Narrow events according to a specific state

[krambono]

Imagine having a state machine like this:

flowchart LR
    A[State A] -->|next| B(State B)
    B -->|prev| A

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After creating an actor from that machine, we can send events:

actor.send({type: 'next'})
actor.send({type: 'prev'})

It would be nice to narrow the events according to the state:

if (actor.matches('State A')) {
  actor.send({type: 'next'})
  actor.send({type: 'prev'}) // <-- compile time error: we can't send event 'prev' from state 'State A'
}

if (actor.matches('State B')) {
  actor.send({type: 'next'}) // <-- compile time error: we can't send event 'next' from state 'State B'
  actor.send({type: 'prev'})
}