flatMap 与 Shapeless 产量 FlatMapper 未找到 [英] flatMap with Shapeless yield FlatMapper not found

问题描述

我正在尝试定义一些这样的结构

I'm trying to define some structure like this

case class Transformer[From, To](
                                 name: String,
                                 get: PaymentEvent => From,
                                 to: From => To

我想过滤名称属于 Set 的元素

I want to filter elements with names that are part of a Set

class filterName(names: Set[String]) extends lowPriority {
  implicit def get[From, To]  = at[Transformer[From, To]]{ trans =>
    if (names.contains(trans.name))
      trans :: HNil
    else
      HNil
  }
}

这是具体的值:

  type HTransformer = Transformer[String, String] :: Transformer[Long, Long] :: HNil

当我想将函数应用到值时

When I want to apply the function to the value

  private def filter(fields: HTransformer, names: Set[String]): HTransformer = {
    fields.flatMap(new filterName(names))
  }

编译器报错:

Error:(42, 19) could not find implicit value for parameter mapper: shapeless.ops.hlist.FlatMapper[f.type,shapeless.::[com.ingenico.datalake.Transformer[String,String],shapeless.::[com.ingenico.datalake.Transformer[Long,Long],shapeless.HNil]]]
    fields.flatMap(new filterName(names))

推荐答案

我猜你误解了类型级计算.

I guess you misunderstand type-level calculations.

如果你想根据一个元素是否是 Set 的一部分来过滤一个 hlist，那么你必须在编译时知道这一点(如果一个元素是 Set 的一部分)，但实际上你只在运行时知道这一点.所以 filterName 将不起作用.

If you want to filter an hlist depending on whether an element is a part of the Set then you must know this (if an element is a part of the Set) at compile time but actually you know this only at runtime. So filterName will not work.

例如，您可以将 hlist 转换为列表，并在运行时将其过滤为普通集合.

For example you can transform the hlist to a list and filter it as an ordinary collection at runtime.

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