Scodec ignore last value when converting codecs between hlist and case class
I'm just starting out with the font-level "scodec" library: https://github.com/scodec/scodec
I found that I used the following function a lot:
/**
* When called on a `Codec[L]` for some `L <: HList`, returns a new codec that encodes/decodes
* `B :: L` but only returns `L`. HList equivalent of `~>`.
* @group hlist
*/
def :~>:[B](codec: Codec[B])(implicit ev: Unit =:= B): Codec[L] = codec.dropLeft(self)
This is useful if I have a case class where I don't want to use every spec value:
case class Example(value1: Int, value3)
implicit val exampleCodec: Codec[Example] = (
("value1" | uint8) ::
("value2" | uint8) :~>: // decode/encode, but dont pass this in when converting from hlist to case class
("value3" | uint8)
).as[Example]
This works well if the value I want to ignore is not the last one in the hlist. Does anyone know how to change the codec if instead I want my case class to be:
case class Example (value1: Int, value2: Int) // ignore value3
Any help is appreciated - thanks!
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You can just use <~
, so instead:
implicit val exampleCodec: Codec[Example] = (
("value1" | uint8) ::
("value2" | uint8).unit(0) :~>:
("value3" | uint8)
).as[Example]
You should write this:
implicit val exampleCodec: Codec[Example] = (
("value1" | uint8) ::
("value3" | uint8) <~
("value2" | uint8).unit(0)
).as[Example]
Note that you explicitly have to make the codec a Codec[Unit]
-I am using .unit(0)
here for example.
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