Encoders, decoders and codecs

We’ve seen how kantan.csv uses encoders and decoders as a convenient way to support new types. This didn’t account for a fairly common scenario, however: types for which one wishes to declare both an encoder and a decoder. It’s certainly possible to write both, but it’s a bit cumbersome. kantan.csv offers an alternative: codecs, which are simply an encoder and a decoder rolled into one.

Cell codecs

We’ve seen before how to create a CellEncoder and a CellDecoder for joda’s DateTime. CellCodec allows you to do the same thing, but in one go:

import kantan.csv._
import kantan.csv.ops._
import org.joda.time.DateTime
import org.joda.time.format.ISODateTimeFormat

implicit val jodaDateTime: CellCodec[DateTime] = {
  val format = ISODateTimeFormat.date()
  CellCodec.from(s => DecodeResult(format.parseDateTime(s)))(d => format.print(d))
}

And with that, we can now both encode and decode DateTime:

val dates = List(
  List(new DateTime(), new DateTime().plusDays(1)),
  List(new DateTime().plusDays(2), new DateTime().plusDays(3))
).asCsv(rfc)
// dates: String = """2022-09-06,2022-09-07
// 2022-09-08,2022-09-09
// """

dates.readCsv[List, List[DateTime]](rfc)
// res0: List[ReadResult[List[DateTime]]] = List(
//   Right(
//     value = List(2022-09-06T00:00:00.000+02:00, 2022-09-07T00:00:00.000+02:00)
//   ),
//   Right(
//     value = List(2022-09-08T00:00:00.000+02:00, 2022-09-09T00:00:00.000+02:00)
//   )
// )

Row codecs

There’s a very similar mechanism for rows: RowCodec. This one is a bit more powerful and a bit more complicated, however: all the helper methods we’ve seen for creating RowDecoder and RowEncoder instances also exist for RowCodec. Let’s take a concrete example with case classes.

case class Person(id: Int, name: String, age: Int)

val ps = List(Person(0, "Nicolas", 38), Person(1, "Kazuma", 1), Person(2, "John", 18))

We want to be able to both encode and decode that, so we can create a RowCodec[Person] instance:

implicit val personCodec: RowCodec[Person] = RowCodec.caseCodec(0, 2, 1)(Person.apply)(Person.unapply)

And with that one line, we’re done:

val csv = ps.asCsv(rfc)
// csv: String = """0,38,Nicolas
// 1,1,Kazuma
// 2,18,John
// """

csv.readCsv[List, Person](rfc)
// res1: List[ReadResult[Person]] = List(
//   Right(value = Person(id = 0, name = "Nicolas", age = 38)),
//   Right(value = Person(id = 1, name = "Kazuma", age = 1)),
//   Right(value = Person(id = 2, name = "John", age = 18))
// )
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