Keeping State in a Purely Functional Language

Question

I am trying to figure out how to do the following, assume that your are working on a controller for a DC motor you want to keep it spinning at a certain speed set by the user,

(def set-point (ref {:sp 90}))

(while true
  (let [curr (read-speed)]
    (controller @set-point curr)))

Now that set-point can change any time via a web a application, I can't think of a way to do this without using ref, so my question is how functional languages deal with this sort of thing? (even though the example is in clojure I am interested in the general idea.)

Answer 1

This will not answer your question but I want to show how these things are done in Clojure. It might help someone reading this later so they don't think they have to read up on monads, reactive programming or other "complicated" subjects to use Clojure.

Clojure is not a purely functional language and in this case it might be a good idea to leave the pure functions aside for a moment and model the inherent state of the system with identities.

In Clojure, you would probably use one of the reference types. There are several to choose from and knowing which one to use might be difficult. The good news is they all support the unified update model so changing the reference type later should be pretty straight forward.

I've chosen an atom but depending on your requirements it might be more appropriate to use a ref or an agent.

The motor is an identity in your program. It is a "label" for some thing that has different values at different times and these values are related to each other (i.e., the speed of the motor). I have put a :validator on the atom to ensure that the speed never drops below zero.

(def motor (atom {:speed 0} :validator (comp not neg? :speed)))

(defn add-speed [n]
  (swap! motor update-in [:speed] + n))

(defn set-speed [n]
  (swap! motor update-in [:speed] (constantly n)))

> (add-speed 10)
> (add-speed -8)
> (add-speed -4) ;; This will not change the state of motor
                 ;; since the speed would drop below zero and
                 ;; the validator does not allow that!
> (:speed @motor)
2
> (set-speed 12)
> (:speed @motor)
12

If you want to change the semantics of the motor identity you have at least two other reference types to choose from.

• If you want to change the speed of the motor asynchronously you would use an agent. Then you need to change swap! with send. This would be useful if, for example, the clients adjusting the motor speed are different from the clients using the motor speed, so that it's fine for the speed to be changed "eventually".

• Another option is to use a ref which would be appropriate if the motor need to coordinate with other identities in your system. If you choose this reference type you change swap! with alter. In addition, all state changes are run in a transaction with dosync to ensure that all identities in the transaction are updated atomically.

Monads are not needed to model identities and state in Clojure!

Answer 2

For this answer, I'm going to interpret "a purely functional language" as meaning "an ML-style language that excludes side effects" which I will interpret in turn as meaning "Haskell" which I'll interpret as meaning "GHC". None of these are strictly true, but given that you're contrasting this with a Lisp derivative and that GHC is rather prominent, I'm guessing this will still get at the heart of your question.

As always, the answer in Haskell is a bit of sleight-of-hand where access to mutable data (or anything with side effects) is structured in such a way that the type system guarantees that it will "look" pure from the inside, while producing a final program that has side effects where expected. The usual business with monads is a large part of this, but the details don't really matter and mostly distract from the issue. In practice, it just means you have to be explicit about where side effects can occur and in what order, and you're not allowed to "cheat".

Mutability primitives are generally provided by the language runtime, and accessed through functions that produce values in some monad also provided by the runtime (often IO, sometimes more specialized ones). First, let's take a look at the Clojure example you provided: it uses ref, which is described in the documentation here:

While Vars ensure safe use of mutable storage locations via thread isolation, transactional references (Refs) ensure safe shared use of mutable storage locations via a software transactional memory (STM) system. Refs are bound to a single storage location for their lifetime, and only allow mutation of that location to occur within a transaction.

Amusingly, that whole paragraph translates pretty directly to GHC Haskell. I'm guessing that "Vars" are equivalent to Haskell's MVar, while "Refs" are almost certainly equivalent to TVar as found in the stm package.

So to translate the example to Haskell, we'll need a function that creates the TVar:

setPoint :: STM (TVar Int)
setPoint = newTVar 90

...and we can use it in code like this:

updateLoop :: IO ()
updateLoop = do tvSetPoint <- atomically setPoint
                sequence_ . repeat $ update tvSetPoint
  where update tv = do curSpeed <- readSpeed
                       curSet   <- atomically $ readTVar tv
                       controller curSet curSpeed

In actual use my code would be far more terse than that, but I've left things more verbose here in hopes of being less cryptic.

I suppose one could object that this code isn't pure and is using mutable state, but... so what? At some point a program is going to run and we'd like it to do input and output. The important thing is that we retain all the benefits of code being pure, even when using it to write code with mutable state. For instance, I've implemented an infinite loop of side effects using the repeat function; but repeat is still pure and behaves reliably and nothing I can do with it will change that.