以 Nat 表示的 HList 类型参数的长度 [英] The length of HList type paremeter in terms of Nat

问题描述

假设我有一个没有参数的方法.如何确定类型参数的长度?

Suppose I have a method without params. How can I determine a length of type parameter?

def func[T <: HList]: Nat = {
  // some magic
}

推荐答案

您可以使用 ops.hlist.Length 操作来计算 的 Nat 长度HList.

You can use ops.hlist.Length operation to calculate the Nat length of an HList.

此外，将其作为不透明的 Nat 并不是很有用，因为您会丢失有关实际数字的所有类型级信息.所以你必须从函数中获取准确的 Nat 类型:

Also, getting it as an opaque Nat is not very useful, because you lose all the type-level information about the actual number. So you have to get the exact Nat type from the function:

import shapeless._
import shapeless.ops.hlist.Length

def func[T <: HList](implicit len: Length[T]): len.Out = len()

用法:

scala> natLen[Int :: String :: HNil]
res1: shapeless.Succ[shapeless.Succ[shapeless._0]] = Succ()

<小时>

获取长度为 Int 似乎更棘手.看来你不能使用 ops.nat.ToInt，因为它需要一个 N <: Nat 类型参数并且基本上使它无用:

---

Getting the length as an Int seems more tricky. It seems you can't use ops.nat.ToInt, because it would require a N <: Nat type parameter and basically make it useless:

def uselessIntLen[T <: HList, N <: Nat](implicit 
  len: Length.Aux[T, N], 
  toInt: ToInt[N]
): Int = toInt()

我使用 HKernel 找到了以下解决方法(当然，也可以手动编写新的类型类 IntLength).也许有人可以帮助提供更直接的内置方法:

I've found the following workaround using HKernel (of course, it's also possible to write a new typeclass IntLength manually). Maybe someone can help with a more direct built-in method:

import shapeless.ops.hlist.HKernelAux 

def intLen[T <: HList](implicit ker: HKernelAux[T]): Int = ker().length

用法:

scala> intLen[Int :: String :: HNil]
res2: Int = 2

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