Kotlin local function with generic

Question

I'm just fooling around with some Functional Programming techniques and tried to implement a generic drop function for a list. However it seems the types are getting shadowed. I'm wondering why I can't re-use a generic type declared.

The IDE doesn't want me to reuse the generic type

sealed class List<out A> {
  
    fun <A> drop(n: Int): List<A> {
        fun go(n: Int, l: List<A>): List<A> = when (n) {
                0 -> l
                else -> go(n - 1, l.tail())
        }
        return go(n, this)
    }
}

The IDE says the following

Type mismatch.
Required:
List<A#1 (type parameter of chapter3.List.drop)>
Found:
List<A#2 (type parameter of chapter3.List)>

Is this not possible with inner local functions?

I think you should move your drop function out of the companion object, and remove the <A> type parameter from it because it's already defined in List<out A>. When you are doing go(n, this) inside drop, this is referring to the companion object, not your list. — marstran
– marstran, Commented Aug 8, 2021 at 15:00
Argh- it actually is outside of the companion object. I was trying to remove non-related functions before posting my question. Still same error though :/ — jimgug
– jimgug, Commented Aug 8, 2021 at 15:06
You still haven't removed the <A> from drop. You are defining a new generic A-type that is not the same as the one in List<out A>. — marstran
– marstran, Commented Aug 8, 2021 at 15:12
Thank you! This was it. ``` fun drop(n: Int): List<A> { fun go(n: Int, l: List<A>): List<A> = when (n) { 0 -> l else -> go(n - 1, l.tail()) } return go(n, this) } ``` — jimgug
– jimgug, Commented Aug 8, 2021 at 15:24

developer_hatch · Accepted Answer · 2021-08-11 03:52:05Z

1

You don't need to redefine you A type again in the function, it cames from the sealed class

sealed class List<out A> {


fun drop(n: Int): List<A> {
    fun go(n: Int, l: List<A>): List<A> = when (n) {
            0 -> l
            else -> go(n - 1, l.tail())
    }
    return go(n, this)
}
}

answered Aug 11, 2021 at 3:52

developer_hatch

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