scala - 无形和注释
问题描述
我想将一些函数应用于案例类中的字段,这些字段用MyAnnotation
. 这个想法是将 typeT
转换为其通用表示,提取注释,压缩,向右(或向左)折叠以重建通用表示并最终返回 type T
。我按照这里提供的答案和这个要点。
我正在使用 scala 2.11.12 和 shapeless 2.3.3。
以下是我的代码:
import shapeless._
import shapeless.ops.hlist._
case class MyAnnotation(func: String) extends scala.annotation.StaticAnnotation
trait Modifier[T] {
def modify(t: T): T
}
object Modifier {
def apply[A: Modifier]: Modifier[A] = implicitly[Modifier[A]]
def create[T](func: T => T): Modifier[T] = new Modifier[T] { override def modify(t: T): T = func(t) }
private def id[T](t: T) = t
implicit val stringModifier: Modifier[String] = create(id)
implicit val booleanModifier: Modifier[Boolean] = create(id)
implicit val byteModifier: Modifier[Byte] = create(id)
implicit val charModifier: Modifier[Char] = create(id)
implicit val doubleModifier: Modifier[Double] = create(id)
implicit val floatModifier: Modifier[Float] = create(id)
implicit val intModifier: Modifier[Int] = create(id)
implicit val longModifier: Modifier[Long] = create(id)
implicit val shortModifier: Modifier[Short] = create(id)
implicit val hnilModifier: Modifier[HNil] = create(id)
implicit def hlistModifier[H, T <: HList, AL <: HList](
implicit
hser: Lazy[Modifier[H]],
tser: Modifier[T]
): Modifier[H :: T] = new Modifier[H :: T] {
override def modify(ht: H :: T): H :: T = {
ht match {
case h :: t =>
hser.value.modify(h) :: tser.modify(t)
}
}
}
implicit val cnilModifier: Modifier[CNil] = create(id)
implicit def coproductModifier[L, R <: Coproduct](
implicit
lser: Lazy[Modifier[L]],
rser: Modifier[R]
): Modifier[L :+: R] = new Modifier[L :+: R] {
override def modify(t: L :+: R): L :+: R = t match {
case Inl(l) => Inl(lser.value.modify(l))
case Inr(r) => Inr(rser.modify(r))
}
}
object Collector extends Poly2 {
implicit def myCase[ACC <: HList, E] = at[(E, Option[MyAnnotation]), ACC] {
case ((e, None), acc) => e :: acc
case ((e, Some(MyAnnotation(func))), acc) => {
println(func)
e :: acc
}
}
}
implicit def genericModifier[T, HL <: HList, AL <: HList, ZL <: HList](
implicit
gen: Generic.Aux[T, HL],
ser: Lazy[Modifier[HL]],
annots: Annotations.Aux[MyAnnotation, T, AL],
zip: Zip.Aux[HL :: AL :: HNil, ZL],
rightFolder: RightFolder[ZL, HNil.type, Collector.type]
): Modifier[T] = new Modifier[T] {
override def modify(t: T): T = {
val generic = gen.to(t)
println(generic)
val annotations = annots()
println(annotations)
val zipped = zip(generic :: annotations :: HNil)
println(zipped)
val modified = zipped.foldRight(HNil)(Collector)
println(modified)
val typed = gen.from(generic) // temporary
typed
}
}
}
上面的代码编译。但是,Modifier
在测试中实例化 a 时:
case class Test(a: String, @MyAnnotation("sha1") b: String)
val test = Test("A", "B")
val modifier: Modifier[Test] = implicitly
测试文件无法编译并给出以下错误:
[error] ambiguous implicit values:
[error] both value StringCanBuildFrom in object Predef of type =>
scala.collection.generic.CanBuildFrom[String,Char,String]
[error] and method $conforms in object Predef of type [A]=> <:<[A,A]
[error] match expected type T
[error] val ser1: Modifier[Test] = implicitly
问题似乎来自正确的文件夹定义:rightFolder
从隐含列表中删除时genericModifier
,它可以工作:
implicit def genericModifier[T, HL <: HList, AL <: HList, ZL <: HList](
implicit
gen: Generic.Aux[T, HL],
ser: Lazy[Modifier[HL]],
annots: Annotations.Aux[MyAnnotation, T, AL],
zip: Zip.Aux[HL :: AL :: HNil, ZL]/*,
rightFolder: RightFolder[ZL, HNil.type, Collector.type]*/
): Modifier[T] = new Modifier[T] {
override def modify(t: T): T = {
val generic = gen.to(t)
println(generic)
val annotations = annots()
println(annotations)
val zipped = zip(generic :: annotations :: HNil)
println(zipped)
/*val modified = zipped.foldRight(HNil)(Collector)
println(modified)*/
val typed = gen.from(generic) // temporary
typed
}
}
怎么了?
解决方案
您的代码中有几个错误:
只定义
Poly
forOption
过于粗略(模式匹配在运行时执行,编译器应该知道 forSome
和None
在编译时的定义)HNil
应该是代替HNil.type
和HNil : HNil
代替HNil
(类型HNil
和HNil.type
不同)编译器不知道
RightFolder
实际返回原始HList
类型,所以你应该使用RightFolder.Aux
type.
正确的代码是
import shapeless.ops.hlist.{RightFolder, Zip}
import shapeless.{::, Annotations, Generic, HList, HNil, Lazy, Poly2}
import scala.annotation.StaticAnnotation
object App {
case class MyAnnotation(func: String) extends StaticAnnotation
object Collector extends Poly2 {
// implicit def myCase[ACC <: HList, E] = at[(E, Option[PII]), ACC] {
// case ((e, None), acc) => e :: acc
// case ((e, Some(MyAnnotation(func))), acc) => {
// println(func)
// e :: acc
// }
// }
implicit def someCase[ACC <: HList, E]: Case.Aux[(E, Some[MyAnnotation]), ACC, E :: ACC] = at {
case ((e, Some(MyAnnotation(func))), acc) =>
println(func)
e :: acc
}
implicit def noneCase[ACC <: HList, E]: Case.Aux[(E, None.type), ACC, E :: ACC] = at {
case ((e, None), acc) => e :: acc
}
}
trait Modifier[T] {
def modify(t: T): T
}
implicit def hListModifier[HL <: HList]: Modifier[HL] = identity(_)
// added as an example, you should replace this with your Modifier for HList
implicit def genericModifier[T, HL <: HList, AL <: HList, ZL <: HList](implicit
gen: Generic.Aux[T, HL],
ser: Lazy[Modifier[HL]],
annots: Annotations.Aux[MyAnnotation, T, AL],
zip: Zip.Aux[HL :: AL :: HNil, ZL],
rightFolder: RightFolder.Aux[ZL, HNil/*.type*/, Collector.type, HL /*added*/]
): Modifier[T] = new Modifier[T] {
override def modify(t: T): T = {
val generic = gen.to(t)
println(generic)
val annotations = annots()
println(annotations)
val zipped = zip(generic :: annotations :: HNil)
println(zipped)
val modified = zipped.foldRight(HNil : HNil /*added*/)(Collector)
println(modified)
val typed = gen.from(modified)
typed
}
}
case class Test(a: String, @MyAnnotation("sha1") b: String)
val test = Test("A", "B")
val modifier: Modifier[Test] = implicitly[Modifier[Test]]
def main(args: Array[String]): Unit = {
val test1 = modifier.modify(test) // prints "sha1"
println(test1) // Test(A,B)
}
}
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