callCC.sc

import $ivy.`org.typelevel::cats-core:2.1.0`
import cats._
import data.ContT
import syntax.all._


// ContT.liftF, currently unreleased.
def ContT_liftF[M[_]: FlatMap, R, A](ma: M[A]): ContT[M, R, A] =
  ContT(ma.flatMap(_))

// Is this stack safe?
def callCC[M[_]: Monad, R, A](f: (M[A] => ContT[M, R, Nothing]) => ContT[M, R, A]): ContT[M, R, A] =
  ContT(cont => f(ContT_liftF(_).withCont(_ => cont)).run(cont))


// Example use
def getCC[M[_]: Monad: Defer, R]: ContT[M, R, ContT[M, R, Nothing]] = {
  callCC[M, R, ContT[M, R, Nothing]] {
    type F[A] = ContT[M, R, A]
    escape => Defer[F].fix[Nothing](self => escape(Monad[M].pure(self)))
  }
}