CaseClass.constructEither

[fommil]

The current implementation of CaseClass.construct means that typeclasses that stop at Functor can be implemented (e.g. Default), but typeclasses with a MonadError (e.g. a JSON decoder) cannot.

The workaround is to use locally scoped exceptions, e.g.:

  private case class Fail(msg: String) extends Exception with NoStackTrace

  def combine[A](ctx: CaseClass[JsDecoder, A]): JsDecoder[A] = {
    case obj @ JsObject(_) =>
      try {
        val success = ctx.construct { p =>
          val value = obj.get(p.label).getOrElse(JsNull)
          p.typeclass.fromJson(value) match {
            case \/-(got)  => got
            case -\/(fail) => throw new Fail(fail) // scalafix:ok
          }
        }
        \/-(success)
      } catch {
        case Fail(msg) => -\/(msg)
      }
    case other => JsDecoder.fail("JsObject", other)
  }

it would be better if you allowed a traversal, even just for Either, allowing

  private case class Fail(msg: String) extends Exception with NoStackTrace

  def combine[A](ctx: CaseClass[JsDecoder, A]): JsDecoder[A] = {
    case obj @ JsObject(_) =>
        ctx.constructEither { p =>
          val value = obj.get(p.label).getOrElse(JsNull)
          p.typeclass.fromJson(value)
        }
    case other => JsDecoder.fail("JsObject", other)
  }

If you produced the boilerplate that called .flatMap then users could even use their own data types, not just Either.

[fommil]

another usecase:

import magnolia._
import org.scalacheck._

import scalaz._, Scalaz._
import scalaz.scalacheck.ScalaCheckBinding._

object ScalacheckMagnolia {
  type Typeclass[A] = Arbitrary[A]

  def combine[A](ctx: CaseClass[Arbitrary, A]): Arbitrary[A] = Arbitrary {
    ctx.parameters.toList.traverse(p => Gen.lzy(p.typeclass)).map { ps =>
      ctx.rawConstruct(ps)
    }
  }

  def dispatch[A](ctx: SealedTrait[Arbitrary, A]): Arbitrary[A] = Arbitrary {
    Gen.frequency(ctx.subtypes.map(s => 1 -> (Gen.lzy(s.typeclass.arbitrary))): _*)
  }

  implicit def gen[A]: Arbitrary[A] = macro Magnolia.gen[A]

}