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We construct a mixture distribution on draws from three bags of marbles. Each bag is associated with a discrete distribution on the three colors blue, green, and red. We draw marbles, with each marble drawn randomly from one of the bags.

(define colors '(blue green red))

(define samples
   200 100

   (define phi (dirichlet '(1 1 1)))
   (define alpha 0.1)
   (define prototype (map (lambda (w) (* alpha w)) phi))

   (define bag->prototype (mem (lambda (bag) (dirichlet prototype))))

   ;;the probability that an observation will come from each bag:
   (define bag-mixture (dirichlet '(1 1 1)))

   ;;each observation (which is named for convenience) comes from one of three bags:
   (define obs->bag
     (mem (lambda (obs-name)
            (multinomial '(bag1 bag2 bag3) bag-mixture))))

   (define draw-marble
     (mem (lambda (obs-name)
            (multinomial colors (bag->prototype (obs->bag obs-name))))))

   ;;did obs1 and obs2 come from the same bag? obs1 and obs3?
   (list (equal? (obs->bag 'obs1) (obs->bag 'obs2))
         (equal? (obs->bag 'obs1) (obs->bag 'obs3)))

    (equal? 'red (draw-marble 'obs1))
    (equal? 'red (draw-marble 'obs2))
    (equal? 'blue (draw-marble 'obs3))
    (equal? 'blue (draw-marble 'obs4))
    (equal? 'red (draw-marble 'obs5))
    (equal? 'blue (draw-marble 'obs6))

(hist (map first samples) "obs1 and obs2 same category?")
(hist (map second samples) "obs1 and obs3 same category?")