Hot potato state in Erlang


December 22, 2022

Erlang is a peculiar language. It is a dynamically typed, functional language, with all the usual features like the preference for recursion over loops, pattern matching, etc. Unlike other languages, it treats all kinds of problems as message passing and truly embraces the concept of concurrency and multiprocessing. As we’ll see below, many problems are solved in Erlang by spawning new processes and communicating with them. Another of Erlang’s oddities is that everything in unmutable, I mean it.

As in any other programming language, in Erlang, you could define a variable X1 and use it to create another one Y.

1> X = 1.
2> X.
3> Y = X + 1.

But, let’s say, that you want to increment X by one.

4> X = X + 1.
** exception error: no match of right hand side value 2

Sorry, you can’t. You need instead to create X1 = X + 1., but this is not a sustainable way of storing the state in the long run. What if you need to preserve a state that changes over time? One way could be to use recursion and pass the value along. A trivial example would be a function that counts From to To while printing the results. At each step, the function checks the Value =:= To2 condition and if the counting is not done, increments the From value in the wildcard _3 case.

count(From, To) ->
    io:format("~w~n", [From]),
    case From of
        Value when Value =:= To ->
        _ ->
            count(From + 1, To)

Notice that the From variable is ephemeral here, it only exists within a function call. When calling count4 recursively, each time we are creating a new variable. But passing everything as arguments of functions does not scale. What if we do want something stateful? That’s where in Erlang you use what I call the hot potato approach. If you don’t know the game, here you are:

Hot potato is a party game that involves players gathering in a circle and tossing a small object such as a beanbag or even a real potato to each other while music plays. The player who is holding the object when the music stops is eliminated.

Same as with using a recursive function, if we want to preserve the state in Erlang, we would be passing it along like a hot potato. The difference would be that the hot potato would be tossed around in an infinite loop in a separate process. Again, we would have a recursive function that passes the State along. The function would be able to receive messages from other processes, where the messages inform it to set the state to the new value, get to answer the sender with the value of the current state, and the bye message that stops the infinite loop. To send a message Message to a Receiver in Erlang, we use the Receiver ! Message syntax.

loop(State) ->
        {From, set, Newstate} ->
            From ! ok,
        {From, get} ->
            From ! {ok, State},
        {From, bye} ->
            From ! ok

Having it, we can spawn a process running the loop that I declared in a potato module.

1> c(potato).
2> Pid = spawn(fun() -> potato:loop(empty) end).

It started with the empty state, we can verify this by sending the message {SenderPid, get} to the process identified by the Pid. I use flush(). here to extract the messages received by the calling process.

3> Pid ! {self(), get}.
4> flush().
Shell got {ok,empty}

We can also send the {SenderPid, set, Value} message to change the state.

5> Pid ! {self(), set, hello}.
6> Pid ! {self(), get}.
7> flush().
Shell got ok
Shell got {ok,hello}

Voilà! We have a mutable state living in an infinite loop of an external process holding it.


  1. Variables in Erlang need to have uppercase names.↩︎

  2. =:=, yes, don’t ask.↩︎

  3. _ is a placeholder for whatever, it is also used for denoting unused variables like _Var.↩︎

  4. The names of the functions need to be in lowercase.↩︎