gistfile1.txt

Error:(26, 56) type mismatch;
 found   : argonaut.DecodeResult[algorithmia.TechSupport.Train]
 required: argonaut.DecodeResult[algorithmia.TechSupport.Input]
Note: algorithmia.TechSupport.Train <: algorithmia.TechSupport.Input, but class DecodeResult is invariant in type A.
You may wish to define A as +A instead. (SLS 4.5)
        (failReason, cursor) => Train.trainCodec.decode(json),

nope.scala

package algorithmia.TechSupport

import algorithmia.TechSupport.Classes.DataPoint
import argonaut.Argonaut._
import argonaut.CodecJson
import argonaut._
/**
  * Created by james on 05/01/17.
  */

sealed trait Input

//TODO: Learn how implicits and type inference work, because this is some weird shit.
//also talk to Jacoby6000 on irc.
object Input {
  implicit def inputCodec: CodecJson[Input] = {

    def encoder(data: Input): Json =
      data match {
        case train: Train => Train.trainCodec.encode(train)
        case predict: Predict => Predict.predictCodec.encode(predict)
      }

    def decoder(json: HCursor): DecodeResult[Input] =
      Predict.predictCodec.decode(json).fold(
        (failReason, cursor) => Train.trainCodec.decode(json),
        successResult => DecodeResult.ok(successResult)
      )


    CodecJson[Input](encoder, decoder)
  }
}

case class Train(labelData: Option[List[DataPoint]], labelFile: Option[String], namespace: Option[String]) extends Input

case class Predict(text: String, namespace: Option[String]) extends Input



object Train{
  implicit def trainCodec: CodecJson[Train] =
    casecodec3(Train.apply, Train.unapply)(
      "labelData",
      "labelFile",
      "namespace"
    )
}

object Predict{
  implicit def predictCodec: CodecJson[Predict] =
    casecodec2(Predict.apply, Predict.unapply)(
      "text",
      "namespace"
    )
}