0

鉴于：

import argonaut._
import Argonaut._
import ArgonautShapeless._

sealed trait Parent
case class Foo(x: Int)    extends Parent
case class Bar(y: String) extends Parent

我试图定义DecodeJson[Parent]：

implicit val parentDecodeJson: DecodeJson[Parent] = 
   DecodeJson(c => c.focus.objectFields match {
       case Some("x" :: _) => implicitly[DecodeJson[Foo]].decode(c)
      case _                => implicitly[DecodeJson[Bar]].decode(c)
   })

但是，这失败了，因为argonaut.DecodeResult它是不变的。

<console>:42: error: type mismatch;
 found   : argonaut.DecodeResult[Foo]
 required: argonaut.DecodeResult[Parent]
Note: Foo <: Parent, but class DecodeResult is invariant in type A.
You may wish to define A as +A instead. (SLS 4.5)
              case Some("x" :: _) => implicitly[DecodeJson[Foo]].decode(c)
                                                                       ^
<console>:43: error: type mismatch;
 found   : argonaut.DecodeResult[Bar]
 required: argonaut.DecodeResult[Parent]
Note: Bar <: Parent, but class DecodeResult is invariant in type A.
You may wish to define A as +A instead. (SLS 4.5)
             case _     => implicitly[DecodeJson[Bar]].decode(c)
                                                             ^

所以，然后我想出了：

implicit val parentDecodeJson: DecodeJson[Parent] = 
   DecodeJson(c => c.focus.objectFields match {
       case Some("x" :: _) => implicitly[DecodeJson[Foo]].decode(c).flatMap{a => DecodeResult.ok(a)}
      case _     => implicitly[DecodeJson[Bar]].decode(c).flatMap{a => DecodeResult.ok(a)}
   })

这似乎有效：

scala> Json.obj( ("x", jNumber(42)) ).as[Parent]
res2: argonaut.DecodeResult[Parent] = DecodeResult(Right(Foo(42)))

scala> Json.obj( ("y", jString("hi!")) ).as[Parent]
res3: argonaut.DecodeResult[Parent] = DecodeResult(Right(Bar(hi!)))

有没有更清洁的方法？

4

1 回答 1

0

您不需要定义自己的隐式：

println(EncodeJson.of[Parent].encode(Bar("a")))//{"Bar":{"y":"a"}}
println(EncodeJson.of[Parent].apply(Bar("a")))//{"Bar":{"y":"a"}}
println(Bar("a").asJson)//{"y":"a"}
println((Bar("a"): Parent).asJson)//{"Bar":{"y":"a"}}
println("{\"Bar\":{\"y\":\"a\"}}".decode[Parent])//Right(Bar(a))
println("{\"Bar\":{\"y\":\"a\"}}".decodeOption[Parent])//Some(Bar(a))

还是我误解了你的目标？

于 2017-10-18T15:42:45.787 回答