Qux.scala

// Before

import io.circe.Decoder

case class Qux(a: Int, b: String)

val decodeQuxMonadic: Decoder[Qux] = Decoder.instance { c =>
  for {
    a <- c.get[Int]("a")
    b <- c.get[String]("b")
  } yield Qux(a, b)
}

// After

import cats.implicits._
import io.circe.Decoder

case class Qux(a: Int, b: String)

val decodeQuxApplicative: Decoder[Qux] = (
  Decoder[Int].prepare(_.downField("a")),
  Decoder[String].prepare(_.downField("b"))
).mapN(Qux.apply _)