CODEWITHSUNDEEP
scalashapeless
softshipper
softshipper

Reputation: 34119

Error in shapeless code: Expression of inst.type does not conform

I am reading the book shapeless and follow the examples.
On chapter 4, there is an example about, how to read the second element of HList.

The code

import shapeless.{HList, ::, HNil}


trait Second[L <: HList] {

  type Out

  def apply(value: L): Out

}

object Second {

  type Aux[L <: HList, O] = Second[L] {type Out = O}

  def apply[L <: HList](implicit inst: Second[L]): Aux[L, inst.Out] =
    inst
}


object Main {

  def main(args: Array[String]): Unit = {
  }

}

The compiler complains:

enter image description here

I copy and paste the code from book one to one and can not configure it out, where is the error.

Please help me to find it.

Upvotes: 0

Views: 116

Answers (1)

Dmytro Mitin
Dmytro Mitin

Reputation: 51723

It's not the compiler but your IDE complains (is it IntelliJ?).

If you compile (Ctrl+Shift+F9 / ⌘+Shift+F9) or run (Ctrl+Shift+F10 / ⌘+Shift+F10) your code you'll see it compiles perfectly fine.

You should get used to have some valid Scala code mis-underlined by IDE in red.

This is the way to convince your IDE:

def apply[L <: HList, O](implicit inst: Second.Aux[L, O]): Aux[L, O] = inst

Upvotes: 1

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