Complementary code for introduction of the Reader in "Dead simple dependency injection video https://youtu.be/ZasXwtTRkio "

reader.scala

package me.reader

/**
  * Created by yuri.zelikov on 1/6/2016.
  */

case class Reader[C, A](g: C => A) {
  def apply(c: C) = g(c)

  def map[B](f: A => B): Reader[C, B] = { c: C =>
    f(g(c))
  }

  def flatMap[B](f: A => Reader[C, B]): Reader[C, B] = { c: C =>
    f(g(c))(c)
  }
}

object Reader {
  implicit def reader[A, B](f: A => B): Reader[A, B] = Reader(f)

  def pure[C, A](a: A): Reader[C, A] = { c: C =>
    a
  }
}

trait KeyValueStore {
  def put(key: String, value: String): Unit

  def get(key: String): String

  def delete(key: String): Unit
}

class StrKeyValueStore extends KeyValueStore {
  var store: Map[String, String] = Map()

  override def put(key: String, value: String): Unit = {
    store = Map[String, String]() + (key -> value) ++ store
  }

  override def get(key: String): String = store(key)

  override def delete(key: String): Unit = {
    store = store - key
  }
}

object RunReader {

  type KvsReader[A] = Reader[KeyValueStore, A]

  def modify[T](k: String, f: String => T): KvsReader[T] = {
    val v: KvsReader[String] = (kvs: KeyValueStore) => kvs.get(k)
    v.map(f)
  }

  // get error: value filter is not a member of me.reader.KeyValueStore
//  def modify2[A](k: String, f: String => String): KvsReader[A] = {
//    for {
//      v: String  <- (kvs: KeyValueStore) => kvs.get(k)
//    } yield f(v)
//  }
}