I'm not too clear on how one would implement a state machine without "spreading the transitions everywhere". Can you share a link to some example code showing the format you prefer?

Typically, we'd see the code that implements the logic for each state gathered together. Since this is where the outgoing transitions get decided, the transitions end up being distributed across their originating states.

erlang/otp gen_fsm maybe?

you define a function with a clause per state (and some other args including the incoming message), and each clause returns a tuple with the new state and some other stuff. the loop is part of the framework, and handles generic otp stuff like updating code in a running system.

That doesn't compute for me. You'd need to give an example for me to understand you.