Error messages lost for recusive decoders

When using emap to create a custom validation for a recusive decoder the error messages are lost for the parts of the object which are decoded recursively:

import io.circe._, io.circe.generic.semiauto._, io.circe.generic.decoding.DerivedDecoder, io.circe.generic.encoding.DerivedObjectEncoder

case class A(i: Int, l: List[A])

def validA(a: A): Boolean = {
    a.i > 0 && a.l.map(validA).foldLeft(true)(_ && _)
}

implicit val ae: Decoder[A] = deriveDecoder[A].emap { a =>
    if(!validA(a)) {
        Left("Bad!")
    } else {
        Right(a)
    }
}

val s = """{"i":1, "l": [{"i": -1, "l": []}, {"i": 2, "l": []}]}"""

parser.parse(s).right.get.as[A]

When running the last line of code you get

io.circe.Decoder.Result[A] = Left(DecodingFailure([A]List[A], List(DownArray, DownField(l))))

Which is a valid decoding failure, but the error message of [A]List[A] isn’t what is expected. I would expect Bad! or something similar to DecodingFailure at .l[0]: Bad!

I’m using circe 0.7.0 for reference. And this issue was originally raised in the gitter channel

[cponomaryov]

I think it is fixed by https://github.com/circe/circe/pull/649

[travisbrown]

@EdgeCaseBerg Normally I’ve been closing issues when the fix hits master. If it’s really urgent I could backport this for an 0.8.1, but I’m guessing 0.9 is less than a week off and I wouldn’t be able to get 0.8.1 out until Saturday anyway.