Decoding JSON into Shapeless's sized collections in Scala with circe:

Decode.scala

// http://stackoverflow.com/questions/39183041/circe-type-level-json-a-function/39183581#39183581
// https://twitter.com/travisbrown/status/769579633771773954

object Main extends App {
  import cats.data.Xor
  import io.circe.{Decoder, DecodingFailure}
  import shapeless.{Nat, Sized}
  import shapeless.ops.nat.ToInt
  import shapeless.syntax.sized._

  implicit def decodeSized[L <: Nat, A](implicit
    dl: Decoder[List[A]],
    ti: ToInt[L]
  ): Decoder[Sized[List[A], L]] = Decoder.instance { c =>
    dl(c).flatMap(as =>
      Xor.fromOption(as.sized[L], DecodingFailure(s"Sized[List[A], _${ti()}]", c.history))
    )
  }

  case class SSN(value: Sized[List[Int], Nat._8])
  implicit val decodeSSN: Decoder[SSN] = Decoder[Sized[List[Int], Nat._8]].map(SSN(_))

  import io.circe.jawn.decode
  val x = decode[SSN]("[1, 2, 3, 4, 5, 6, 7, 8]")
  println(x)
  val y = decode[SSN]("[1, 2, 3, 4, 5, 6, 7]")
  println(y)

  // Right(SSN(List(1, 2, 3, 4, 5, 6, 7, 8)))
  // Left(DecodingFailure(Sized[List[A], _8], List()))
}

/*
libraryDependencies ++= Seq(
  "org.typelevel" %% "cats" % "0.7.0",
  "com.chuusai" %% "shapeless" % "2.3.2"
)

val circeVersion = "0.5.0-M3"
libraryDependencies ++= Seq(
  "io.circe" %% "circe-core",
  "io.circe" %% "circe-generic",
  "io.circe" %% "circe-parser",
  "io.circe" %% "circe-literal"
).map(_ % circeVersion)
*/