filterMN

gistfile1.scala

import scalaz._, syntax.monad._

final def filterMN[A, M[_]:Monad, N[_]:Monad](as: List[A])(p: A => N[M[Boolean]]): N[M[List[A]]] =
  as match {
    case Nil => Monad[N].point(Monad[M].point(Nil))
    case h :: t =>
      for {
        mb <- p(h)
        mg <- filterMN(t)(p)
      } yield {
        Monad[M].bind(mb)(b => if (b) Monad[M].map(mg)(tt => h :: tt) else mg)
      }
  }