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Erlang comes with gen_fsm behavior because often when writting network protocols and servers, a state machine is a useful abstraction.

Relevant:

"Rage Against The Finite-State Machine" from "Learn you some Erlang For Great Good"

In Haskell, you can represent a state machine and its current state using an iteratee:

An iteratee is essentially just a function that takes an input symbol and returns either a final state value or another iteratee.