Using lambda with generic type in Scala

Question

The second argument of myFunc is a function with complex arguments:

def myFunc(list : List[String],
           combine: (Map[String, ListBuffer[String]], String, String) => Unit) = {
    // body of myFunc is just a stub and doesn't matter
    val x = Map[String, ListBuffer[String]]()

    list.foreach ((e:String) => {
       val spl = e.split(" ")
       combine(x, spl(0), spl(1))
    })

    x
}

I need to pass second argument to myFunc, so it can be used with various types A, B instead of specific String, ListBuffer[String].

def myFunc(list : List[A], combine: (Map[A, B], A, A) => Unit) = {

    val x = Map[A, B]()

    list.foreach(e => {          
        combine(x, e)
    })
}

How to declare and call such construct?

You need to specify that A and B are type parameters, like this: def myFunc[A, B](list: List[A], combine: (Map[A, B], A, A) => Unit) — marstran
– marstran, Commented Aug 11, 2016 at 10:30

diziaq · Accepted Answer · 2016-08-12 05:25:48Z

2

You can do the following,

def myFunc[A, B](list : List[A], combine: (Map[A, B], A, A) => Unit) = {
  val x = Map[A, B]()
  list.foreach (e => combine(x, e, e))
  x
}

Ad use it like

myFunc[String, Int](List("1","2","3"), (obj, k, v) => obj.put(k, v.toInt) )

edited Aug 12, 2016 at 5:25

diziaq

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answered Aug 11, 2016 at 10:31

sarveshseri

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Comments

justinpc · Accepted Answer · 2016-08-11 10:52:48Z

It seems that you are looking to generalise the container being used. Were you looking for something like this? Here we import scala.language.higherKinds so that we can take Container, a kind which takes a single type parameter as a type parameter to addPair.

import scala.language.higherKinds

def addPair[K, V, Container[_]](map: Map[K, Container[V]],
                                addToContainer: (Container[V], V) => Container[V],
                                emptyContainer: => Container[V],
                                pair: (K, V)): Map[K, Container[V]] = {
    val (key, value) = pair
    val existingValues = map.getOrElse(key, emptyContainer)
    val newValues = addToContainer(existingValues, value)

    map + (key -> newValues)
}

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