How to convert a generic method into generic function

Question

Question

How to convert the method timed into a function?

val timing = new StringBuffer
def timed[T](label: String, code: => T): T = {
  val start = System.currentTimeMillis()
  val result = code
  val stop = System.currentTimeMillis()
  timing.append(s"Processing $label took ${stop - start} ms.\n")
  result
}

Below causes "error: not found: type T"

val timing = new StringBuffer
val timed: (String, => T) => T = (label, code) => {
    val start = System.currentTimeMillis()
    val result = code
    val stop = System.currentTimeMillis()
    timing.append(s"Processing $label took ${stop - start} ms.\n")
    result
}

Dmytro Mitin · Accepted Answer · 2019-11-18 22:55:25Z

3

There is no such thing as generic function in Scala (and generic value at all), only generic methods are.

Generic functions will appear in Scala 3.

https://github.com/lampepfl/dotty/pull/4672

http://dotty.epfl.ch/docs/reference/overview.html#new-constructs

val timing = new StringBuffer
val timed: [T] => (String, /*=>*/ T) => T = [T] => (label: String, code: /*=>*/ T) => {
  val start = System.currentTimeMillis()
  val result = code
  val stop = System.currentTimeMillis()
  timing.append(s"Processing $label took ${stop - start} ms.\n")
  result
}

in Dotty 0.20.0-RC1.

edited Nov 18, 2019 at 22:55

answered Nov 18, 2019 at 22:45

Dmytro Mitin

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Comments

slouc · Accepted Answer · 2019-11-18 23:09:46Z

Scala doesn't have polymorphic functions.

Let me use an example from an excellent blog post by Miles Sabin:

def singleton[T](t: T) = Set(t)

// eta-expanded to Int => Set[Int]
List(1, 2, 3) map singleton

// eta-expanded to String => Set[String]
List("foo", "bar", "baz") map singleton

Method singleton() gets eta-expanded into a function at the point of usage, and at that point its types are fixed into inferred concrete types. So it can easily "become" (be expanded into) a function Int => Set[Int] or String => Set[String], but cannot stay something like T => Set[T]. There's no built-in way of expressing polymorphic functions in the language itself (Scala 3 might change that).

However, there is a way to have polymorphic functions via natural transformations and/or shapeless's Poly with a bit of type gymnastics. Both are based on the same principle of encoding the function as a trait with higher kinded types for input/output and having a generic apply[T]() method with an independent type T which is fixed to a different concrete type every time the function is invoked (in our case those would be Int, String etc.).

Something like this:

trait PolymorphicFunction[F[_], G[_]] {
  def apply[T](f: F[T]): G[T]
}

But instead of attempting to explain it in details here, I would rather refer you to shapeless documentation and the aforementioned short series of blog posts.

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