Decoding: Configuration.default.withDefaults makes it use the default value when the decoding the field failed

I’m using the Configuration.default.withDefaults feature and lots of my case classes have defaults values for members. However, when decoding a json where there is an error (e.g. type mismatch) it uses the default value and swallows the error:

import io.circe.generic.extras.Configuration
import io.circe.generic.extras.auto._

implicit val customConfig: Configuration = Configuration.default.withDefaults
final case class Foo(number: Int = 999)
io.circe.parser.decode[Foo]("""{"number": "not a number!"}""")

res0: Either[io.circe.Error,Foo] = Right(Foo(999))

Ideally, I’d like the default value to be used only if the field was missing from the json, and get an error when it was there but incorrectly.

Can I change this behavior somewhere central so that I don’t need to have custom decoders for all my case classes with defaults?

[exoego]

I think that “fallback to default value if decoding failed” is a good option to have. But it should not be default, since I think

• If value of wrong type is given in JSON, decoding should fail and report developers what is wrong.
• Default values should be used only if key is missing in JSON.

I think popular JSON-based Web-APIs works like this.

[froth]

Took the time to try the ApplicativeError idea it out and found it was more complicated than helpful. But finished the pull request anyway while doing so. 😉 Please have a look.