How to use pattern matching with function values in Scala

Question

I have several functions with same input and output types initialized in an object

object Utils {
  def f1(value: Int): Double = ???
  def f2(value: Int): Double = ???
  def f3(value: Int): Double = ???
}

I have a list of higher order values for those functions:

val x = List(Utils.f1, Utils.f2)

How can I use pattern matching to check which functions of those declared in the object are contained in x? I would like to obtain something similar to the following code:

x(0) match {
  case Utils.f1 => ...
  case Utils.f2 => ...
}

This is not possible in the general sense, function equality is a NP-Hard problem, but they may be subtle workarounds. However, needing to know which function you have feels weird, this seems like an XY-Problem, care to explain why exactly do you want to do this, what is the meta-problem behind the question? — Luis Miguel Mejía Suárez
– Luis Miguel Mejía Suárez, Commented Oct 27, 2020 at 15:23
In my project I have a set of objects where one of their value is a higher order function, this value describes the behaviour of the object to some events happening. Based on this value, I want to represent these objects differently in the GUI. — Snorlite
– Snorlite, Commented Oct 27, 2020 at 15:33
You can create your own ADT to encapsulate those functions, so you can pattern match against that to determine which function you have. — Luis Miguel Mejía Suárez
– Luis Miguel Mejía Suárez, Commented Oct 27, 2020 at 15:39

Dmytro Mitin · Accepted Answer · 2020-10-27 15:37:28Z

4

This will be possible if you make f1, f2, f3 vals. Please notice that upon pattern matching the equality by reference will be used

object Utils {
  val f1: Int => Double = _ * 10.0
  val f2: Int => Double = _ * 20.0
  val f3: Int => Double = _ * 30.0
}

val x: Seq[Int => Double] = List(Utils.f1, Utils.f2)

import Utils._

x(0) match {
  case `f1` => println(1)
  case `f2` => println(2)
  case `f3` => println(3)
}

If you keep f1, f2, f3 defs then

object Utils {
  def f1(value: Int): Double = value * 10.0
  def f2(value: Int): Double = value * 20.0
  def f3(value: Int): Double = value * 30.0
}

val x: Seq[Int => Double] = List(Utils.f1, Utils.f2)
  
val f1: Int => Double = Utils.f1
val f2: Int => Double = Utils.f2
val f3: Int => Double = Utils.f3

x(0) match {
  case `f1` => println(1)
  case `f2` => println(2)
  case `f3` => println(3)
}

produces MatchError.

edited Oct 27, 2020 at 15:37

answered Oct 27, 2020 at 15:29

Dmytro Mitin

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4 Comments

Snorlite Over a year ago

What if the input type is generic? def standardFoodEffect[A <: Int](value: A): Set[Double] How can i convert this into val?

Dmytro Mitin Over a year ago

@Snorlite In Scala 2 there are no polymorphic functions as first-class citizens. You'll have to emulate them with wrappers trait Poly { type B; def apply[A](a: A): B } val standardFoodEffect = new Poly { type B = Set[Double]; def apply[A <: Int](a: A) = ??? } like shapeless.Poly.

Dmytro Mitin Over a year ago

Or trait Poly[Upper] {type B; def apply[A <: Upper](a: A): B}, val standardFoodEffect = new Poly[Int]...

Dmytro Mitin Over a year ago

github.com/milessabin/shapeless/wiki/…

Martijn · Accepted Answer · 2020-10-27 15:23:49Z

4

You can't match against functions, it has no meaningfully defined equality.

answered Oct 27, 2020 at 15:23

Martijn

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