scala-zio2-partially-applied-types-workshop

• references
  • Zymposium - ZIO API Design Techniques
  • https://circe.github.io/circe/codec.html
  • https://www.baeldung.com/scala/circe-json
  • https://typelevel.org/cats/typeclasses/show.html

preface

• goals of this workshop
• workshop plan
  1. implement ZIOCustom.serviceWithZIO to infer R, E, A types
     • it may be valuable to first take a look at: https://github.com/mtumilowicz/scala-cats-functional-dependency-injection-workshop#izumi-tag
  2. implement TransactionService.inTransaction to infer R, E, A types

partially applied arguments

• good analogy to set intuition on partially applied types
• example
  • problem

    def map[A, B](list: List[A], f: A => B): List[B] =
      list.map(f)
    
    val nums = List(1,2,3)
    
    mapping function(nums, _ + 1) // not compiling
    mapping function(nums, (i: Int) => i + 1) // compiling
    

  • solution - partial application of arguments

    def map[A, B](list: List[A])(f: A => B): List[B] =
      list.map(f)
    
    val nums = List(1,2,3)
    
    mapping function(nums)(_ + 1)
    

partially applied types

• in scala, you can specify either all types or none params
• example
  • problem

    def serviceWithZIO[Service: Tag, R, E, A](f: Service => ZIO[R, E, A]): ZIO[R with Service, E, A] =
      ZIO.service[Service].flatMap(service => f(service))
    

    • Service and R, E, A are not connected - Service has to be specified, rest can be inferred
  • solution - partial application of types

    def serviceWithZIO2[Service] =
      new ServiceWithZIOPartiallyApplied[Service]
    
    final class ServiceWithZIOPartiallyApplied[Service]() {
      def apply[R, E, A](f: Service => ZIO[R, E, A])(implicit tag: Tag[Service]): ZIO[R with Service, E, A] =
        ZIO.service[Service].flatMap(service => f(service)) // tag is needed by ZIO.service[Service]
    }
    

• usually used for wrappers, where we have groups of unconnected types
• performance
  • to keep it at zero cost, we have to use 'AnyVal'

    final class ServiceWithZIOPartiallyApplied[Service](private val dummy: Boolean = true) extends AnyVal {
        ...
    }