Support for default parameter values in generic

This appears to be mostly shapeless related, but @travisbrown asked me to file this for posterity…

case class Foo(s: String, i: Int = 10)
val json = """{"s": "Hello"}"""

// this obviously works
decode[Int => Foo](json).map(_(10))

// but what about cases where you have many default parameters of the same type?
case class Bar(i1: Int, i2: Int = 2, i3: Int = 3, i4: Int = 4)
val json2 = """{"i1": 1}"""

// i would like the following behavior...
"""{"i1": 1, "i4": 44}""".as[Bar] // => Bar(1, 2, 3, 44)

// this would work kind of work, but it overrides the default values
decode[(Int, Int, Int) => Bar].map(_(2, 3, 44))

// alternatively, I could do this to ignore the potentially incoming values:
val dec = Decoder.instance[Bar](c =>
  for {
    i1 <- c.downField("i1").as[Int]
  } yield Bar(i1)
)

// or change my datatype completely:
case class Bar2(i1: Int, i2: Option[Int], i3: Option[Int], i4: Option[Int])
val dec2 = Decoder.instance[Bar2](c =>
  for {
    i1 <- c.downField("i1").as[Int]
    i2 <- c.downField("i2").as[Option[Int]]
    i3 <- c.downField("i3").as[Option[Int]]
    i4 <- c.downField("i4").as[Option[Int]]
  } yield Bar2(i1, i2, i3, i4)
)

[travisbrown]

This is now available in circe-extras in 0.6.0-RC1.

[travisbrown]

@trane It’s not necessarily obvious that a derived decoder will use default constructor values (when they’re needed), and if that’s not the behavior you want, it could be an unpleasant surprise. But I agree—I think it makes sense to use them (with plenty of documentation) in auto, and to provide something like decoderWithoutDefaults in semiauto.