A probability monad for the bootstrap particle filter

Introduction

In the previous post I showed how to write your own general-purpose monadic probabilistic programming language from scratch in 50 lines of (Scala) code. That post is a pre-requisite for this one, so if you haven’t read it, go back and have a quick skim through it before proceeding. In that post I tried to keep everything as simple as possible, but at the expense of both elegance and efficiency. In this post I’ll address one problem with the implementation from that post – the memory (and computational) overhead associated with forming the Cartesian product of particle sets during monadic binding (flatMap). So if particle sets are typically of size N, then the Cartesian product is of size N^2, and multinomial resampling is applied to this set of size N^2 in order to sample back down to a set of size N. But this isn’t actually necessary. We can directly construct a set of size N, certainly saving memory, but also potentially saving computation time if the conditional distribution (on the right of the monadic bind) can be efficiently sampled. If we do this we will have a probability monad encapsulating the logic of a bootstrap particle filter, such as is often used for computing the filtering distribution of a state-space model in time series analysis. This simple change won’t solve the computational issues associated with deep monadic binding, but does solve the memory problem, and can lead to computationally efficient algorithms so long as care is taken in the formulation of probabilistic programs to ensure that deep monadic binding doesn’t occur. We’ll discuss that issue in the context of state-space models later, once we have our new SMC-based probability monad.

Materials for this post can be found in my blog repo, and a draft of this post itself can be found in the form of an executable tut document.

An SMC-based monad

The idea behind the approach to binding used in this monad is to mimic the “predict” step of a bootstrap particle filter. Here, for each particle in the source distribution, exactly one particle is drawn from the required conditional distribution and paired with the source particle, preserving the source particle’s original weight. So, in order to operationalise this, we will need a draw method adding into our probability monad. It will also simplify things to add a flatMap method to our Particle type constructor.

To follow along, you can type sbt console from the min-ppl2 directory of my blog repo, then paste blocks of code one at a time.

  import breeze.stats.{distributions => bdist}
  import breeze.linalg.DenseVector
  import cats._
  import cats.implicits._

  implicit val numParticles = 2000

  case class Particle[T](v: T, lw: Double) { // value and log-weight
    def map[S](f: T => S): Particle[S] = Particle(f(v), lw)
    def flatMap[S](f: T => Particle[S]): Particle[S] = {
      val ps = f(v)
      Particle(ps.v, lw + ps.lw)
    }
  }

I’ve added a dependence on cats here, so that we can use some derived methods, later. To take advantage of this, we must provide evidence that our custom types conform to standard type class interfaces. For example, we can provide evidence that Particle[_] is a monad as follows.

  implicit val particleMonad = new Monad[Particle] {
    def pure[T](t: T): Particle[T] = Particle(t, 0.0)
    def flatMap[T,S](pt: Particle[T])(f: T => Particle[S]): Particle[S] = pt.flatMap(f)
    def tailRecM[T,S](t: T)(f: T => Particle[Either[T,S]]): Particle[S] = ???
  }

The technical details are not important for this post, but we’ll see later what this can give us.

We can now define our Prob[_] monad in the following way.

  trait Prob[T] {
    val particles: Vector[Particle[T]]
    def draw: Particle[T]
    def mapP[S](f: T => Particle[S]): Prob[S] = Empirical(particles map (_ flatMap f))
    def map[S](f: T => S): Prob[S] = mapP(v => Particle(f(v), 0.0))
    def flatMap[S](f: T => Prob[S]): Prob[S] = mapP(f(_).draw)
    def resample(implicit N: Int): Prob[T] = {
      val lw = particles map (_.lw)
      val mx = lw reduce (math.max(_,_))
      val rw = lw map (lwi => math.exp(lwi - mx))
      val law = mx + math.log(rw.sum/(rw.length))
      val ind = bdist.Multinomial(DenseVector(rw.toArray)).sample(N)
      val newParticles = ind map (i => particles(i))
      Empirical(newParticles.toVector map (pi => Particle(pi.v, law)))
    }
    def cond(ll: T => Double): Prob[T] = mapP(v => Particle(v, ll(v)))
    def empirical: Vector[T] = resample.particles.map(_.v)
  }

  case class Empirical[T](particles: Vector[Particle[T]]) extends Prob[T] {
    def draw: Particle[T] = {
      val lw = particles map (_.lw)
      val mx = lw reduce (math.max(_,_))
      val rw = lw map (lwi => math.exp(lwi - mx))
      val law = mx + math.log(rw.sum/(rw.length))
      val idx = bdist.Multinomial(DenseVector(rw.toArray)).draw
      Particle(particles(idx).v, law)
    }
  }

As before, if you are pasting code blocks into the REPL, you will need to use :paste mode to paste these two definitions together.

The essential structure is similar to that from the previous post, but with a few notable differences. Most fundamentally, we now require any concrete implementation to provide a draw method returning a single particle from the distribution. Like before, we are not worrying about purity of functional code here, and using a standard random number generator with a globally mutating state. We can define a mapP method (for “map particle”) using the new flatMap method on Particle, and then use that to define map and flatMap for Prob[_]. Crucially, draw is used to define flatMap without requiring a Cartesian product of distributions to be formed.

We add a draw method to our Empirical[_] implementation. This method is computationally intensive, so it will still be computationally problematic to chain several flatMaps together, but this will no longer be N^2 in memory utilisation. Note that again we carefully set the weight of the drawn particle so that its raw weight is the average of the raw weight of the empirical distribution. This is needed to propagate conditioning information correctly back through flatMaps. There is obviously some code duplication between the draw method on Empirical and the resample method on Prob, but I’m not sure it’s worth factoring out.

It is worth noting that neither flatMap nor cond triggers resampling, so the user of the library is now responsible for resampling when appropriate.

We can provide evidence that Prob[_] forms a monad just like we did Particle[_].

  implicit val probMonad = new Monad[Prob] {
    def pure[T](t: T): Prob[T] = Empirical(Vector(Particle(t, 0.0)))
    def flatMap[T,S](pt: Prob[T])(f: T => Prob[S]): Prob[S] = pt.flatMap(f)
    def tailRecM[T,S](t: T)(f: T => Prob[Either[T,S]]): Prob[S] = ???
  }

Again, we’ll want to be able to create a distribution from an unweighted collection of values.

  def unweighted[T](ts: Vector[T], lw: Double = 0.0): Prob[T] =
    Empirical(ts map (Particle(_, lw)))

We will again define an implementation for distributions with tractable likelihoods, which are therefore easy to condition on. They will typically also be easy to draw from efficiently, and we will use this fact, too.

  trait Dist[T] extends Prob[T] {
    def ll(obs: T): Double
    def ll(obs: Seq[T]): Double = obs map (ll) reduce (_+_)
    def fit(obs: Seq[T]): Prob[T] = mapP(v => Particle(v, ll(obs)))
    def fitQ(obs: Seq[T]): Prob[T] = Empirical(Vector(Particle(obs.head, ll(obs))))
    def fit(obs: T): Prob[T] = fit(List(obs))
    def fitQ(obs: T): Prob[T] = fitQ(List(obs))
  }

We can give implementations of this for a few standard distributions.

  case class Normal(mu: Double, v: Double)(implicit N: Int) extends Dist[Double] {
    lazy val particles = unweighted(bdist.Gaussian(mu, math.sqrt(v)).
      sample(N).toVector).particles
    def draw = Particle(bdist.Gaussian(mu, math.sqrt(v)).draw, 0.0)
    def ll(obs: Double) = bdist.Gaussian(mu, math.sqrt(v)).logPdf(obs)
  }

  case class Gamma(a: Double, b: Double)(implicit N: Int) extends Dist[Double] {
    lazy val particles = unweighted(bdist.Gamma(a, 1.0/b).
      sample(N).toVector).particles
    def draw = Particle(bdist.Gamma(a, 1.0/b).draw, 0.0)
    def ll(obs: Double) = bdist.Gamma(a, 1.0/b).logPdf(obs)
  }

  case class Poisson(mu: Double)(implicit N: Int) extends Dist[Int] {
    lazy val particles = unweighted(bdist.Poisson(mu).
      sample(N).toVector).particles
    def draw = Particle(bdist.Poisson(mu).draw, 0.0)
    def ll(obs: Int) = bdist.Poisson(mu).logProbabilityOf(obs)
  }

Note that we now have to provide an (efficient) draw method for each implementation, returning a single draw from the distribution as a Particle with a (raw) weight of 1 (log weight of 0).

We are done. It’s a few more lines of code than that from the previous post, but this is now much closer to something that could be used in practice to solve actual inference problems using a reasonable number of particles. But to do so we will need to be careful do avoid deep monadic binding. This is easiest to explain with a concrete example.

Using the SMC-based probability monad in practice

Monadic binding and applicative structure

As explained in the previous post, using Scala’s for-expressions for monadic binding gives a natural and elegant PPL for our probability monad “for free”. This is fine, and in general there is no reason why using it should lead to inefficient code. However, for this particular probability monad implementation, it turns out that deep monadic binding comes with a huge performance penalty. For a concrete example, consider the following specification, perhaps of a prior distribution over some independent parameters.

    val prior = for {
      x <- Normal(0,1)
      y <- Gamma(1,1)
      z <- Poisson(10)
    } yield (x,y,z)

Don’t paste that into the REPL – it will take an age to complete!

Again, I must emphasise that there is nothing wrong with this specification, and there is no reason in principle why such a specification can’t be computationally efficient – it’s just a problem for our particular probability monad. We can begin to understand the problem by thinking about how this will be de-sugared by the compiler. Roughly speaking, the above will de-sugar to the following nested flatMaps.

    val prior2 =
      Normal(0,1) flatMap {x =>
        Gamma(1,1) flatMap {y =>
          Poisson(10) map {z =>
            (x,y,z)}}}

Again, beware of pasting this into the REPL.

So, although written from top to bottom, the nesting is such that the flatMaps collapse from the bottom-up. The second flatMap (the first to collapse) isn’t such a problem here, as the Poisson has a O(1) draw method. But the result is an empirical distribution, which has an O(N) draw method. So the first flatMap (the second to collapse) is an O(N^2) operation. By extension, it’s easy to see that the computational cost of nested flatMaps will be exponential in the number of monadic binds. So, looking back at the for expression, the problem is that there are three <-. The last one isn’t a problem since it corresponds to a map, and the second last one isn’t a problem, since the final distribution is tractable with an O(1) draw method. The problem is the first <-, corresponding to a flatMap of one empirical distribution with respect to another. For our particular probability monad, it’s best to avoid these if possible.

The interesting thing to note here is that because the distributions are independent, there is no need for them to be sequenced. They could be defined in any order. In this case it makes sense to use the applicative structure implied by the monad.

Now, since we have told cats that Prob[_] is a monad, it can provide appropriate applicative methods for us automatically. In Cats, every monad is assumed to be also an applicative functor (which is true in Cartesian closed categories, and Cats implicitly assumes that all functors and monads are defined over CCCs). So we can give an alternative specification of the above prior using applicative composition.

 val prior3 = Applicative[Prob].tuple3(Normal(0,1), Gamma(1,1), Poisson(10))
// prior3: Wrapped.Prob[(Double, Double, Int)] = Empirical(Vector(Particle((-0.057088546468105204,0.03027578552505779,9),0.0), Particle((-0.43686658266043743,0.632210127012762,14),0.0), Particle((-0.8805715148936012,3.4799656228544706,4),0.0), Particle((-0.4371726407147289,0.0010707859994652403,12),0.0), Particle((2.0283297088320755,1.040984491158822,10),0.0), Particle((1.2971862986495886,0.189166705596747,14),0.0), Particle((-1.3111333817551083,0.01962422606642761,9),0.0), Particle((1.6573851896142737,2.4021836368401415,9),0.0), Particle((-0.909927220984726,0.019595551644771683,11),0.0), Particle((0.33888133893822464,0.2659823344145805,10),0.0), Particle((-0.3300797295729375,3.2714740256437667,10),0.0), Particle((-1.8520554352884224,0.6175322756460341,10),0.0), Particle((0.541156780497547...

This one is mathematically equivalent, but safe to paste into your REPL, as it does not involve deep monadic binding, and can be used whenever we want to compose together independent components of a probabilistic program. Note that “tupling” is not the only possibility – Cats provides a range of functions for manipulating applicative values.

This is one way to avoid deep monadic binding, but another strategy is to just break up a large for expression into separate smaller for expressions. We can examine this strategy in the context of state-space modelling.

Particle filtering for a non-linear state-space model

We can now re-visit the DGLM example from the previous post. We began by declaring some observations and a prior.

    val data = List(2,1,0,2,3,4,5,4,3,2,1)
// data: List[Int] = List(2, 1, 0, 2, 3, 4, 5, 4, 3, 2, 1)

    val prior = for {
      w <- Gamma(1, 1)
      state0 <- Normal(0.0, 2.0)
    } yield (w, List(state0))
// prior: Wrapped.Prob[(Double, List[Double])] = Empirical(Vector(Particle((4.220683377724395,List(0.37256749723762683)),0.0), Particle((0.4436668049925418,List(-1.0053578391265572)),0.0), Particle((0.9868899648436931,List(-0.6985099310193449)),0.0), Particle((0.13474375773634908,List(0.9099291736792412)),0.0), Particle((1.9654021747685184,List(-0.042127103727998175)),0.0), Particle((0.21761202474220223,List(1.1074616830012525)),0.0), Particle((0.31037163527711015,List(0.9261849914020324)),0.0), Particle((1.672438830781466,List(0.01678529855289384)),0.0), Particle((0.2257151759143097,List(2.5511304854128354)),0.0), Particle((0.3046489890769499,List(3.2918304533361398)),0.0), Particle((1.5115941814057159,List(-1.633612165168878)),0.0), Particle((1.4185906813831506,List(-0.8460922678989864))...

Looking carefully at the for-expression, there are just two <-, and the distribution on the RHS of the flatMap is tractable, so this is just O(N). So far so good.

Next, let’s look at the function to add a time point, which previously looked something like the following.

    def addTimePointSIS(current: Prob[(Double, List[Double])],
      obs: Int): Prob[(Double, List[Double])] = {
      println(s"Conditioning on observation: $obs")
      for {
        tup <- current
        (w, states) = tup
        os = states.head
        ns <- Normal(os, w)
        _ <- Poisson(math.exp(ns)).fitQ(obs)
      } yield (w, ns :: states)
    }
// addTimePointSIS: (current: Wrapped.Prob[(Double, List[Double])], obs: Int)Wrapped.Prob[(Double, List[Double])]

Recall that our new probability monad does not automatically trigger resampling, so applying this function in a fold will lead to a simple sampling importance sampling (SIS) particle filter. Typically, the bootstrap particle filter includes resampling after each time point, giving a special case of a sampling importance resampling (SIR) particle filter, which we could instead write as follows.

    def addTimePointSimple(current: Prob[(Double, List[Double])],
      obs: Int): Prob[(Double, List[Double])] = {
      println(s"Conditioning on observation: $obs")
      val updated = for {
        tup <- current
        (w, states) = tup
        os = states.head
        ns <- Normal(os, w)
        _ <- Poisson(math.exp(ns)).fitQ(obs)
      } yield (w, ns :: states)
      updated.resample
    }
// addTimePointSimple: (current: Wrapped.Prob[(Double, List[Double])], obs: Int)Wrapped.Prob[(Double, List[Double])]

This works fine, but we can see that there are three <- in this for expression. This leads to a flatMap with an empirical distribution on the RHS, and hence is O(N^2). But this is simple enough to fix, by separating the updating process into separate “predict” and “update” steps, which is how people typically formulate particle filters for state-space models, anyway. Here we could write that as

    def addTimePoint(current: Prob[(Double, List[Double])],
      obs: Int): Prob[(Double, List[Double])] = {
      println(s"Conditioning on observation: $obs")
      val predict = for {
        tup <- current
        (w, states) = tup
        os = states.head
        ns <- Normal(os, w)
      }
      yield (w, ns :: states)
      val updated = for {
        tup <- predict
        (w, states) = tup
        st = states.head
        _ <- Poisson(math.exp(st)).fitQ(obs)
      } yield (w, states)
      updated.resample
    }
// addTimePoint: (current: Wrapped.Prob[(Double, List[Double])], obs: Int)Wrapped.Prob[(Double, List[Double])]

By breaking the for expression into two: the first for the “predict” step and the second for the “update” (conditioning on the observation), we get two O(N) operations, which for large N is clearly much faster. We can then run the filter by folding over the observations.

  import breeze.stats.{meanAndVariance => meanVar}
// import breeze.stats.{meanAndVariance=>meanVar}

  val mod = data.foldLeft(prior)(addTimePoint(_,_)).empirical
// Conditioning on observation: 2
// Conditioning on observation: 1
// Conditioning on observation: 0
// Conditioning on observation: 2
// Conditioning on observation: 3
// Conditioning on observation: 4
// Conditioning on observation: 5
// Conditioning on observation: 4
// Conditioning on observation: 3
// Conditioning on observation: 2
// Conditioning on observation: 1
// mod: Vector[(Double, List[Double])] = Vector((0.24822528144246606,List(0.06290285371838457, 0.01633338109272575, 0.8997103339551227, 1.5058726341571411, 1.0579925693609091, 1.1616536515200064, 0.48325623593870665, 0.8457351097543767, -0.1988290999293708, -0.4787511341321954, -0.23212497417019512, -0.15327432440577277)), (1.111430233331792,List(0.6709342824443849, 0.009092797044165657, -0.13203367846117453, 0.4599952735399485, 1.3779288637042504, 0.6176597963402879, 0.6680455419800753, 0.48289163013446945, -0.5994001698510807, 0.4860969602653898, 0.10291798193078927, 1.2878325765987266)), (0.6118925941009055,List(0.6421161986636132, 0.679470360928868, 1.0552459559203342, 1.200835166087372, 1.3690372269589233, 1.8036766847282912, 0.6229883551656629, 0.14872642198313774, -0.122700856878725...

  meanVar(mod map (_._1)) // w
// res0: breeze.stats.meanAndVariance.MeanAndVariance = MeanAndVariance(0.2839184023932576,0.07391602428256917,2000)

  meanVar(mod map (_._2.reverse.head)) // initial state
// res1: breeze.stats.meanAndVariance.MeanAndVariance = MeanAndVariance(0.26057368528422714,0.4802810202354611,2000)

  meanVar(mod map (_._2.head)) // final state
// res2: breeze.stats.meanAndVariance.MeanAndVariance = MeanAndVariance(0.5448036669181697,0.28293080584600894,2000)

Summary and conclusions

Here we have just done some minor tidying up of the rather naive probability monad from the previous post to produce an SMC-based probability monad with improved performance characteristics. Again, we get an embedded probabilistic programming language “for free”. Although the language itself is very flexible, allowing us to construct more-or-less arbitrary probabilistic programs for Bayesian inference problems, we saw that a bug/feature of this particular inference algorithm is that care must be taken to avoid deep monadic binding if reasonable performance is to be obtained. In most cases this is simple to achieve by using applicative composition or by breaking up large for expressions.

There are still many issues and inefficiencies associated with this PPL. In particular, if the main intended application is to state-space models, it would make more sense to tailor the algorithms and implementations to exactly that case. OTOH, if the main concern is a generic PPL, then it would make sense to make the PPL independent of the particular inference algorithm. These are both potential topics for future posts.

Software

  • min-ppl2 – code associated with this blog post
  • Rainier – a more efficient PPL with similar syntax
  • monad-bayes – a Haskell library exploring related ideas

Write your own general-purpose monadic probabilistic programming language from scratch in 50 lines of (Scala) code

Background

In May I attended a great workshop on advances and challenges in machine learning languages at the CMS in Cambridge. There was an a good mix of people from different disciplines, and a bit of a theme around probabilistic programming. The workshop schedule includes links to many of the presentations, and is generally worth browsing. In particular, it includes a link to the slides for my presentation on a compositional approach to scalable Bayesian computation and probabilistic programming. I’ve given a few talks on this kind of thing over the last couple of years, at Newcastle, at the Isaac Newton Institute in Cambridge (twice), and at CIRM in France. But I think I explained things best at this workshop at the CMS, though my impression could partly have been a reflection of the more interested and relevant audience. In the talk I started with a basic explanation of why ideas from category theory and functional programming can help to solve problems in statistical computing in a more composable and scalable way, before moving on to discuss probability monads and their fundamental connection to probabilistic programming. The take home message from the talk is that if you have a generic inference algorithm, expressing the logic in the context of probability monads can give you an embedded probabilistic programming language (PPL) for that inference algorithm essentially “for free”.

So, during my talk I said something a little fool-hardy. I can’t remember my exact words, but while presenting the idea behind an SMC-based probability monad I said something along the lines of “one day I will write a blog post on how to write a probabilistic programming language from scratch in 50 lines of code, and this is how I’ll do it“! Rather predictably (with hindsight), immediately after my talk about half a dozen people all pleaded with me to urgently write the post! I’ve been a little busy since then, but now that things have settled down a little for the summer, I’ve some time to think and code, so here is that post.

Introduction

The idea behind this post is to show that, if you think about the problem in the right way, and use a programming language with syntactic support for monadic composition, then producing a flexible, general, compositional, embedded domain specific language (DSL) for probabilistic programming based on a given generic inference algorithm is no more effort than hard-coding two or three illustrative examples. You would need to code up two or three examples for a paper anyway, but providing a PPL is way more useful. There is also an interesting converse to this, which is that if you can’t easily produce a PPL for your “general” inference algorithm, then perhaps it isn’t quite as “general” as you thought. I’ll try to resist exploring that here…

To illustrate these principles I want to develop a fairly minimal PPL, so that the complexities of the inference algorithm don’t hide the simplicity of the PPL embedding. Importance sampling with resampling is probably the simplest useful generic Bayesian inference algorithm to implement, so that’s what I’ll use. Note that there are many limitations of the approach that I will adopt, which will make it completely unsuitable for “real” problems. In particular, this implementation is: inefficient, in terms of both compute time and memory usage, statistically inefficient for deep nesting and repeated conditioning, due to the particle degeneracy problem, specific to a particular probability monad, strictly evaluated, impure (due to mutation of global random number state), etc. All of these things are easily fixed, but all at the expense of greater abstraction, complexity and lines of code. I’ll probably discuss some of these generalisations and improvements in future posts, but for this post I want to keep everything as short and simple as practical. It’s also worth mentioning that there is nothing particularly original here. Many people have written about monadic embedded PPLs, and several have used an SMC-based monad for illustration. I’ll give some pointers to useful further reading at the end.

The language, in 50 lines of code

Without further ado, let’s just write the PPL. I’m using plain Scala, with just a dependency on the Breeze scientific library, which I’m going to use for simulating random numbers from standard distributions, and evaluation of their log densities. I have a directory of materials associated with this post in a git repo. This post is derived from an executable tut document (so you know it works), which can be found here. If you just want to follow along copying code at the command prompt, just run sbt from an empty or temp directory, and copy the following to spin up a Scala console with the Breeze dependency:

set libraryDependencies += "org.scalanlp" %% "breeze" % "1.0-RC4"
set libraryDependencies += "org.scalanlp" %% "breeze-natives" % "1.0-RC4"
set scalaVersion := "2.13.0"
console

We start with a couple of Breeze imports

import breeze.stats.{distributions => bdist}
import breeze.linalg.DenseVector

which are not strictly necessary, but clean up the subsequent code. We are going to use a set of weighted particles to represent a probability distribution empirically, so we’ll start by defining an appropriate ADT for these:

implicit val numParticles = 300

case class Particle[T](v: T, lw: Double) { // value and log-weight
  def map[S](f: T => S): Particle[S] = Particle(f(v), lw)
}

We also include a map method for pushing a particle through a transformation, and a default number of particles for sampling and resampling. 300 particles are enough for illustrative purposes. Ideally it would be good to increase this for more realistic experiments. We can use this particle type to build our main probability monad as follows.

trait Prob[T] {
  val particles: Vector[Particle[T]]
  def map[S](f: T => S): Prob[S] = Empirical(particles map (_ map f))
  def flatMap[S](f: T => Prob[S]): Prob[S] = {
    Empirical((particles map (p => {
      f(p.v).particles.map(psi => Particle(psi.v, p.lw + psi.lw))
    })).flatten).resample
  }
  def resample(implicit N: Int): Prob[T] = {
    val lw = particles map (_.lw)
    val mx = lw reduce (math.max(_,_))
    val rw = lw map (lwi => math.exp(lwi - mx))
    val law = mx + math.log(rw.sum/(rw.length))
    val ind = bdist.Multinomial(DenseVector(rw.toArray)).sample(N)
    val newParticles = ind map (i => particles(i))
    Empirical(newParticles.toVector map (pi => Particle(pi.v, law)))
  }
  def cond(ll: T => Double): Prob[T] =
    Empirical(particles map (p => Particle(p.v, p.lw + ll(p.v))))
  def empirical: Vector[T] = resample.particles.map(_.v)
}

case class Empirical[T](particles: Vector[Particle[T]]) extends Prob[T]

Note that if you are pasting into the Scala REPL you will need to use :paste mode for this. So Prob[_] is our base probability monad trait, and Empirical[_] is our simplest implementation, which is just a collection of weighted particles. The method flatMap forms the naive product of empirical measures and then resamples in order to stop an explosion in the number of particles. There are two things worth noting about the resample method. The first is that the log-sum-exp trick is being used to avoid overflow and underflow when the log weights are exponentiated. The second is that although the method returns an equally weighted set of particles, the log weights are all set in order that the average raw weight of the output set matches the average raw weight of the input set. This is a little tricky to explain, but it turns out to be necessary in order to correctly propagate conditioning information back through multiple monadic binds (flatMaps). The cond method allows conditioning of a distribution using an arbitrary log-likelihood. It is included for comparison with some other implementations I will refer to later, but we won’t actually be using it, so we could save two lines of code here if necessary. The empirical method just extracts an unweighted set of values from a distribution for subsequent analysis.

It will be handy to have a function to turn a bunch of unweighted particles into a set of particles with equal weights (a sort-of inverse of the empirical method just described), so we can define that as follows.

def unweighted[T](ts: Vector[T], lw: Double = 0.0): Prob[T] =
  Empirical(ts map (Particle(_, lw)))

Probabilistic programming is essentially trivial if we only care about forward sampling. But interesting PPLs allow us to condition on observed values of random variables. In the context of SMC, this is simplest when the distribution being conditioned has a tractable log-likelihood. So we can now define an extension of our probability monad for distributions with a tractable log-likelihood, and define a bunch of convenient conditioning (or “fitting”) methods using it.

trait Dist[T] extends Prob[T] {
  def ll(obs: T): Double
  def ll(obs: Seq[T]): Double = obs map (ll) reduce (_+_)
  def fit(obs: Seq[T]): Prob[T] =
    Empirical(particles map (p => Particle(p.v, p.lw + ll(obs))))
  def fitQ(obs: Seq[T]): Prob[T] = Empirical(Vector(Particle(obs.head, ll(obs))))
  def fit(obs: T): Prob[T] = fit(List(obs))
  def fitQ(obs: T): Prob[T] = fitQ(List(obs))
}

The only unimplemented method is ll(). The fit method re-weights a particle set according to the observed log-likelihood. For convenience, it also returns a particle cloud representing the posterior-predictive distribution of an iid value from the same distribution. This is handy, but comes at the expense of introducing an additional particle cloud. So, if you aren’t interested in the posterior predictive, you can avoid this cost by using the fitQ method (for “fit quick”), which doesn’t return anything useful. We’ll see examples of this in practice, shortly. Note that the fitQ methods aren’t strictly required for our “minimal” PPL, so we can save a couple of lines by omitting them if necessary. Similarly for the variants which allow conditioning on a collection of iid observations from the same distribution.

At this point we are essentially done. But for convenience, we can define a few standard distributions to help get new users of our PPL started. Of course, since the PPL is embedded, it is trivial to add our own additional distributions later.

case class Normal(mu: Double, v: Double)(implicit N: Int) extends Dist[Double] {
  lazy val particles = unweighted(bdist.Gaussian(mu, math.sqrt(v)).sample(N).toVector).particles
  def ll(obs: Double) = bdist.Gaussian(mu, math.sqrt(v)).logPdf(obs) }

case class Gamma(a: Double, b: Double)(implicit N: Int) extends Dist[Double] {
  lazy val particles = unweighted(bdist.Gamma(a, 1.0/b).sample(N).toVector).particles
  def ll(obs: Double) = bdist.Gamma(a, 1.0/b).logPdf(obs) }

case class Poisson(mu: Double)(implicit N: Int) extends Dist[Int] {
  lazy val particles = unweighted(bdist.Poisson(mu).sample(N).toVector).particles
  def ll(obs: Int) = bdist.Poisson(mu).logProbabilityOf(obs) }

Note that I’ve parameterised the Normal and Gamma the way that statisticians usually do, and not the way they are usually parameterised in scientific computing libraries (such as Breeze).

That’s it! This is a complete, general-purpose, composable, monadic PPL, in 50 (actually, 48, and fewer still if you discount trailing braces) lines of code. Let’s now see how it works in practice.

Examples

Normal random sample

We’ll start off with just about the simplest slightly interesting example I can think of: Bayesian inference for the mean and variance of a normal distribution from a random sample.

import breeze.stats.{meanAndVariance => meanVar}
// import breeze.stats.{meanAndVariance=>meanVar}

val mod = for {
  mu <- Normal(0, 100)
  tau <- Gamma(1, 0.1)
  _ <- Normal(mu, 1.0/tau).fitQ(List(8.0,9,7,7,8,10))
} yield (mu,tau)
// mod: Wrapped.Prob[(Double, Double)] = Empirical(Vector(Particle((8.718127116254472,0.93059589932682),-15.21683812389373), Particle((7.977706390420308,1.1575288208065433),-15.21683812389373), Particle((7.977706390420308,1.1744750937611985),-15.21683812389373), Particle((7.328100552769214,1.1181787982959164),-15.21683812389373), Particle((7.977706390420308,0.8283737237370494),-15.21683812389373), Particle((8.592847414557049,2.2934836446009026),-15.21683812389373), Particle((8.718127116254472,1.498741032928539),-15.21683812389373), Particle((8.592847414557049,0.2506065368748732),-15.21683812389373), Particle((8.543283880264225,1.127386759627675),-15.21683812389373), Particle((7.977706390420308,1.3508728798704925),-15.21683812389373), Particle((7.977706390420308,1.1134430556990933),-15.2168...

val modEmp = mod.empirical
// modEmp: Vector[(Double, Double)] = Vector((7.977706390420308,0.8748006833362748), (6.292345096890432,0.20108091703626174), (9.15330820843396,0.7654238730107492), (8.960935105658741,1.027712984079369), (7.455292602273359,0.49495749079351836), (6.911716909394562,0.7739749058662421), (6.911716909394562,0.6353785792877397), (7.977706390420308,1.1744750937611985), (7.977706390420308,1.1134430556990933), (8.718127116254472,1.166399872049532), (8.763777227034538,1.0468304705769353), (8.718127116254472,0.93059589932682), (7.328100552769214,1.6166695922250236), (8.543283880264225,0.4689300351248357), (8.543283880264225,2.0028918490755094), (7.536025958690963,0.6282318170458533), (7.328100552769214,1.6166695922250236), (7.049843463553113,0.20149378088848635), (7.536025958690963,2.3565657669819897...

meanVar(modEmp map (_._1)) // mu
// res0: breeze.stats.meanAndVariance.MeanAndVariance = MeanAndVariance(8.311171010932343,0.4617800639333532,300)

meanVar(modEmp map (_._2)) // tau
// res1: breeze.stats.meanAndVariance.MeanAndVariance = MeanAndVariance(0.940762723934599,0.23641881704888842,300)

Note the use of the empirical method to turn the distribution into an unweighted set of particles for Monte Carlo analysis. Anyway, the main point is that the syntactic sugar for monadic binds (flatMaps) provided by Scala’s for-expressions (similar to do-notation in Haskell) leads to readable code not so different to that in well-known general-purpose PPLs such as BUGS, JAGS, or Stan. There are some important differences, however. In particular, the embedded DSL has probabilistic programs as regular values in the host language. These may be manipulated and composed like other values. This makes this probabilistic programming language more composable than the aforementioned languages, which makes it much simpler to build large, complex probabilistic programs from simpler, well-tested, components, in a scalable way. That is, this PPL we have obtained “for free” is actually in many ways better than most well-known PPLs.

Noisy measurements of a count

Here we’ll look at the problem of inference for a discrete count given some noisy iid continuous measurements of it.

val mod = for {
  count <- Poisson(10)
  tau <- Gamma(1, 0.1)
  _ <- Normal(count, 1.0/tau).fitQ(List(4.2,5.1,4.6,3.3,4.7,5.3))
} yield (count, tau)
// mod: Wrapped.Prob[(Int, Double)] = Empirical(Vector(Particle((5,4.488795220669575),-11.591037521513753), Particle((5,1.7792314573063672),-11.591037521513753), Particle((5,2.5238021156137673),-11.591037521513753), Particle((4,3.280754333896923),-11.591037521513753), Particle((5,2.768438569482849),-11.591037521513753), Particle((4,1.3399975573518912),-11.591037521513753), Particle((5,1.1792835858615431),-11.591037521513753), Particle((5,1.989491156206883),-11.591037521513753), Particle((4,0.7825254987152054),-11.591037521513753), Particle((5,2.7113936834028793),-11.591037521513753), Particle((5,3.7615196800240387),-11.591037521513753), Particle((4,1.6833300961124709),-11.591037521513753), Particle((5,2.749183220798113),-11.591037521513753), Particle((5,2.1074062883430202),-11.591037521513...

val modEmp = mod.empirical
// modEmp: Vector[(Int, Double)] = Vector((4,3.243786594839479), (4,1.5090869158886693), (4,1.280656912383482), (5,2.0616356908358195), (5,3.475433097869503), (5,1.887582611202514), (5,2.8268877720514745), (5,0.9193261688050818), (4,1.7063629502805908), (5,2.116414832864841), (5,3.775508828984636), (5,2.6774941123762814), (5,2.937859946593459), (5,1.2047689975166402), (5,2.5658806161572656), (5,1.925890364268593), (4,1.0194093176888832), (5,1.883288825936725), (5,4.9503779454422965), (5,0.9045613180858916), (4,1.5795027943928661), (5,1.925890364268593), (5,2.198539449287062), (5,1.791363956348445), (5,0.9853760689818026), (4,1.6541388923071607), (5,2.599899960899971), (4,1.8904423810277957), (5,3.8983183765907836), (5,1.9242319515895554), (5,2.8268877720514745), (4,1.772120802027519), (5,2...

meanVar(modEmp map (_._1.toDouble)) // count
// res2: breeze.stats.meanAndVariance.MeanAndVariance = MeanAndVariance(4.670000000000004,0.23521739130434777,300)

meanVar(modEmp map (_._2)) // tau
// res3: breeze.stats.meanAndVariance.MeanAndVariance = MeanAndVariance(1.9678279101913874,0.9603971613375548,300)

I’ve included this mainly as an example of inference for a discrete-valued parameter. There are people out there who will tell you that discrete parameters are bad/evil/impossible. This isn’t true – discrete parameters are cool!

Linear model

Because our PPL is embedded, we can take full advantage of the power of the host programming language to build our models. Let’s explore this in the context of Bayesian estimation of a linear model. We’ll start with some data.

val x = List(1.0,2,3,4,5,6)
// x: List[Double] = List(1.0, 2.0, 3.0, 4.0, 5.0, 6.0)

val y = List(3.0,2,4,5,5,6)
// y: List[Double] = List(3.0, 2.0, 4.0, 5.0, 5.0, 6.0)

val xy = x zip y
// xy: List[(Double, Double)] = List((1.0,3.0), (2.0,2.0), (3.0,4.0), (4.0,5.0), (5.0,5.0), (6.0,6.0))

Now, our (simple) linear regression model will be parameterised by an intercept, alpha, a slope, beta, and a residual variance, v. So, for convenience, let’s define an ADT representing a particular linear model.

case class Param(alpha: Double, beta: Double, v: Double)
// defined class Param

Now we can define a prior distribution over models as follows.

val prior = for {
  alpha <- Normal(0,10)
  beta <- Normal(0,4)
  v <- Gamma(1,0.1)
} yield Param(alpha, beta, v)
// prior: Wrapped.Prob[Param] = Empirical(Vector(Particle(Param(-2.392517550699654,-3.7516090283880095,1.724680963054379),0.0), Particle(Param(7.60982717067903,-1.4318199629361292,2.9436745225038545),0.0), Particle(Param(-1.0281832158124837,-0.2799562317845073,4.05125312048092),0.0), Particle(Param(-1.0509321093485073,-2.4733837587060448,0.5856868459456287),0.0), Particle(Param(7.678898742733517,0.15616204936412104,5.064540017623097),0.0), Particle(Param(-3.392028985658713,-0.694412176170572,7.452625596437611),0.0), Particle(Param(3.0310535934425324,-2.97938526497514,2.138446100857938),0.0), Particle(Param(3.016959696424399,1.3370878561954143,6.18957854813488),0.0), Particle(Param(2.6956505371497066,1.058845844793446,5.257973123790336),0.0), Particle(Param(1.496225540527873,-1.573936445746...

Since our language doesn’t include any direct syntactic support for fitting regression models, we can define our own function for conditioning a distribution over models on a data point, which we can then apply to our prior as a fold over the available data.

def addPoint(current: Prob[Param], obs: (Double, Double)): Prob[Param] = for {
    p <- current
    (x, y) = obs
    _ <- Normal(p.alpha + p.beta * x, p.v).fitQ(y)
  } yield p
// addPoint: (current: Wrapped.Prob[Param], obs: (Double, Double))Wrapped.Prob[Param]

val mod = xy.foldLeft(prior)(addPoint(_,_)).empirical
// mod: Vector[Param] = Vector(Param(1.4386051853067798,0.8900831186754122,4.185564696221981), Param(0.5530582357040271,1.1296886766045509,3.468527573093037), Param(0.6302560079049571,0.9396563485293532,3.7044543917875927), Param(3.68291303096638,0.4781372802435529,5.151665328789926), Param(3.016959696424399,0.4438016738989412,1.9988914122633519), Param(3.016959696424399,0.4438016738989412,1.9988914122633519), Param(0.6302560079049571,0.9396563485293532,3.7044543917875927), Param(0.6302560079049571,0.9396563485293532,3.7044543917875927), Param(3.68291303096638,0.4781372802435529,5.151665328789926), Param(3.016959696424399,0.4438016738989412,1.9988914122633519), Param(0.6302560079049571,0.9396563485293532,3.7044543917875927), Param(0.6302560079049571,0.9396563485293532,3.7044543917875927), ...

meanVar(mod map (_.alpha))
// res4: breeze.stats.meanAndVariance.MeanAndVariance = MeanAndVariance(1.5740812481283812,1.893684802867127,300)

meanVar(mod map (_.beta))
// res5: breeze.stats.meanAndVariance.MeanAndVariance = MeanAndVariance(0.7690238868623273,0.1054479268115053,300)

meanVar(mod map (_.v))
// res6: breeze.stats.meanAndVariance.MeanAndVariance = MeanAndVariance(3.5240853748668695,2.793386340338213,300)

We could easily add syntactic support to our language to enable the fitting of regression-style models, as is done in Rainier, of which more later.

Dynamic generalised linear model

The previous examples have been fairly simple, so let’s finish with something a bit less trivial. Our language is quite flexible enough to allow the analysis of a dynamic generalised linear model (DGLM). Here we’ll fit a Poisson DGLM with a log-link and a simple Brownian state evolution. More complex models are more-or-less similarly straightforward. The model is parameterised by an initial state, state0, and and evolution variance, w.

val data = List(2,1,0,2,3,4,5,4,3,2,1)
// data: List[Int] = List(2, 1, 0, 2, 3, 4, 5, 4, 3, 2, 1)

val prior = for {
  w <- Gamma(1, 1)
  state0 <- Normal(0.0, 2.0)
} yield (w, List(state0))
// prior: Wrapped.Prob[(Double, List[Double])] = Empirical(Vector(Particle((0.12864918092587044,List(-2.862479260552014)),0.0), Particle((1.1706344622093179,List(1.6138397233532091)),0.0), Particle((0.757288087950638,List(-0.3683499919402798)),0.0), Particle((2.755201217523856,List(-0.6527488751780317)),0.0), Particle((0.7535085397802043,List(0.5135562407906502)),0.0), Particle((1.1630726564525629,List(0.9703146201262348)),0.0), Particle((1.0080345715326213,List(-0.375686732266234)),0.0), Particle((4.603723117526974,List(-1.6977366375222938)),0.0), Particle((0.2870669117815037,List(2.2732160435099433)),0.0), Particle((2.454675218313211,List(-0.4148287542786906)),0.0), Particle((0.3612534201761152,List(-1.0099270904161748)),0.0), Particle((0.29578453393473114,List(-2.4938128878051966)),0.0)...

We can define a function to create a new hidden state, prepend it to the list of hidden states, and condition on the observed value at that time point as follows.

def addTimePoint(current: Prob[(Double, List[Double])],
  obs: Int): Prob[(Double, List[Double])] = for {
  tup <- current
  (w, states) = tup
  os = states.head
  ns <- Normal(os, w)
  _ <- Poisson(math.exp(ns)).fitQ(obs)
} yield (w, ns :: states)
// addTimePoint: (current: Wrapped.Prob[(Double, List[Double])], obs: Int)Wrapped.Prob[(Double, List[Double])]

We then run our (augmented state) particle filter as a fold over the time series.

val mod = data.foldLeft(prior)(addTimePoint(_,_)).empirical
// mod: Vector[(Double, List[Double])] = Vector((0.053073252551193446,List(0.8693030057529023, 1.2746526177834938, 1.020307245610461, 1.106341696651584, 1.070777529635013, 0.8749041525303247, 0.9866999164354662, 0.4082577920509255, 0.06903234462140699, -0.018835642776197814, -0.16841912034400547, -0.08919045681401294)), (0.0988871875952762,List(-0.24241948109998607, 0.09321618969352086, 0.9650532206325375, 1.1738734442767293, 1.2272325310228442, 0.9791695328246326, 0.5576319082578128, -0.0054280215024367084, 0.4256621012454391, 0.7486862644576158, 0.8193517409118243, 0.5928750312493785)), (0.16128799384962295,List(-0.30371187329667104, -0.3976854602292066, 0.5869357473774455, 0.9881090696832543, 1.2095181380307558, 0.7211231597865506, 0.8085486452269925, 0.2664373341459165, -0.627344024142...

meanVar(mod map (_._1)) // w
// res7: breeze.stats.meanAndVariance.MeanAndVariance = MeanAndVariance(0.29497487517435844,0.0831412016262515,300)

meanVar(mod map (_._2.reverse.head)) // state0 (initial state)
// res8: breeze.stats.meanAndVariance.MeanAndVariance = MeanAndVariance(0.04617218427664018,0.372844704533101,300)

meanVar(mod map (_._2.head)) // stateN (final state)
// res9: breeze.stats.meanAndVariance.MeanAndVariance = MeanAndVariance(0.4937178761565612,0.2889287607470016,300)

Summary, conclusions, and further reading

So, we’ve seen how we can build a fully functional, general-purpose, compositional, monadic PPL from scratch in 50 lines of code, and we’ve seen how we can use it to solve real, analytically intractable Bayesian inference problems of non-trivial complexity. Of course, there are many limitations to using exactly this PPL implementation in practice. The algorithm becomes intolerably slow for deeply nested models, and uses unreasonably large amounts of RAM for large numbers of particles. It also suffers from a particle degeneracy problem if there are too many conditioning events. But it is important to understand that these are all deficiencies of the naive inference algorithm used, not the PPL itself. The PPL is flexible and compositional and can be used to build models of arbitrary size and complexity – it just needs to be underpinned by a better, more efficient, inference algorithm. Rainier is a Scala library I’ve blogged about previously which uses a very similar PPL to the one described here, but is instead underpinned by a fast, efficient, HMC algorithm. With my student Jonny Law, we have recently arXived a paper on Functional probabilistic programming for scalable Bayesian modelling, discussing some of these issues, and exploring the compositional nature of monadic PPLs (somewhat glossed over in this post).

Since the same PPL can be underpinned by different inference algorithms encapsulated as probability monads, an obvious question is whether it is possible to abstract the PPL away from the inference algorithm implementation. Of course, the answer is “yes”, and this has been explored to great effect in papers such as Practical probabilistic programming with monads and Functional programming for modular Bayesian inference. Note that they use the cond approach to conditioning, which looks a bit unwieldy, but is equivalent to fitting. As well as allowing alternative inference algorithms to be applied to the same probabilistic program, it also enables the composing of inference algorithms – for example, composing a MH algorithm with an SMC algorithm in order to get a PMMH algorithm. The ideas are implemented in an embedded DSL for Haskell, monad-bayes. If you are not used to Haskell, the syntax will probably seem a bit more intimidating than Scala’s, but the semantics are actually quite similar, with the main semantic difference being that Scala is strictly evaluated by default, whereas Haskell is lazily evaluated by default. Both languages support both lazy and strict evaluation – the difference relates simply to default behaviour, but is important nevertheless.

Papers

Software

  • min-ppl – code associated with this blog post
  • Rainier – a more efficient PPL with similar syntax
  • monad-bayes – a Haskell library exploring related ideas

Bayesian hierarchical modelling with Rainier

Introduction

In the previous post I gave a brief introduction to Rainier, a new HMC-based probabilistic programming library/DSL for Scala. In that post I assumed that people were using the latest source version of the library. Since then, version 0.1.1 of the library has been released, so in this post I will demonstrate use of the released version of the software (using the binaries published to Sonatype), and will walk through a slightly more interesting example – a dynamic linear state space model with unknown static parameters. This is similar to, but slightly different from, the DLM example in the Rainier library. So to follow along with this post, all that is required is SBT.

An interactive session

First run SBT from an empty directory, and paste the following at the SBT prompt:

set libraryDependencies  += "com.stripe" %% "rainier-plot" % "0.1.1"
set scalaVersion := "2.12.4"
console

This should give a Scala REPL with appropriate dependencies (rainier-plot has all of the relevant transitive dependencies). We’ll begin with some imports, and then simulating some synthetic data from a dynamic linear state space model with an AR(1) latent state and Gaussian noise on the observations.

import com.stripe.rainier.compute._
import com.stripe.rainier.core._
import com.stripe.rainier.sampler._

implicit val rng = ScalaRNG(1)
val n = 60 // number of observations/time points
val mu = 3.0 // AR(1) mean
val a = 0.95 // auto-regressive parameter
val sig = 0.2 // AR(1) SD
val sigD = 3.0 // observational SD
val state = Stream.
  iterate(0.0)(x => mu + (x - mu) * a + sig * rng.standardNormal).
  take(n).toVector
val obs = state.map(_ + sigD * rng.standardNormal)

Now we have some synthetic data, let’s think about building a probabilistic program for this model. Start with a prior.

case class Static(mu: Real, a: Real, sig: Real, sigD: Real)
val prior = for {
  mu <- Normal(0, 10).param
  a <- Normal(1, 0.1).param
  sig <- Gamma(2,1).param
  sigD <- Gamma(2,2).param
  sp <- Normal(0, 50).param
} yield (Static(mu, a, sig, sigD), List(sp))

Note the use of a case class for wrapping the static parameters. Next, let’s define a function to add a state and associated observation to an existing model.

def addTimePoint(current: RandomVariable[(Static, List[Real])],
                     datum: Double) = for {
  tup <- current
  static = tup._1
  states = tup._2
  os = states.head
  ns <- Normal(((Real.one - static.a) * static.mu) + (static.a * os),
                 static.sig).param
  _ <- Normal(ns, static.sigD).fit(datum)
} yield (static, ns :: states)

Given this, we can generate the probabilistic program for our model as a fold over the data initialised with the prior.

val fullModel = obs.foldLeft(prior)(addTimePoint(_, _))

If we don’t want to keep samples for all of the variables, we can focus on the parameters of interest, wrapping the results in a Map for convenient sampling and plotting.

val model = for {
  tup <- fullModel
  static = tup._1
  states = tup._2
} yield
  Map("mu" -> static.mu,
  "a" -> static.a,
  "sig" -> static.sig,
  "sigD" -> static.sigD,
  "SP" -> states.reverse.head)

We can sample with

val out = model.sample(HMC(3), 100000, 10000 * 500, 500)

(this will take several minutes) and plot some diagnostics with

import com.cibo.evilplot.geometry.Extent
import com.stripe.rainier.plot.EvilTracePlot._

val truth = Map("mu" -> mu, "a" -> a, "sigD" -> sigD,
  "sig" -> sig, "SP" -> state(0))
render(traces(out, truth), "traceplots.png",
  Extent(1200, 1400))
render(pairs(out, truth), "pairs.png")

This generates the following diagnostic plots:

Everything looks good.

Summary

Rainier is a monadic embedded DSL for probabilistic programming in Scala. We can use standard functional combinators and for-expressions for building models to sample, and then run an efficient HMC algorithm on the resulting probability monad in order to obtain samples from the posterior distribution of the model.

See the Rainier repo for further details.

Monadic probabilistic programming in Scala with Rainier

Introduction

Rainier is an interesting new probabilistic programming library for Scala recently open-sourced by Stripe. Probabilistic programming languages provide a computational framework for building and fitting Bayesian models to data. There are many interesting probabilistic programming languages, and there is currently a lot of interesting innovation happening with probabilistic programming languages embedded in strongly typed functional programming languages such as Scala and Haskell. However, most such languages tend to be developed by people lacking expertise in statistics and numerics, leading to elegant, composable languages which work well for toy problems, but don’t scale well to the kinds of practical problems that applied statisticians are interested in. Conversely, there are a few well-known probabilistic programming languages developed by and for statisticians which have efficient inference engines, but are hampered by inflexible, inelegant languages and APIs. Rainier is interesting because it is an attempt to bridge the gap between these two worlds: it has a functional, composable, extensible, monadic API, yet is backed by a very efficient, high-performance scalable inference engine, using HMC and a static compute graph for reverse-mode AD. Clearly there will be some loss of generality associated with choosing an efficient inference algorithm (eg. for HMC, there needs to be a fixed number of parameters and they must all be continuous), but it still covers a large proportion of the class of hierarchical models commonly used in applied statistical modelling.

In this post I’ll give a quick introduction to Rainier using an interactive session requiring only that SBT is installed and the Rainier repo is downloaded or cloned.

Interactive session

To follow along with this post just clone, or download and unpack, the Rainier repo, and run SBT from the top-level Rainier directory and paste commands. First start a Scala REPL.

project rainierPlot
console

Before we start building models, we need some data. For this post we will focus on a simple logistic regression model, and so we will begin by simulating some synthetic data consistent with such a model.

val r = new scala.util.Random(0)
val N = 1000
val beta0 = 0.1
val beta1 = 0.3
val x = (1 to N) map { i =>
  3.0 * r.nextGaussian
}
val theta = x map { xi =>
  beta0 + beta1 * xi
}
def expit(x: Double): Double = 1.0 / (1.0 + math.exp(-x))
val p = theta map expit
val y = p map (pi => (r.nextDouble < pi))

Now we have some synthetic data, we can fit the model and see if we are able to recover the “true” parameters used to generate the synthetic data. In Rainier, we build models by declaring probabilistic programs for the model and the data, and then run an inference engine to generate samples from the posterior distribution.

Start with a bunch of Rainier imports:

import com.stripe.rainier.compute._
import com.stripe.rainier.core._
import com.stripe.rainier.sampler._
import com.stripe.rainier.repl._

Now we want to build a model. We do so by describing the joint distribution of parameters and data. Rainier has a few built-in distributions, and these can be combined using standard functional monadic combinators such as map, zip, flatMap, etc., to create a probabilistic program representing a probability monad for the model. Due to the monadic nature of such probabilistic programs, it is often most natural to declare them using a for-expression.

val model = for {
  beta0 <- Normal(0, 5).param
  beta1 <- Normal(0, 5).param
  _ <- Predictor.from{x: Double =>
      {
        val theta = beta0 + beta1 * x
        val p = Real(1.0) / (Real(1.0) + (Real(0.0) - theta).exp)
        Categorical.boolean(p)
      }
    }.fit(x zip y)
} yield Map("b0"->beta0, "b1"->beta1)

This kind of construction is very natural for anyone familiar with monadic programming in Scala, but will no doubt be a little mysterious otherwise. RandomVariable is the probability monad used for HMC sampling, and these can be constructed from Distributions using .param (for unobserved parameters) and .fit (for variables with associated observations). Predictor is just a convenience for observations corresponding to covariate information. model is therefore a RandomVariable over beta0 and beta1, the two unobserved parameters of interest. Note that I briefly discussed this kind of pure functional approach to describing probabilistic programs (using Rand from Breeze) in my post on MCMC as a stream.

Now we have our probabilistic program, we can sample from it using HMC as follows.

implicit val rng = ScalaRNG(3)
val its = 10000
val thin = 5
val out = model.sample(HMC(5), 10000, its*thin, thin)
println(out.take(10))

The argument to HMC() is the number of leapfrog steps to take per iteration.

Finally, we can use EvilPlot to look at the HMC output and check that we have managed to reasonably recover the true parameters associated with our synthetic data.

import com.cibo.evilplot.geometry.Extent
import com.stripe.rainier.plot.EvilTracePlot._

render(traces(out, truth = Map("b0" -> beta0, "b1" -> beta1)),
  "traceplots.png", Extent(1200, 1000))
render(pairs(out, truth = Map("b0" -> beta0, "b1" -> beta1)), "pairs.png")

Everything looks good, and the sampling is very fast!

Further reading

For further information, see the Rainier repo. In particular, start with the tour of Rainier’s core, which gives a more detailed introduction to how Rainier works than this post. Those interested in how the efficient AD works may want to read about the compute graph, and the implementation notes explain how it all fits together. There is some basic ScalaDoc for the core package, and also some examples (including this one), and there’s a gitter channel for asking questions. This is a very new project, so there are a few minor bugs and wrinkles in the initial release, but development is progressing rapidly, so I fully expect the library to get properly battle-hardened over the next few months.

For those unfamiliar with the monadic approach to probabilistic programming, then Ścibior et al (2015) is probably a good starting point.

MCMC as a Stream

Introduction

This weekend I’ve been preparing some material for my upcoming Scala for statistical computing short course. As part of the course, I thought it would be useful to walk through how to think about and structure MCMC codes, and in particular, how to think about MCMC algorithms as infinite streams of state. This material is reasonably stand-alone, so it seems suitable for a blog post. Complete runnable code for the examples in this post are available from my blog repo.

A simple MH sampler

For this post I will just consider a trivial toy Metropolis algorithm using a Uniform random walk proposal to target a standard normal distribution. I’ve considered this problem before on my blog, so if you aren’t very familiar with Metropolis-Hastings algorithms, you might want to quickly review my post on Metropolis-Hastings MCMC algorithms in R before continuing. At the end of that post, I gave the following R code for the Metropolis sampler:

metrop3<-function(n=1000,eps=0.5) 
{
        vec=vector("numeric", n)
        x=0
        oldll=dnorm(x,log=TRUE)
        vec[1]=x
        for (i in 2:n) {
                can=x+runif(1,-eps,eps)
                loglik=dnorm(can,log=TRUE)
                loga=loglik-oldll
                if (log(runif(1)) < loga) { 
                        x=can
                        oldll=loglik
                        }
                vec[i]=x
        }
        vec
}

I will begin this post with a fairly direct translation of this algorithm into Scala:

def metrop1(n: Int = 1000, eps: Double = 0.5): DenseVector[Double] = {
    val vec = DenseVector.fill(n)(0.0)
    var x = 0.0
    var oldll = Gaussian(0.0, 1.0).logPdf(x)
    vec(0) = x
    (1 until n).foreach { i =>
      val can = x + Uniform(-eps, eps).draw
      val loglik = Gaussian(0.0, 1.0).logPdf(can)
      val loga = loglik - oldll
      if (math.log(Uniform(0.0, 1.0).draw) < loga) {
        x = can
        oldll = loglik
      }
      vec(i) = x
    }
    vec
}

This code works, and is reasonably fast and efficient, but there are several issues with it from a functional programmers perspective. One issue is that we have committed to storing all MCMC output in RAM in a DenseVector. This probably isn’t an issue here, but for some big problems we might prefer to not store the full set of states, but to just print the states to (say) the console, for possible re-direction to a file. It is easy enough to modify the code to do this:

def metrop2(n: Int = 1000, eps: Double = 0.5): Unit = {
    var x = 0.0
    var oldll = Gaussian(0.0, 1.0).logPdf(x)
    (1 to n).foreach { i =>
      val can = x + Uniform(-eps, eps).draw
      val loglik = Gaussian(0.0, 1.0).logPdf(can)
      val loga = loglik - oldll
      if (math.log(Uniform(0.0, 1.0).draw) < loga) {
        x = can
        oldll = loglik
      }
      println(x)
    }
}

But now we have two version of the algorithm. One for storing results locally, and one for streaming results to the console. This is clearly unsatisfactory, but we shall return to this issue shortly. Another issue that will jump out at functional programmers is the reliance on mutable variables for storing the state and old likelihood. Let’s fix that now by re-writing the algorithm as a tail-recursion.

@tailrec
def metrop3(n: Int = 1000, eps: Double = 0.5, x: Double = 0.0, oldll: Double = Double.MinValue): Unit = {
    if (n > 0) {
      println(x)
      val can = x + Uniform(-eps, eps).draw
      val loglik = Gaussian(0.0, 1.0).logPdf(can)
      val loga = loglik - oldll
      if (math.log(Uniform(0.0, 1.0).draw) < loga)
        metrop3(n - 1, eps, can, loglik)
      else
        metrop3(n - 1, eps, x, oldll)
    }
  }

This has eliminated the vars, and is just as fast and efficient as the previous version of the code. Note that the @tailrec annotation is optional – it just signals to the compiler that we want it to throw an error if for some reason it cannot eliminate the tail call. However, this is for the print-to-console version of the code. What if we actually want to keep the iterations in RAM for subsequent analysis? We can keep the values in an accumulator, as follows.

@tailrec
def metrop4(n: Int = 1000, eps: Double = 0.5, x: Double = 0.0, oldll: Double = Double.MinValue, acc: List[Double] = Nil): DenseVector[Double] = {
    if (n == 0)
      DenseVector(acc.reverse.toArray)
    else {
      val can = x + Uniform(-eps, eps).draw
      val loglik = Gaussian(0.0, 1.0).logPdf(can)
      val loga = loglik - oldll
      if (math.log(Uniform(0.0, 1.0).draw) < loga)
        metrop4(n - 1, eps, can, loglik, can :: acc)
      else
        metrop4(n - 1, eps, x, oldll, x :: acc)
    }
}

Factoring out the updating logic

This is all fine, but we haven’t yet addressed the issue of having different versions of the code depending on what we want to do with the output. The problem is that we have tied up the logic of advancing the Markov chain with what to do with the output. What we need to do is separate out the code for advancing the state. We can do this by defining a new function.

def newState(x: Double, oldll: Double, eps: Double): (Double, Double) = {
    val can = x + Uniform(-eps, eps).draw
    val loglik = Gaussian(0.0, 1.0).logPdf(can)
    val loga = loglik - oldll
    if (math.log(Uniform(0.0, 1.0).draw) < loga) (can, loglik) else (x, oldll)
}

This function takes as input a current state and associated log likelihood and returns a new state and log likelihood following the execution of one step of a MH algorithm. This separates the concern of state updating from the rest of the code. So now if we want to write code that prints the state, we can write it as

  @tailrec
  def metrop5(n: Int = 1000, eps: Double = 0.5, x: Double = 0.0, oldll: Double = Double.MinValue): Unit = {
    if (n > 0) {
      println(x)
      val ns = newState(x, oldll, eps)
      metrop5(n - 1, eps, ns._1, ns._2)
    }
  }

and if we want to accumulate the set of states visited, we can write that as

  @tailrec
  def metrop6(n: Int = 1000, eps: Double = 0.5, x: Double = 0.0, oldll: Double = Double.MinValue, acc: List[Double] = Nil): DenseVector[Double] = {
    if (n == 0) DenseVector(acc.reverse.toArray) else {
      val ns = newState(x, oldll, eps)
      metrop6(n - 1, eps, ns._1, ns._2, ns._1 :: acc)
    }
  }

Both of these functions call newState to do the real work, and concentrate on what to do with the sequence of states. However, both of these functions repeat the logic of how to iterate over the sequence of states.

MCMC as a stream

Ideally we would like to abstract out the details of how to do state iteration from the code as well. Most functional languages have some concept of a Stream, which represents a (potentially infinite) sequence of states. The Stream can embody the logic of how to perform state iteration, allowing us to abstract that away from our code, as well.

To do this, we will restructure our code slightly so that it more clearly maps old state to new state.

def nextState(eps: Double)(state: (Double, Double)): (Double, Double) = {
    val x = state._1
    val oldll = state._2
    val can = x + Uniform(-eps, eps).draw
    val loglik = Gaussian(0.0, 1.0).logPdf(can)
    val loga = loglik - oldll
    if (math.log(Uniform(0.0, 1.0).draw) < loga) (can, loglik) else (x, oldll)
}

The "real" state of the chain is just x, but if we want to avoid recalculation of the old likelihood, then we need to make this part of the chain’s state. We can use this nextState function in order to construct a Stream.

  def metrop7(eps: Double = 0.5, x: Double = 0.0, oldll: Double = Double.MinValue): Stream[Double] =
    Stream.iterate((x, oldll))(nextState(eps)) map (_._1)

The result of calling this is an infinite stream of states. Obviously it isn’t computed – that would require infinite computation, but it captures the logic of iteration and computation in a Stream, that can be thought of as a lazy List. We can get values out by converting the Stream to a regular collection, being careful to truncate the Stream to one of finite length beforehand! eg. metrop7().drop(1000).take(10000).toArray will do a burn-in of 1,000 iterations followed by a main monitoring run of length 10,000, capturing the results in an Array. Note that metrop7().drop(1000).take(10000) is a Stream, and so nothing is actually computed until the toArray is encountered. Conversely, if printing to console is required, just replace the .toArray with .foreach(println).

The above stream-based approach to MCMC iteration is clean and elegant, and deals nicely with issues like burn-in and thinning (which can be handled similarly). This is how I typically write MCMC codes these days. However, functional programming purists would still have issues with this approach, as it isn’t quite pure functional. The problem is that the code isn’t pure – it has a side-effect, which is to mutate the state of the under-pinning pseudo-random number generator. If the code was pure, calling nextState with the same inputs would always give the same result. Clearly this isn’t the case here, as we have specifically designed the function to be stochastic, returning a randomly sampled value from the desired probability distribution. So nextState represents a function for randomly sampling from a conditional probability distribution.

A pure functional approach

Now, ultimately all code has side-effects, or there would be no point in running it! But in functional programming the desire is to make as much of the code as possible pure, and to push side-effects to the very edges of the code. So it’s fine to have side-effects in your main method, but not buried deep in your code. Here the side-effect is at the very heart of the code, which is why it is potentially an issue.

To keep things as simple as possible, at this point we will stop worrying about carrying forward the old likelihood, and hard-code a value of eps. Generalisation is straightforward. We can make our code pure by instead defining a function which represents the conditional probability distribution itself. For this we use a probability monad, which in Breeze is called Rand. We can couple together such functions using monadic binds (flatMap in Scala), expressed most neatly using for-comprehensions. So we can write our transition kernel as

def kernel(x: Double): Rand[Double] = for {
    innov <- Uniform(-0.5, 0.5)
    can = x + innov
    oldll = Gaussian(0.0, 1.0).logPdf(x)
    loglik = Gaussian(0.0, 1.0).logPdf(can)
    loga = loglik - oldll
    u <- Uniform(0.0, 1.0)
} yield if (math.log(u) < loga) can else x

This is now pure – the same input x will always return the same probability distribution – the conditional distribution of the next state given the current state. We can draw random samples from this distribution if we must, but it’s probably better to work as long as possible with pure functions. So next we need to encapsulate the iteration logic. Breeze has a MarkovChain object which can take kernels of this form and return a stochastic Process object representing the iteration logic, as follows.

MarkovChain(0.0)(kernel).
  steps.
  drop(1000).
  take(10000).
  foreach(println)

The steps method contains the logic of how to advance the state of the chain. But again note that no computation actually takes place until the foreach method is encountered – this is when the sampling occurs and the side-effects happen.

Metropolis-Hastings is a common use-case for Markov chains, so Breeze actually has a helper method built-in that will construct a MH sampler directly from an initial state, a proposal kernel, and a (log) target.

MarkovChain.
  metropolisHastings(0.0, (x: Double) =>
  Uniform(x - 0.5, x + 0.5))(x =>
  Gaussian(0.0, 1.0).logPdf(x)).
  steps.
  drop(1000).
  take(10000).
  toArray

Note that if you are using the MH functionality in Breeze, it is important to make sure that you are using version 0.13 (or later), as I fixed a few issues with the MH code shortly prior to the 0.13 release.

Summary

Viewing MCMC algorithms as infinite streams of state is useful for writing elegant, generic, flexible code. Streams occur everywhere in programming, and so there are lots of libraries for working with them. In this post I used the simple Stream from the Scala standard library, but there are much more powerful and flexible stream libraries for Scala, including fs2 and Akka-streams. But whatever libraries you are using, the fundamental concepts are the same. The most straightforward approach to implementation is to define impure stochastic streams to consume. However, a pure functional approach is also possible, and the Breeze library defines some useful functions to facilitate this approach. I’m still a little bit ambivalent about whether the pure approach is worth the additional cognitive overhead, but it’s certainly very interesting and worth playing with and thinking about the pros and cons.

Complete runnable code for the examples in this post are available from my blog repo.

A scalable particle filter in Scala

Introduction

Many modern algorithms in computational Bayesian statistics have at their heart a particle filter or some other sequential Monte Carlo (SMC) procedure. In this blog I’ve discussed particle MCMC algorithms which use a particle filter in the inner-loop in order to compute a (noisy, unbiased) estimate of the marginal likelihood of the data. These algorithms are often very computationally intensive, either because the forward model used to propagate the particles is expensive, or because the likelihood associated with each particle/observation is expensive (or both). In this case it is desirable to parallelise the particle filter to run on all available cores of a machine, or in some cases, it would even be desirable to distribute the the particle filter computation across a cluster of machines.

Parallelisation is difficult when using the conventional imperative programming languages typically used in scientific and statistical computing, but is much easier using modern functional languages such as Scala. In fact, in languages such as Scala it is possible to describe algorithms at a higher level of abstraction, so that exactly the same algorithm can run in serial, run in parallel across all available cores on a single machine, or run in parallel across a cluster of machines, all without changing any code. Doing so renders parallelisation a non-issue. In this post I’ll talk through how to do this for a simple bootstrap particle filter, but the same principle applies for a large range of statistical computing algorithms.

Typeclasses and monadic collections

In the previous post I gave a quick introduction to the monad concept, and to monadic collections in particular. Many computational tasks in statistics can be accomplished using a sequence of operations on monadic collections. We would like to write code that is independent of any particular implementation of a monadic collection, so that we can switch to a different implementation without changing the code of our algorithm (for example, switching from a serial to a parallel collection). But in strongly typed languages we need to know at compile time that the collection we use has the methods that we require. Typeclasses provide a nice solution to this problem. I don’t want to get bogged down in a big discussion about Scala typeclasses here, but suffice to say that they describe a family of types conforming to a particular interface in an ad hoc loosely coupled way (they are said to provide ad hoc polymorphism). They are not the same as classes in traditional O-O languages, but they do solve a similar problem to the adaptor design pattern, in a much cleaner way. We can describe a simple typeclass for our monadic collection as follows:

trait GenericColl[C[_]] {
  def map[A, B](ca: C[A])(f: A => B): C[B]
  def reduce[A](ca: C[A])(f: (A, A) => A): A
  def flatMap[A, B, D[B] <: GenTraversable[B]](ca: C[A])(f: A => D[B]): C[B]
  def zip[A, B](ca: C[A])(cb: C[B]): C[(A, B)]
  def length[A](ca: C[A]): Int
}

In the typeclass we just list the methods that we expect our generic collection to provide, but do not say anything about how they are implemented. For example, we know that operations such as map and reduce can be executed in parallel, but this is a separate concern. We can now write code that can be used for any collection conforming to the requirements of this typeclass. The full code for this example is provided in the associated github repo for this blog, and includes the obvious syntax for this typeclass, and typeclass instances for the Scala collections Vector and ParVector, that we will exploit later in the example.

SIR step for a bootstrap filter

We can now write some code for a single observation update of a bootstrap particle filter.

def update[S: State, O: Observation, C[_]: GenericColl](
  dataLik: (S, O) => LogLik, stepFun: S => S
)(x: C[S], o: O): (LogLik, C[S]) = {
  val xp = x map (stepFun(_))
  val lw = xp map (dataLik(_, o))
  val max = lw reduce (math.max(_, _))
  val rw = lw map (lwi => math.exp(lwi - max))
  val srw = rw reduce (_ + _)
  val l = rw.length
  val z = rw zip xp
  val rx = z flatMap (p => Vector.fill(Poisson(p._1 * l / srw).draw)(p._2))
  (max + math.log(srw / l), rx)
}

This is a very simple bootstrap filter, using Poisson resampling for simplicity and data locality, but does include use of the log-sum-exp trick to prevent over/underflow of raw weight calculations, and tracks the marginal (log-)likelihood of the observation. With this function we can now pass in a “prior” particle distribution in any collection conforming to our typeclass, together with a propagator function, an observation (log-)likelihood, and an observation, and it will return back a new collection of particles of exactly the same type that was provided for input. Note that all of the operations we require can be accomplished with the standard monadic collection operations declared in our typeclass.

Filtering as a functional fold

Once we have a function for executing one step of a particle filter, we can produce a function for particle filtering as a functional fold over a sequence of observations:

def pFilter[S: State, O: Observation, C[_]: GenericColl, D[O] <: GenTraversable[O]](
  x0: C[S], data: D[O], dataLik: (S, O) => LogLik, stepFun: S => S
): (LogLik, C[S]) = {
  val updater = update[S, O, C](dataLik, stepFun) _
  data.foldLeft((0.0, x0))((prev, o) => {
    val next = updater(prev._2, o)
    (prev._1 + next._1, next._2)
  })
}

Folding data structures is a fundamental concept in functional programming, and is exactly what is required for any kind of filtering problem. Note that Brian Beckman has recently written a series of articles on Kalman filtering as a functional fold.

Marginal likelihoods and parameter estimation

So far we haven’t said anything about parameters or parameter estimation, but this is appropriate, since parametrisation is a separate concern from filtering. However, once we have a function for particle filtering, we can produce a function concerned with evaluating marginal likelihoods trivially:

def pfMll[S: State, P: Parameter, O: Observation, 
            C[_]: GenericColl, D[O] <: GenTraversable[O]](
  simX0: P => C[S], stepFun: P => S => S, 
  dataLik: P => (S, O) => LogLik, data: D[O]
): (P => LogLik) = (th: P) => 
       pFilter(simX0(th), data, dataLik(th), stepFun(th))._1

Note that this higher-order function does not return a value, but instead a function which will accept a parameter as input and return a (log-)likelihood as output. This can then be used for parameter estimation purposes, perhaps being used in a PMMH pMCMC algorithm, or something else. Again, this is a separate concern.

Example

Here I’ll just give a completely trivial toy example, purely to show how the functions work. For avoidance of doubt, I know that there are many better/simpler/easier ways to tackle this problem! Here we will just look at inferring the auto-regression parameter of a linear Gaussian AR(1)-plus-noise model using the functions we have developed.

First we can simulate some synthetic data from this model, using a value of 0.8 for the auto-regression parameter:

val inNoise = Gaussian(0.0, 1.0).sample(99)
val state = DenseVector(inNoise.scanLeft(0.0)((s, i) => 0.8 * s + i).toArray)
val noise = DenseVector(Gaussian(0.0, 2.0).sample(100).toArray)
val data = (state + noise).toArray.toList

Now assuming that we don’t know the auto-regression parameter, we can construct a function to evaluate the likelihood of different parameter values as follows:

val mll = pfMll(
  (th: Double) => Gaussian(0.0, 10.0).sample(10000).toVector.par,
  (th: Double) => (s: Double) => Gaussian(th * s, 1.0).draw,
  (th: Double) => (s: Double, o: Double) => Gaussian(s, 2.0).logPdf(o),
  data
)

Note that the 4 characters “.par” at the end of line 2 are the only difference between running this code serially or in parallel! Now we can run this code by calling the returned function with different values. So, hopefully mll(0.8) will return a larger log-likelihood than (say) mll(0.6) or mll(0.9). The example code in the github repo plots the results of calling mll() for a range of values (note that if that was the genuine use-case, then it would be much better to parallellise the parameter range than the particle filter, due to providing better parallelisation granularity, but many other examples require parallelisation of the particle filter itself). In this particular example, both the forward model and the likelihood are very cheap operations, so there is little to be gained from parallelisation. Nevertheless, I still get a speedup of more than a factor of two using the parallel version on my laptop.

Conclusion

In this post we have shown how typeclasses can be used in Scala to write code that is parallelisation-agnostic. Code written in this way can be run on one or many cores as desired. We’ve illustrated the concept with a scalable particle filter, but nothing about the approach is specific to that application. It would be easy to build up a library of statistical routines this way, all of which can effectively exploit available parallel hardware. Further, although we haven’t demonstrated it here, it is trivial to extend this idea to allow code to be distribution over a cluster of parallel machines if necessary. For example, if an Apache Spark cluster is available, it is easy to make a Spark RDD instance for our generic collection typeclass, that will then allow us to run our (unmodified) particle filter code over a Spark cluster. This emphasises the fact that Spark can be useful for distributing computation as well as just processing “big data”. I’ll say more about Spark in subsequent posts.

First steps with monads in Scala

Introduction

In the previous post I gave a quick introduction to some important concepts in functional programming, such as HOFs, closures, currying and partial application, and hopefully gave some insight into why these concepts might be useful in the context of scientific computing. Another concept that is very important in modern functional programming is that of the monad. Monads are one of those concepts that turns out to be very simple and intuitive once you “get it”, but completely impenetrable until you do! Now, there zillions of monad tutorials out there, and I don’t think that I have anything particularly insightful to add to the discussion. That said, most of the tutorials focus on problems and examples that are some way removed from the interests of statisticians and scientific programmers. So in this post I want to try and give a very informal and intuitive introduction to the monad concept in a way that I hope will resonate with people from a more scientific computing background.

The term “monad” is borrowed from that of the corresponding concept in category theory. The connection between functional programming and category theory is strong and deep. I intend to expore this more in future posts, but for this post the connection is not important and no knowledge of category theory is assumed (or imparted!).

Functors and Monads

Maps and Functors

All of the code used in this post in contained in the first-monads directory of my blog repo. The best way to follow this post is to copy-and-paste commands one-at-a-time from this post to a Scala REPL or sbt console. Note that only the numerical linear algebra examples later in this post require any non-standard dependencies.

The map method is one of the first concepts one meets when beginning functional programming. It is a higher order method on many (immutable) collection and other container types. Let’s start by looking at how map operates on Lists.

val x = (0 to 4).toList
// x: List[Int] = List(0, 1, 2, 3, 4)
val x2 = x map { x => x * 3 }
// x2: List[Int] = List(0, 3, 6, 9, 12)
val x3 = x map { _ * 3 }
// x3: List[Int] = List(0, 3, 6, 9, 12)
val x4 = x map { _ * 0.1 }
// x4: List[Double] = List(0.0, 0.1, 0.2, 0.30000000000000004, 0.4)

The last example shows that a List[T] can be converted to a List[S] if map is passed a function of type T => S. Of course there’s nothing particularly special about List here. It works with other collection types in the same way, as the following example with (immutable) Vector illustrates:

val xv = x.toVector
// xv: Vector[Int] = Vector(0, 1, 2, 3, 4)
val xv2 = xv map { _ * 0.2 }
// xv2: scala.collection.immutable.Vector[Double] = Vector(0.0, 0.2, 0.4, 0.6000000000000001, 0.8)
val xv3 = for (xi <- xv) yield (xi * 0.2)
// xv3: scala.collection.immutable.Vector[Double] = Vector(0.0, 0.2, 0.4, 0.6000000000000001, 0.8)

Note here that the for comprehension generating xv3 is exactly equivalent to the map call generating xv2 – the for-comprehension is just syntactic sugar for the map call. The benefit of this syntax will become apparent in the more complex examples we consider later.

Many collection and other container types have a map method that behaves this way. Any parametrised type that does have a map method like this is known as a Functor. Again, the name is due to category theory, but that doesn’t matter for this post. From a Scala-programmer perspective, a functor can be thought of as a trait, in pseudo-code as

trait F[T] {
  def map(f: T => S): F[S]
}

with F representing the functor. In fact it turns out to be better to think of a functor as a type class, but that is yet another topic for a future post… Also note that to be a functor in the strict sense (from a category theory perspective), the map method must behave sensibly – that is, it must satisfy the functor laws. But again, I’m keeping things informal and intuitive for this post – there are plenty of other monad tutorials which emphasise the category theory connections.

FlatMap and Monads

Once we can map functions over elements of containers, we soon start mapping functions which themselves return values of the container type. eg. we can map a function returning a List over the elements of a List, as illustrated below.

val x5 = x map { x => List(x - 0.1, x + 0.1) }
// x5: List[List[Double]] = List(List(-0.1, 0.1), List(0.9, 1.1), List(1.9, 2.1), List(2.9, 3.1), List(3.9, 4.1))

Clearly this returns a list-of-lists. Sometimes this is what we want, but very often we actually want to flatten down to a single list so that, for example, we can subsequently map over all of the elements of the base type with a single map. We could take the list-of-lists and then flatten it, but this pattern is so common that the act of mapping and then flattening is often considered to be a basic operation, often known in Scala as flatMap. So for our toy example, we could carry out the flatMap as follows.

val x6 = x flatMap { x => List(x - 0.1, x + 0.1) }
// x6: List[Double] = List(-0.1, 0.1, 0.9, 1.1, 1.9, 2.1, 2.9, 3.1, 3.9, 4.1)

The ubiquity of this pattern becomes more apparent when we start thinking about iterating over multiple collections. For example, suppose now that we have two lists, x and y, and that we want to iterate over all pairs of elements consisting of one element from each list.

val y = (0 to 12 by 2).toList
// y: List[Int] = List(0, 2, 4, 6, 8, 10, 12)
val xy = x flatMap { xi => y map { yi => xi * yi } }
// xy: List[Int] = List(0, 0, 0, 0, 0, 0, 0, 0, 2, 4, 6, 8, 10, 12, 0, 4, 8, 12, 16, 20, 24, 0, 6, 12, 18, 24, 30, 36, 0, 8, 16, 24, 32, 40, 48)

This pattern of having one or more nested flatMaps followed by a final map in order to iterate over multiple collections is very common. It is exactly this pattern that the for-comprehension is syntactic sugar for. So we can re-write the above using a for-comprehension

val xy2 = for {
  xi <- x
  yi <- y
} yield (xi * yi)
// xy2: List[Int] = List(0, 0, 0, 0, 0, 0, 0, 0, 2, 4, 6, 8, 10, 12, 0, 4, 8, 12, 16, 20, 24, 0, 6, 12, 18, 24, 30, 36, 0, 8, 16, 24, 32, 40, 48)

This for-comprehension (usually called a for-expression in Scala) has an intuitive syntax reminiscent of the kind of thing one might write in an imperative language. But it is important to remember that <- is not actually an imperative assignment. The for-comprehension really does expand to the pure-functional nested flatMap and map call given above.

Recalling that a functor is a parameterised type with a map method, we can now say that a monad is just a functor which also has a flatMap method. We can write this in pseudo-code as

trait M[T] {
  def map(f: T => S): M[S]
  def flatMap(f: T => M[S]): M[S]
}

Not all functors can have a flattening operation, so not all functors are monads, but all monads are functors. Monads are therefore more powerful than functors. Of course, more power is not always good. The principle of least power is one of the main principles of functional programming, but monads are useful for sequencing dependent computations, as illustrated by for-comprehensions. In fact, since for-comprehensions de-sugar to calls to map and flatMap, monads are precisely what are required in order to be usable in for-comprehensions. Collections supporting map and flatMap are referred to as monadic. Most Scala collections are monadic, and operating on them using map and flatMap operations, or using for-comprehensions is referred to as monadic-style. People will often refer to the monadic nature of a collection (or other container) using the word monad, eg. the “List monad”.

So far the functors and monads we have been working with have been collections, but not all monads are collections, and in fact collections are in some ways atypical examples of monads. Many monads are containers or wrappers, so it will be useful to see examples of monads which are not collections.

Option monad

One of the first monads that many people encounter is the Option monad (referred to as the Maybe monad in Haskell, and Optional in Java 8). You can think of it as being a strange kind of “collection” that can contain at most one element. So it will either contain an element or it won’t, and so can be used to wrap the result of a computation which might fail. If the computation succeeds, the value computed can be wrapped in the Option (using the type Some), and if it fails, it will not contain a value of the required type, but simply be the value None. It provides a referentially transparent and type-safe alternative to raising exceptions or returning NULL references. We can transform Options using map.

val three = Option(3)
// three: Option[Int] = Some(3)
val twelve = three map (_ * 4)
// twelve: Option[Int] = Some(12)

But when we start combining the results of multiple computations that could fail, we run into exactly the same issues as before.

val four = Option(4)
// four: Option[Int] = Some(4)
val twelveB = three map (i => four map (i * _))
// twelveB: Option[Option[Int]] = Some(Some(12))

Here we have ended up with an Option wrapped in another Option, which is not what we want. But we now know the solution, which is to replace the first map with flatMap, or better still, use a for-comprehension.

val twelveC = three flatMap (i => four map (i * _))
// twelveC: Option[Int] = Some(12)
val twelveD = for {
  i <- three
  j <- four
} yield (i * j)
// twelveD: Option[Int] = Some(12)

Again, the for-comprehension is a little bit easier to understand than the chaining of calls to flatMap and map. Note that in the for-comprehension we don’t worry about whether or not the Options actually contain values – we just concentrate on the “happy path”, where they both do, safe in the knowledge that the Option monad will take care of the failure cases for us. Two of the possible failure cases are illustrated below.

val oops: Option[Int] = None
// oops: Option[Int] = None
val oopsB = for {
  i <- three
  j <- oops
} yield (i * j)
// oopsB: Option[Int] = None
val oopsC = for {
  i <- oops
  j <- four
} yield (i * j)
// oopsC: Option[Int] = None

This is a typical benefit of code written in a monadic style. We chain together multiple computations thinking only about the canonical case and trusting the monad to take care of any additional computational context for us.

IEEE floating point and NaN

Those with a background in scientific computing are probably already familiar with the NaN value in IEEE floating point. In many regards, this value and the rules around its behaviour mean that Float and Double types in IEEE compliant languages behave as an Option monad with NaN as the None value. This is simply illustrated below.

val nan = Double.NaN
3.0 * 4.0
// res0: Double = 12.0
3.0 * nan
// res1: Double = NaN
nan * 4.0
// res2: Double = NaN

The NaN value arises naturally when computations fail. eg.

val nanB = 0.0 / 0.0
// nanB: Double = NaN

This Option-like behaviour of Float and Double means that it is quite rare to see examples of Option[Float] or Option[Double] in Scala code. But there are some disadvantages of the IEEE approach, as discussed elsewhere. Also note that this only works for Floats and Doubles, and not for other types, including, say, Int.

val nanC=0/0
// This raises a runtime exception!

Option for matrix computations

Good practical examples of scientific computations which can fail crop up frequently in numerical linear algebra, so it’s useful to see how Option can simplify code in that context. Note that the code in this section requires the Breeze library, so should be run from an sbt console using the sbt build file associated with this post.

In statistical applications, one often needs to compute the Cholesky factorisation of a square symmetric matrix. This operation is built into Breeze as the function cholesky. However the factorisation will fail if the matrix provided is not positive semi-definite, and in this case the cholesky function will throw a runtime exception. We will use the built in cholesky function to create our own function, safeChol (using a monad called Try which is closely related to the Option monad) returning an Option of a matrix rather than a matrix. This function will not throw an exception, but instead return None in the case of failure, as illustrated below.

import breeze.linalg._
def safeChol(m: DenseMatrix[Double]): Option[DenseMatrix[Double]] = scala.util.Try(cholesky(m)).toOption
val m = DenseMatrix((2.0, 1.0), (1.0, 3.0))
val c = safeChol(m)
// c: Option[breeze.linalg.DenseMatrix[Double]] =
// Some(1.4142135623730951  0.0
// 0.7071067811865475  1.5811388300841898  )

val m2 = DenseMatrix((1.0, 2.0), (2.0, 3.0))
val c2 = safeChol(m2)
// c2: Option[breeze.linalg.DenseMatrix[Double]] = None

A Cholesky factorisation is often followed by a forward or backward solve. This operation may also fail, independently of whether the Cholesky factorisation fails. There doesn’t seem to be a forward solve function directly exposed in the Breeze API, but we can easily define one, which I call dangerousForwardSolve, as it will throw an exception if it fails, just like the cholesky function. But just as for the cholesky function, we can wrap up the dangerous function into a safe one that returns an Option.

import com.github.fommil.netlib.BLAS.{getInstance => blas}
def dangerousForwardSolve(A: DenseMatrix[Double], y: DenseVector[Double]): DenseVector[Double] = {
  val yc = y.copy
  blas.dtrsv("L", "N", "N", A.cols, A.toArray, A.rows, yc.data, 1)
  yc
}
def safeForwardSolve(A: DenseMatrix[Double], y: DenseVector[Double]): Option[DenseVector[Double]] = scala.util.Try(dangerousForwardSolve(A, y)).toOption

Now we can write a very simple function which chains these two operations together, as follows.

def safeStd(A: DenseMatrix[Double], y: DenseVector[Double]): Option[DenseVector[Double]] = for {
  L <- safeChol(A)
  z <- safeForwardSolve(L, y)
} yield z

safeStd(m,DenseVector(1.0,2.0))
// res15: Option[breeze.linalg.DenseVector[Double]] = Some(DenseVector(0.7071067811865475, 0.9486832980505138))

Note how clean and simple this function is, concentrating purely on the “happy path” where both operations succeed and letting the Option monad worry about the three different cases where at least one of the operations fails.

The Future monad

Let’s finish with a monad for parallel and asynchronous computation, the Future monad. The Future monad is used for wrapping up slow computations and dispatching them to another thread for completion. The call to Future returns immediately, allowing the calling thread to continue while the additional thread processes the slow work. So at that stage, the Future will not have completed, and will not contain a value, but at some (unpredictable) time in the future it (hopefully) will (hence the name). In the following code snippet I construct two Futures that will each take at least 10 seconds to complete. On the main thread I then use a for-comprehension to chain the two computations together. Again, this will return immediately returning another Future that at some point in the future will contain the result of the derived computation. Then, purely for illustration, I force the main thread to stop and wait for that third future (f3) to complete, printing the result to the console.

import scala.concurrent.duration._
import scala.concurrent.{Future,ExecutionContext,Await}
import ExecutionContext.Implicits.global
val f1=Future{
  Thread.sleep(10000)
  1 }
val f2=Future{
  Thread.sleep(10000)
  2 }
val f3=for {
  v1 <- f1
  v2 <- f2
  } yield (v1+v2)
println(Await.result(f3,30.second))

When you paste this into your console you should observe that you get the result in 10 seconds, as f1 and f2 execute in parallel on separate threads. So the Future monad is one (of many) ways to get started with parallel and async programming in Scala.

Summary

In this post I’ve tried to give a quick informal introduction to the monad concept, and tried to use examples that will make sense to those interested in scientific and statistical computing. There’s loads more to say about monads, and there are many more commonly encountered useful monads that haven’t been covered in this post. I’ve skipped over lots of details, especially those relating to the formal definitions of functors and monads, including the laws that map and flatMap must satisfy and why. But those kinds of details can be easily picked up from other monad tutorials. Anyone interested in pursuing the formal connections may be interested in a page of links I’m collating on category theory for FP. In particular, I quite like the series of blog posts on category theory for programmers. As I’ve mentioned in previous posts, I also really like the book Functional Programming in Scala, which I strongly recommend to anyone who wants to improve their Scala code. In a subsequent post I’ll explain how monadic style is relevant to issues relating to the statistical analysis of big data, as exemplified in Apache Spark. It’s probably also worth mentioning that there is another kind of functor that turns out to be exceptionally useful in functional programming: the applicative functor. This is more powerful than a basic functor, but less powerful than a monad. It turns out to be useful for computations which need to be sequenced but are not sequentially dependent (context-free rather than context-sensitive), and is a little bit more general and flexible than a monad in cases where it is appropriate.