hmm.wppl

var transition = function(s) {
  return s ? flip(0.7) : flip(0.3)
}

var observe = function(s) {
  return s ? flip(0.9) : flip(0.1)
}

var hmm = function(n) {
  var prev = (n == 1) ? {states: [true], observations: []} : hmm(n - 1);
  var newState = transition(prev.states[prev.states.length - 1]);
  var newObs = observe(newState);
  return {
    states: prev.states.concat([newState]),
    observations: prev.observations.concat([newObs])
  };
}

var trueObservations = [false, false, false];

var arrayEq = function(a, b) {
  return (a.length == 0) ? true : (a[0] == b[0] && arrayEq(a.slice(1), b.slice(1)))
}

Enumerate(function() {
  var r = hmm(3);
  factor(arrayEq(r.observations, trueObservations) ? 0 : -Infinity);
  return r.states;
})