我正在尝试定义一种制作字符串类型的通用方法。例如,我想
case class Foo1(s: String) extends AnyVal
case class Foo2(s: String) extends AnyVal
...
有scalaz.Show,scalaz.Equal,argonaut.CodecJson等实例。 我知道这可能是使用hacky方法,比如抓住case类生成的apply / unapply函数,但我希望能找到一个类型安全,无样板的无形解决方案。这是我提出的最好的:
import scalaz._, Scalaz._
import argonaut._, Argonaut._
import shapeless._
trait HasName[A] {
def to(v: A): String
def fr(v: String): A
}
object HasName {
def apply[A](implicit instance: HasName[A]): HasName[A] = instance
def instance[A](f: A => String, g: String => A): HasName[A] = new HasName[A] { def to(v: A) = f(v); def fr(v: String) = g(v) }
implicit val hlist: HasName[String :: HNil] = new HasName[String :: HNil] {
def to(v: String :: HNil) = v.head
def fr(v: String) = v :: HNil
}
implicit def generic[A, R](implicit F: Generic.Aux[A, R], G: HasName[R]): HasName[A] = instance(
v => G.to(F.to(v)),
v => F.from(G.fr(v))
)
}
trait Name[A] {
val F: HasName[A]
implicit val show: Show[A] = Show.shows(F.to)
implicit val read: Read[A] = Read.readUnsafe(F.fr)
implicit val json: CodecJson[A] = CodecJson[A](v => jString(F.to(v)), c => c.as[String] map F.fr)
implicit val equal: Equal[A] = Equal.equalA[A]
}
然后用户可以
case class Foo1(s: String) extends AnyVal
object Foo1 extends Name[Foo1] {
val F = cachedImplicit[HasName[Foo1]]
}
这不是太多样板,但仍然有那种讨厌的F.我试过这个:
class Name[A](implicit F: HasName[A]) {
implicit val show: Show[A] = Show.shows(F.to)
implicit val read: Read[A] = Read.readUnsafe(F.fr)
implicit val json: CodecJson[A] = CodecJson[A](v => jString(F.to(v)), c => c.as[String] map F.fr)
implicit val equal: Equal[A] = Equal.equalA[A]
}
在呼叫站点会更好:
object Foo1 extends Name[Foo1]
但它不起作用;你不能拥有隐式的名字参数,也不能传递非名字的循环引用。
关于如何保持调用者和被调用者代码的任何想法都很好?
答案 0 :(得分:1)
您可以使用HasName
隐含在范围内的事实来执行以下操作
trait Name[A] {
implicit def show(implicit F: HasName[A]): Show[A] = Show.shows(F.to)
implicit def read(implicit F: HasName[A]): Read[A] = Read.readUnsafe(F.fr)
implicit def json(implicit F: HasName[A]): CodecJson[A] = CodecJson[A](v => jString(F.to(v)), c => c.as[String] map F.fr)
implicit def equal(implicit F: HasName[A]): Equal[A] = Equal.equalA[A]
}