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Matt Moore
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Circe

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How to use Circe - which imports to use, auto vs semiauto derivation, etc…

Circe Standalone
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Auto Derivation
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Auto derivation is typically used. Circe will using shapeless for derivation of types to determine how to encode and decode.

Semiauto Derivation
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Semiauto derivation is very similar to auto but allows for selecting whether to encode or just decode.

With http4s / cats-effect
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Auto Derivation
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//> using scala "3.2.2"

//> using dep "io.circe::circe-core:0.14.5"
//> using dep "io.circe::circe-generic:0.14.5"
//> using dep "io.circe::circe-parser:0.14.5"

//> using dep "org.http4s::http4s-circe:0.23.18"
//> using dep "org.http4s::http4s-dsl:0.23.18"
//> using dep "org.http4s::http4s-ember-client:0.23.18"
//> using dep "org.http4s::http4s-ember-server:0.23.18"

//> using dep "org.typelevel::cats-effect:3.4.10"

//> using dep "ch.qos.logback:logback-classic:1.4.7"

import cats.effect.*

import com.comcast.ip4s.*

import io.circe.*
import io.circe.syntax.*
import io.circe.generic.auto.*

import org.http4s.*
import org.http4s.circe.*
import org.http4s.dsl.io.*
import org.http4s.implicits.*
import org.http4s.ember.server.*

case class HelloResponse(greeting: String)

object Main extends IOApp {
  val helloWorldService = HttpRoutes.of[IO] {
    case GET -> Root / "hello" / name =>
      Ok(HelloResponse(s"Hello, $name").asJson)
  }.orNotFound

  def run(args: List[String]): IO[ExitCode] =
    EmberServerBuilder
      .default[IO]
      .withHost(ipv4"0.0.0.0")
      .withPort(port"8080")
      .withHttpApp(helloWorldService)
      .build
      .use(_ => IO.never)
      .as(ExitCode.Success)
}

Semiauto Derivation
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//> using scala "3.2.2"

//> using dep "io.circe::circe-core:0.14.5"
//> using dep "io.circe::circe-generic:0.14.5"
//> using dep "io.circe::circe-parser:0.14.5"

//> using dep "org.http4s::http4s-circe:0.23.18"
//> using dep "org.http4s::http4s-dsl:0.23.18"
//> using dep "org.http4s::http4s-ember-client:0.23.18"
//> using dep "org.http4s::http4s-ember-server:0.23.18"

//> using dep "org.typelevel::cats-effect:3.4.10"

//> using dep "ch.qos.logback:logback-classic:1.4.7"

import cats.effect.*

import com.comcast.ip4s.*

import io.circe.*
import io.circe.syntax.*
import io.circe.generic.semiauto.*

import org.http4s.*
import org.http4s.circe.*
import org.http4s.dsl.io.*
import org.http4s.implicits.*
import org.http4s.ember.server.*

case class HelloResponse(greeting: String)

given Encoder[HelloResponse] = deriveEncoder
given Decoder[HelloResponse] = deriveDecoder
// Both decoder and encoder as a codec
// given Codec[HelloResponse] = deriveCodec[HelloResponse]

object Main extends IOApp {
  val helloWorldService = HttpRoutes.of[IO] {
    case GET -> Root / "hello" / name =>
      Ok(HelloResponse(s"Hello, $name").asJson)
  }.orNotFound

  def run(args: List[String]): IO[ExitCode] =
    EmberServerBuilder
      .default[IO]
      .withHost(ipv4"0.0.0.0")
      .withPort(port"8080")
      .withHttpApp(helloWorldService)
      .build
      .use(_ => IO.never)
      .as(ExitCode.Success)
}