字符串定义可用于无形镜片 [英] Shapeless lenses usage with a string definition

问题描述

我想使用无形透镜来通过String定义访问案例类字段的值.

I would like use shapeless lenses to access value of the case class field by a String definition.

我知道这段代码有效.

case class Test(id: String, calc: Long)
val instance = Test("123232", 3434L)

val lens = lens[Test] >> 'id

val valueOfFieldId = lens.get(instance)

但是我想做的是:

 val fieldName = "id"

 val lens = lens[Test] >> fieldName.witness
//I typed .witness because it was expecting a witness (if I am not wrong)

 val valueOfFieldId = lens.get(instance)

但是使用此代码，我得到了这个错误.

But with this code, I am getting this error.

找不到参数mkLens的隐式值:shapeless.MkFieldLens [A $ A148.this.Test，A $ A148.this.str.type] def get $$ instance $$ lll = lll;/* ### worksheet ###生成$$ end $$ */lazy val lens = lens [Test] >> str.witness

Could not find implicit value for parameter mkLens: shapeless.MkFieldLens[A$A148.this.Test,A$A148.this.str.type] def get$$instance$$lll = lll;/* ###worksheet### generated $$end$$ */ lazy val lens = lens[Test] >> str.witness

是否可以使用String定义获取case类字段的值?

Is it possible to get the value of case class field with a String definition?

谢谢.

推荐答案

您应该在这里使用Symbol('id)而不是String("id").

You are supposed to use Symbol ('id) here rather than String ("id").

从String

Symbol(fieldName)

是运行时操作，Shapeless在编译时操作.

is runtime operation and Shapeless operates in compile time.

为什么不能使用符号?

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