prob-monad-h

gistfile1.txt

module Dist where

import System.Random

data Dist a = Dist {
    support :: [a],
    gen :: StdGen -> (a, StdGen),
    expect :: (a -> Float) -> Float
}

instance Monad Dist where
  return a = Dist 
    {
      support = [a], 
      gen = \g -> (a, g), 
      expect = \f -> f a
    }

  da >>= fdb = Dist 
    {
      support = concat [support (fdb a) | a <- support da],
      gen = \g -> let (a, g') = (gen da g)  in  (gen (fdb a) g'),
      expect = \f -> expect da (\a -> expect (fdb a) f)
    }