Dead-simple dependency injection based on Scalaz Reader Monad

Injector.scala

import scalaz.Reader

/**
  * Trait providing helper methods for handling application-wide dependency injection using the
  * ''reader monad'' functional pattern.
  *
  * @author Vaclav Svejcar ([email protected])
  */
trait Injector {

  /**
    * Injects the specified dependency into the given ''reader monad''.
    *
    * = Example of use: =
    * {{{
    *   def testReader() = Reader((config: Config) => config.getString("foo.bar"))
    *   def withConfig[R] = inject[Config, R](configToInject) _
    *   val result: String = withConfig(testReader())
    * }}}
    * 
    * = Or using implicit conversion: =
    * {{{
    *   def testReader() = Reader((config: Config) => config.getString("foo.bar"))
    *   implicit def withConfig[R]: (Reader[Config, R] => R) = inject[Config, R](configToInject)
    *   val result: String = testReader()
    * }}}
    *
    * @param dependency dependency to be injected into the ''reader monad''
    * @param reader     ''reader monad''
    * @tparam D type of the dependency
    * @tparam R type of the result
    * @return result of the ''reader monad'' operation
    */
  protected def inject[D, R](dependency: D)(reader: Reader[D, R]): R = reader(dependency)
}