Pass function of function as parameter in Scala

Question

I have a method setBuildQuery and I want that it can receive as parameter a function x. The function x should take as input an indefinite number of parameters and output another function y. Function y takes as input two dates and outputs a string.

Examples using a function notation

x = (one_f)(from_date, to_date) => string                      or
x = (one_f, two_f)(from_date, to_date) => string               or
x = (one_f, two_f, ..., n_f)(from_date, to_date) => string

How can I model this in Scala (i.e. how can I say to a function to accept a function x of this type? How the user of my app can specify this function as a val ?

I was thinking something like function of function or high order functions. I am not too familiar with them in Scala though.

Brian McCutchon · Accepted Answer · 2018-08-09 17:29:16Z

1

You can't have a function that takes an arbitrary number of parameters.* The best you could do is take a function that takes a Seq:

def setBuildQuery(f: Seq[YourType] => (Date, Date) => String)

You could then define a function it accepts like this:

val f: Seq[YourType] => (Date, Date) => String =
    ls => (from, to) => ???

* You can have a method that takes an arbitrary number of parameters, but that doesn't help here.

answered Aug 9, 2018 at 17:29

Brian McCutchon

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Sebastian Celestino · Accepted Answer · 2018-08-09 16:52:46Z

0

There are many way to do that, you can use Partial Application to define your function.

The function will not execute until you invoke it with all its parameters

def x(head:String, tail:String*)(from:Date, to:Date): String = {
  println(head) // it doesnt exec until from and to are provided
  "result"
}

val y = x("string1", "string2") _

y(new Date, new Date)

or you can return a function

def x(head:String, tail:String*): (Date,Date) => String = {
  println(head) // it exec before from and to are provided
  (from:Date, to:Date) => {
    "result"
  }
}

val y = x("string1", "string2")

y(new Date, new Date)

answered Aug 9, 2018 at 16:52

Sebastian Celestino

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3 Comments

Brian McCutchon Over a year ago

That's not partial application, that's currying/multiple argument lists.

Sebastian Celestino Over a year ago

x is partially applied here val y = x("string1", "string2") _

Brian McCutchon Over a year ago

Yes, but you're not using "Partial Application to define your function."

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