prob-monad-sz

gistfile1.txt

import scalaz._

object distribMonad {

  import Scalaz._
  import scala.util.Random

  abstract class Dist[A] {
    def support: List[A]
    def gen: Random => (A, Random)
    def expect: (A => Double) => Double
  }

  def DistMonad = new Monad[Dist] {
    def pure[A](a: => A): Dist[A] = new Dist[A] {
      def support = List(a)
      def gen(g: Random) = (a, g)
      def expect(f: (A => Double)) = f(a)
    }
    def bind[A,B](da:Dist[A],fdb:A=>Dist[B]):Dist[B] = new Dist[B] {
      def support = (for (a <- da.support) yield fdb(a).support).flatten
      def gen(g: Random) = da.gen(g) match { case (a, g1) => (fdb(a)).gen(g1)}
      def expect(f: (B => Double)) = da.expect {a => (fdb(a)).expect(f)}
    }
  }

}