How does circe parse a generic type object to Json?

Question

Here is what I am trying to do ``` case class MessageModel (time: Long, content: String) {} val message = MessageModel(123, "Hello World") def jsonParser[A] (obj: A) : String = obj.asJson.noSpaces println jsonParser[MessageModel](message) ``` this doesn't work, because it will complain Error:(13, 8) could not find implicit value for parameter encoder: io.circe.Encoder[A] obj.asJson.noSpaces ^ I kind of understand why it is happening, but is there a way to work around it? Thanks

Accepted Answer

Encoding and decoding in circe are provided by type classes, which means that you have to be able to prove at compile time that you have a type class instance for `A` if you want to encode (or decode) a value of type `A`.

This means that when you write something like this:

```
import io.circe.syntax._

def jsonPrinter[A](obj: A): String = obj.asJson.noSpaces
```

You're not providing enough information about `A` for circe to be able to print values of that type. You can fix this with a context bound:

```
import io.circe.Encoder
import io.circe.syntax._

def jsonPrinter[A: Encoder](obj: A): String = obj.asJson.noSpaces
```

Which is Scala's syntactic sugar for something like this:

```
def jsonPrinter[A](obj: A)(implicit encoder: Encoder[A]): String =
  obj.asJson.noSpaces
```

Both of these will compile, and you can pass them a value of any type that has an implicit `Encoder` instance. For your `MessageModel` specifically, you could use circe's generic derivation, since it's a case class:

```
scala> import io.circe.generic.auto._
import io.circe.generic.auto._

scala> case class MessageModel(time: Long, content: String)
defined class MessageModel

scala> val message = MessageModel(123, "Hello World")
message: MessageModel = MessageModel(123,Hello World)

scala> jsonPrinter(message)
res0: String = {"time":123,"content":"Hello World"}
```

Note that this wouldn't work without the `auto` import, which is providing `Encoder` instances for any case class (or sealed trait hierarchy) whose members are all also encodeable.

How does circe parse a generic type object to Json?

Solution

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