Configuring implicits in 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?

Why not require that any method which wants to do processing will have an implicit ev: Processor[T]? What is this ApplicationContext object? — Yuval Itzchakov
– Yuval Itzchakov, Commented Nov 22, 2017 at 13:03
@YuvalItzchakov ApplicationContext object contains typeclasses needed to be imported. — St.Antario
– St.Antario, Commented Nov 22, 2017 at 13:05
@YuvalItzchakov Even if I require implict ev: Processor[T], I need to configure the DbProcessor anyway. — St.Antario
– St.Antario, Commented Nov 22, 2017 at 13:06
Ok, do you have to use an object to place all these implicits there? I'd separate these concerns out and have the outmost top place create an instance of the right processor I need. — Yuval Itzchakov
– Yuval Itzchakov, Commented Nov 22, 2017 at 13:07
@YuvalItzchakov The reason I put all these implicits there was the following: I wanted to generify my processing engine which using some of these implicits and to modify its behavior just import the configured application context. So I abstraced away constructurs and configuration of the specific implicit in my application. — St.Antario
– St.Antario, Commented Nov 22, 2017 at 13:12

curious · Accepted Answer · 2017-11-22 13:06:32Z

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

SergGr · Accepted Answer · 2017-11-22 15:07:26Z

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.

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