Every action has two results (Erlang edition)

Every action has two results: a set of side effects on the world, and the next version of ourselves.

I learned this from Erlang, a purely functional yet stateful programming language. Erlang uses actor-based concurrency. The language is fully immutable, yet the programs are not: every time an actor receives a message, it can send messages to other actors, and then return a different version of itself, instantiated with different state.

Here’s an example from the tutorial in the Erlang docs:

%%% This is the server process for the "messenger"
%%% the user list has the format [{ClientPid1, Name1},{ClientPid22, Name2},...]
server(User_List) ->
    receive
        {From, logon, Name} ->
            New_User_List = server_logon(From, Name, User_List),
            server(New_User_List);
%%% ...
   end.

This defines a server process (that is, an actor) which receives a logon message. When it does that, it builds a new list of users including the one it just received and all the ones it knew about before. Then it constructs a new version of itself with the new list! (That’s implicitly returned from receive.) The next message will be received by the new server.

It’s like that with us, too. Today I made coffee, with a side effect of some dirty cups and fewer coffee beans, and a next version of me that was more alert. Today I checked twitter, with a side effect of nothing observable, and a next version of me ready to write this post. Now I’m writing this post, which will have side effects with unknown consequences, depending on what y’all do with it.

This works in our teams, too. Every task we complete changes the world, and it changes us. Maybe we add tests or implement a feature. In the process, we learn about the software system we participate in. Did we do this as a team, or will we catch each other up later? Is changing the software more safe or harder than before?

When “productivity” measures focus on externally-visible outcomes, sometimes the internal system is left in a terrible state. Burnout in people, “technical debt” in code, and a degeneration of the mental models that connect us with the code we care for.

The consequences of our work matter now. The next version of us matters for the whole future, for everything after now.

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