The Kalman filter (or Kalman smoother) is a Hidden Markov Model with Gaussian transition and observation functions. The model here is based on the Kalman filter in Anglican.

```
(define xs '(1 2 3 4 5 7 8 9 10))
(define ys '(0.38 0.49 0.47 0.28 0.24 0.33 0.47 0.40 0.44))
(define samples
(mh-query
10000 1
;; Measurement noise variance
(define m-noise 0.01)
;; State transition model which is applied to the previous state
(define A 1)
;; Observation model which maps the true state space into the observed space
(define H 1)
;; Process noise variance
(define p-noise 0.00001)
;; Transition function
(define f
(mem
(lambda (t)
(if (<= t 1)
(gaussian 0 1)
(gaussian (* A (f (- t 1))) p-noise)))))
;; Query expression
(f 6)
;; Condition
(all
(map (lambda (x y) (= (gaussian (* H (f x)) m-noise y) y))
xs
ys))))
(density samples "(f 6)" #t)
```

References: