This is a tangent > [in functional programming] you don't have to think of an internal state, like in OOP. You still have to but less than with OOP languages.

This is a tangent > [in functional programming] you don't have to think of an internal state, like in OOP. You still have to but less than with OOP languages. For example I've got an Elixir GenServer that uploads files to Google Storage and keeps retrying of something goes wrong. The internal state of the server is the list of files to upload. Files get added to the list when the server receives a message with a file and removed when the file is uploaded. Tests have to deal with the internal state, as in OOP. This kind of stateful "functions" are implemented by recursively passing the state to the same function. Tail call optimization helps not overflowing the stack. Of course we have state only where it matters so the servers are few and sparse. But sometimes we have to pass large data structures between functions. They would be the internal state of an object in OOP. Check Ecto's Multi for transactions.