"dependency injection" monad (for the lulz)

Inject.scala

case class Inject[-E, +A](inject: E => A) {

  // covariant functor and monad
  def map[B](f: A => B): Inject[E, B] =
    Inject { e => f(inject(e)) }
  def flatMap[ε <: E, B](f: A => Inject[ε, B]): Inject[ε, B] =
    Inject { e => f(inject(e)).inject(e) }

  // to satisfy for-comprehension desugaring in scala < 2.10
  def filter(f: A => Boolean): Inject[E, A] =
    map { a => if (f(a)) a else throw new MatchError(a) }

  // contravariant functor: adapt this injector to work on some other environment
  def module[ε](f: ε => E): Inject[ε, A] =
    Inject { e => inject(f(e)) }

}

object Inject {
  def ask[E]: Inject[E, E] = Inject { e => e }
  def const[A](a: A): Inject[Any, A] = Inject { _ => a }
}

def ask[E]: Inject[E, E] = Inject { identity }

:)