How does it becomes to Id type?

Question

I have the following code:

import cats._
import cats.Functor

object StudyIt {

  def main(args: Array[String]): Unit = {
    val one: Int = 1
    val a = Functor[Id].map(one)(_ + 1)
    println(a)
  }

}

As you can see, map expects the type of Id[Int] but I just past Int to map, why it is possible?

score 5 · Answer 1 · answered Mar 04 '18 at 20:11

5

That's because Id is just a type alias, which gets completely inlined at runtime. It looks like this:

type Id[A] = A

So in essence Id[Int] and Int are the exact same type.

answered Mar 04 '18 at 20:11

Luka Jacobowitz

22,795
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What does `completely inlined at runtime` mean? – softshipper Mar 04 '18 at 20:21
It means that everywhere there's an `Id[A]` it will instead turn it into an `A`. – Luka Jacobowitz Mar 04 '18 at 21:04

score 2 · Answer 2 · answered Mar 04 '18 at 20:18

2

That's because Id[A] is literally defined as just A

type Id[A] = A

so that Id[Int] is just an alias for Int.

Here is it, as one of the first definitions (line 34): Github link.

answered Mar 04 '18 at 20:18

Andrey Tyukin

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How does it becomes to Id type?

2 Answers2