def ifThird[T](action: C => T): Option[T] = None
}
case class First[A](x: A) extends Either3[A, Nothing, Nothing] {
override def ifFirst[T](action: A => T): Option[T] = Some(action(x))
}
case class Second[A](x: A) extends Either3[Nothing, A, Nothing] {
override def ifSecond[T](action: A => T): Option[T] = Some(action(x))
}
case class Third[A](x: A) extends Either3[Nothing, Nothing, A] {
override def ifThird[T](action: A => T): Option[T] = Some(action(x))
}
now having
def getUserParam3(userId: String): Map[String, Either3[Boolean, String, Int]] = {
//do something
Map(
"isExist" -> First(true),
"userDataA" -> Second("String"),
"userDataB" -> Third(1)
)
}
val foo3 = getUserParam3("bar")
you can use your values as
foo3("userdata").ifSecond(doSomething)
I have a generic map with values, some of which can be in turn lists of values.
I'm trying to process a given key and convert the results to the type expected by an outside caller, like this:
// A map with some values being other collections.
val map: Map[String, Any] = Map("foo" -> 1, "bar" -> Seq('a', 'b'. 'a'))
// A generic method with a "specialization" for collections (pseudocode)
def cast[T](key: String) = map.get(key).map(_.asInstanceOf[T])
def cast[C <: Iterable[T]](key: String) = map.get(key).map(list => list.to[C].map(_.asIntanceOf[T]))
// Expected usage
cast[Int]("foo") // Should return 1:Int
cast[Set[Char]]("bar") // Should return Set[Char]('a', 'b')
This is to show what I would like to do, but it does not work. The compiler error complains (correctly, about 2 possible matches). I've also tried to make this a single function with some sort of pattern match on the type to no avail.
I've been reading on #specialized, TypeTag, CanBuildFrom and other scala functionality, but I failed to find a simple way to put it all together. Separate examples I've found address different pieces and some ugly workarounds, but nothing that would simply allow an external user to call cast and get an exception is the cast was invalid. Some stuff is also old, I'm using Scala 2.10.5.
This appears to work but it has a some problems.
def cast[T](m: Map[String, Any], k: String):T = m(k) match {
case x: T => x
}
With the right input you get the correct output.
scala> cast[Int](map,"foo")
res18: Int = 1
scala> cast[Set[Char]](map,"bar")
res19: Set[Char] = Set(a, b)
But it throws if the type is wrong for the key or if the map has no such key (of course).
You can do this via implicit parameters:
val map: Map[String, Any] = Map("foo" -> 1, "bar" -> Set('a', 'b'))
abstract class Casts[B] {def cast(a: Any): B}
implicit val doubleCast = new Casts[Double] {
override def cast(a: Any): Double = a match {
case x: Int => x.toDouble
}
}
implicit val intCast = new Casts[Int] {
override def cast(a: Any): Int = a match {
case x: Int => x
case x: Double => x.toInt
}
}
implicit val seqCharCast = new Casts[Seq[Char]] {
override def cast(a: Any): Seq[Char] = a match {
case x: Set[Char] => x.toSeq
case x: Seq[Char] => x
}
}
def cast[T](key: String)(implicit p:Casts[T]) = p.cast(map(key))
println(cast[Double]("foo")) // <- 1.0
println(cast[Int]("foo")) // <- 1
println(cast[Seq[Char]]("bar")) // <- ArrayBuffer(a, b) which is Seq(a, b)
But you still need to iterate over all type-to-type options, which is reasonable as Set('a', 'b').asInstanceOf[Seq[Char]] throws, and you cannot use a universal cast, so you need to handle such cases differently.
Still it sounds like an overkill, and you may need to review your approach from global perspective
I'm trying to implement a new type, Chunk, that is similar to a Map. Basically, a "Chunk" is either a mapping from String -> Chunk, or a string itself.
Eg it should be able to work like this:
val m = new Chunk("some sort of value") // value chunk
assert(m.getValue == "some sort of value")
val n = new Chunk("key" -> new Chunk("value"), // nested chunks
"key2" -> new Chunk("value2"))
assert(n("key").getValue == "value")
assert(n("key2").getValue == "value2")
I have this mostly working, except that I am a little confused by how the + operator works for immutable maps.
Here is what I have now:
class Chunk(_map: Map[String, Chunk], _value: Option[String]) extends Map[String, Chunk] {
def this(items: (String, Chunk)*) = this(items.toMap, None)
def this(k: String) = this(new HashMap[String, Chunk], Option(k))
def this(m: Map[String, Chunk]) = this(m, None)
def +[B1 >: Chunk](kv: (String, B1)) = throw new Exception(":( do not know how to make this work")
def -(k: String) = new Chunk(_map - k, _value)
def get(k: String) = _map.get(k)
def iterator = _map.iterator
def getValue = _value.get
def hasValue = _value.isDefined
override def toString() = {
if (hasValue) getValue
else "Chunk(" + (for ((k, v) <- this) yield k + " -> " + v.toString).mkString(", ") + ")"
}
def serialize: String = {
if (hasValue) getValue
else "{" + (for ((k, v) <- this) yield k + "=" + v.serialize).mkString("|") + "}"
}
}
object main extends App {
val m = new Chunk("message_info" -> new Chunk("message_type" -> new Chunk("boom")))
val n = m + ("c" -> new Chunk("boom2"))
}
Also, comments on whether in general this implementation is appropriate would be appreciated.
Thanks!
Edit: The algebraic data types solution is excellent, but there remains one issue.
def +[B1 >: Chunk](kv: (String, B1)) = Chunk(m + kv) // compiler hates this
def -(k: String) = Chunk(m - k) // compiler is pretty satisfied with this
The - operator here seems to work, but the + operator really wants me to return something of type B1 (I think)? It fails with the following issue:
overloaded method value apply with alternatives: (map: Map[String,Chunk])MapChunk <and> (elems: (String, Chunk)*)MapChunk cannot be applied to (scala.collection.immutable.Map[String,B1])
Edit2:
Xiefei answered this question -- extending map requires that I handle + with a supertype (B1) of Chunk, so in order to do this I have to have some implementation for that, so this will suffice:
def +[B1 >: Chunk](kv: (String, B1)) = m + kv
However, I don't ever really intend to use that one, instead, I will also include my implementation that returns a chunk as follows:
def +(kv: (String, Chunk)):Chunk = Chunk(m + kv)
How about an Algebraic data type approach?
abstract sealed class Chunk
case class MChunk(elems: (String, Chunk)*) extends Chunk with Map[String,Chunk] {
val m = Map[String, Chunk](elems:_*)
def +[B1 >: Chunk](kv: (String, B1)) = m + kv
def -(k: String) = m - k
def iterator = m.iterator
def get(s: String) = m.get(s)
}
case class SChunk(s: String) extends Chunk
// A 'Companion' object that provides 'constructors' and extractors..
object Chunk {
def apply(s: String) = SChunk(s)
def apply(elems: (String, Chunk)*) = MChunk(elems: _*)
// just a couple of ideas...
def unapply(sc: SChunk) = Option(sc).map(_.value)
def unapply(smc: (String, MChunk)) = smc match {
case (s, mc) => mc.get(s)
}
}
Which you can use like:
val simpleChunk = Chunk("a")
val nestedChunk = Chunk("b" -> Chunk("B"))
// Use extractors to get the values.
val Chunk(s) = simpleChunk // s will be the String "a"
val Chunk(c) = ("b" -> nestedChunk) // c will be a Chunk: Chunk("B")
val Chunk(c) = ("x" -> nestedChunk) // will throw a match error, because there's no "x"
// pattern matching:
("x" -> mc) match {
case Chunk(w) => Some(w)
case _ => None
}
The unapply extractors are just a suggestion; hopefully you can mess with this idea till you get what you want.
The way it's written, there's no way to enforce that it can't be both a Map and a String at the same time. I would be looking at capturing the value using Either and adding whatever convenience methods you require:
case class Chunk(value:Either[Map[String,Chunk],String]) {
...
}
That will also force you to think about what you really need to do in situations such as adding a key/value pair to a Chunk that represents a String.
Have you considered using composition instead of inheritance? So, instead of Chunk extending Map[String, Chunk] directly, just have Chunk internally keep an instance of Map[String, Chunk] and provide the extra methods that you need, and otherwise delegating to the internal map's methods.
def +(kv: (String, Chunk)):Chunk = new Chunk(_map + kv, _value)
override def +[B1 >: Chunk](kv: (String, B1)) = _map + kv
What you need is a new + method, and also implement the one declared in Map trait.