Basic port of Control.Monad.ContT

ContT.scala

package cats
package data

import cats.syntax.all._

trait ContT[R, M[_], A] {
  def run(f: A => M[R]): M[R]

  def mapResult(f: M[R] => M[R]): ContT[R, M, A] =
    ContT(c => f(run(c)))

  def map[B](f: A => B): ContT[R, M, B] =
    ContT(c => run(a => c(f(a))))

  def flatMap[B](f: A => ContT[R, M, B]): ContT[R, M, B] =
    ContT(c => run(a => f(a).run(c)))

  def with_[B](f: (B => M[R]) => (A => M[R])): ContT[R, M, B] =
    ContT(c => run(f(c)))

  def liftLocal[R1](ask: M[R1], local: (R1 => R1, M[R]) => M[R], f: R1 => R1)(implicit M: Monad[M]): ContT[R, M, A] =
    ContT(c => ask.flatMap(r => local(f, run(a => local(_ => r, c(a))))))
}

object ContT {

  def apply[R, M[_], A](c: (A => M[R]) => M[R]): ContT[R, M, A] =
    new ContT[R, M, A] {
      def run(f: A => M[R]): M[R] = c(f)
    }

  def eval[R, M[_]: Monad](co: ContT[R, M, R]): M[R] =
    co.run(Monad[M].pure)

  def callCC[R, M[_], A, B](f: (A => ContT[R, M, B]) => ContT[R, M, A]): ContT[R, M, A] =
    ContT(c => f(a => ContT(_ => c(a))).run(c))

  def shift[R, M[_]: Monad, A](f: (A => M[R]) => ContT[R, M, R]): ContT[R, M, A] =
    ContT(c => ContT.eval(f(c)))

  def reset[R, M[_]: Monad, R1](co: ContT[R, M, R]): ContT[R1, M, R] =
    ContT(c => eval(co).flatMap(c))

  implicit def monad[R, M[_]: Monad]: Monad[ContT[R, M, ?]] =
    new Monad[ContT[R, M, ?]] {
      def pure[A](a: A): ContT[R, M, A] =
        ContT(c => c(a))
      override def ap[A, B](co: ContT[R, M, A])(cof: ContT[R, M, A => B]): ContT[R, M, B] =
        ContT(c => cof.run(f => co.run(a => c(f(a)))))
      override def map[A, B](co: ContT[R, M, A])(f: A => B): ContT[R, M, B] =
        co.map(f)
      override def map2[A, B, Z](fa: ContT[R, M, A], fb: ContT[R, M, B])(f: (A, B) => Z): ContT[R, M, Z] =
        fa.flatMap(a => fb.map(b => f(a, b)))
      override def flatten[A](co: ContT[R, M, ContT[R, M, A]]): ContT[R, M, A] =
        ContT(c => co.run(_.run(c)))
      def flatMap[A, B](co: ContT[R, M, A])(f: A => ContT[R, M, B]): ContT[R, M, B] =
        co.flatMap(f)
    }
}