Configuring implicits in Scala - scala

Question

I have typeclass:
trait ProcessorTo[T]{
def process(s: String): T
}
and its implementation
class DefaultProcessor extends ProcessorTo[String]{
def process(s: String): String = s
}
trait DefaultProcessorSupport{
implicit val p: Processor[String] = new DefaultProcessor
}
To make it available for using I created
object ApplicationContext
extends DefaultProcessorSupport
with //Some other typeclasses
But now I have to add a processor which performs some DataBase - read. The DB URL etc are placed in condifguration file that is available only a runtime. For now I did the following.
class DbProcessor extends ProcessorTo[Int]{
private var config: Config = _
def start(config: Config) = //set the configuration, open connections etc
//Other implementation
}
object ApplicationContext{
implicit val p: ProcessorTo[Int] = new DbProcessor
def configure(config: Config) = p.asInstanceOf[DbProcessor].start(config)
}
It works for me, but I'm not sure about this technique. Looks strange for me a little bit. Is it a bad practice? If so, what would be a good solution?

Answer 1

I am a bit confused by the requirements as DbProcessor is missing the process implementation(???) and trait ProcessorTo[T] is missing start method which is defined in DbProcessor. So, I will assume the following while answering: the type class has both process and start methods
Define a type class:
trait ProcessorTo[T]{
def start(config: Config): Unit
def process(s: String): T
}
Provide implementations for the type class in the companion objects:
object ProcessorTo {
implicit object DbProcessor extends ProcessorTo[Int] {
override def start(config: Config): Unit = ???
override def process(s: String): Int = ???
}
implicit object DefaultProcessor extends ProcessorTo[String] {
override def start(config: Config): Unit = ???
override def process(s: String): String = s
}
}
and use it in your ApplicationContext as follows:
object ApplicationContext {
def configure[T](config: Config)(implicit ev: ProcessorTo[T]) = ev.start(config)
}
This is a nice blog post about Type Classes: http://danielwestheide.com/blog/2013/02/06/the-neophytes-guide-to-scala-part-12-type-classes.html

Answer 2

I don't really see why you need start. If your implicit DbProcessor has a dependency, why not make it an explicit dependency via constructor? I mean something like this:
class DbConfig(val settings: Map[String, Object]) {}
class DbProcessor(config: DbConfig) extends ProcessorTo[Int] {
// here goes actual configuration of the processor using config
private val mappings: Map[String, Int] = config.settings("DbProcessor").asInstanceOf[Map[String, Int]]
override def process(s: String): Int = mappings.getOrElse(s, -1)
}
object ApplicationContext {
// first create config then pass it explicitly
val config = new DbConfig(Map[String, Object]("DbProcessor" -> Map("1" -> 123)))
implicit val p: ProcessorTo[Int] = new DbProcessor(config)
}
Or if you like Cake pattern, you can do something like this:
trait DbConfig {
def getMappings(): Map[String, Int]
}
class DbProcessor(config: DbConfig) extends ProcessorTo[Int] {
// here goes actual configuration of the processor using config
private val mappings: Map[String, Int] = config.getMappings()
override def process(s: String): Int = mappings.getOrElse(s, -1)
}
trait DbProcessorSupport {
self: DbConfig =>
implicit val dbProcessor: ProcessorTo[Int] = new DbProcessor(self)
}
object ApplicationContext extends DbConfig with DbProcessorSupport {
override def getMappings(): Map[String, Int] = Map("1" -> 123)
}
So the only thing you do in your ApplicationContext is providing actual implementation of the DbConfig trait.