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package magnolia.examples
import magnolia._
import scala.language.experimental.macros
/** very basic decoder for converting strings to other types */
trait Decoder[T] { def decode(str: String): T }
/** derivation object (and companion object) for [[Decoder]] instances */
object Decoder {
/** decodes strings */
implicit val string: Decoder[String] = new Decoder[String] {
def decode(str: String): String = str
}
/** decodes ints */
implicit val int: Decoder[Int] = new Decoder[Int] { def decode(str: String): Int = str.toInt }
/** binds the Magnolia macro to this derivation object */
implicit def gen[T]: Decoder[T] = macro Magnolia.gen[T]
/** type constructor for new instances of the typeclass */
type Typeclass[T] = Decoder[T]
/** defines how new [[Decoder]]s for case classes should be constructed */
def combine[T](ctx: CaseClass[Decoder, T]): Decoder[T] = new Decoder[T] {
def decode(value: String) = {
val (_, values) = parse(value)
ctx.construct { param =>
param.typeclass.decode(values(param.label))
}
}
}
/** defines how to choose which subtype of the sealed trait to use for decoding */
def dispatch[T](ctx: SealedTrait[Decoder, T]): Decoder[T] = new Decoder[T] {
def decode(param: String) = {
val (name, _) = parse(param)
val subtype = ctx.subtypes.find(_.typeName.full == name).get
subtype.typeclass.decode(param)
}
}
/** very simple extractor for grabbing an entire parameter value, assuming matching parentheses */
private def parse(value: String): (String, Map[String, String]) = {
val end = value.indexOf('(')
val name = value.substring(0, end)
def parts(value: String,
idx: Int = 0,
depth: Int = 0,
collected: List[String] = List("")): List[String] = {
def plus(char: Char): List[String] = collected.head + char :: collected.tail
if (idx == value.length) collected
else
value(idx) match {
case '(' =>
parts(value, idx + 1, depth + 1, plus('('))
case ')' =>
if (depth == 1) plus(')')
else parts(value, idx + 1, depth - 1, plus(')'))
case ',' =>
if (depth == 0) parts(value, idx + 1, depth, "" :: collected)
else parts(value, idx + 1, depth, plus(','))
case char =>
parts(value, idx + 1, depth, plus(char))
}
}
def keyValue(str: String): (String, String) = {
val List(label, value) = str.split("=", 2).to[List]
(label, value)
}
(name, parts(value.substring(end + 1, value.length - 1)).map(keyValue).toMap)
}
}
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