Is it dangerous to share references between states?

[symbiont-sean-lyons]

When transitioning a model using Command.NextState -> commands.State, is it dangerous to share pointers or reference types between model states?

For example,

type State struct {
  index map[string]string
}

func (writeFooBarCommand) NextState(s commands.State) commands.State {
  current := s.(State)
  next := current
  next.index["foo"] = "bar"
  return next
}

I believe that the existing model state and the new model state will share a reference to the underlying map, meaning manipulations via the old state instance or the new state instance will reflect in both objects. I currently make it a habit to deep copy my model states when transitioning, but my question is whether or not the deep copy is necessary.

[untoldwind]

In this specific case it should be no problem mutating the state as long as InitialStateGen always generates a new instance.

The main loop running a sequence of commands is done in this function:

gopter/commands/actions.go

Lines 41 to 53 in 8de0d74

	func (a *actions) run(systemUnderTest SystemUnderTest) *gopter.PropResult {
	state := a.initialStateProvider()
	propResult := &gopter.PropResult{Status: gopter.PropTrue}
	for _, shrinkableCommand := range a.sequentialCommands {
	if !shrinkableCommand.command.PreCondition(state) {
	return &gopter.PropResult{Status: gopter.PropFalse}
	}
	result := shrinkableCommand.command.Run(systemUnderTest)
	state = shrinkableCommand.command.NextState(state)
	propResult = propResult.And(shrinkableCommand.command.PostCondition(state, result))
	}
	return propResult
	}

As you can see the previous state is just forgotten after the command has been applied, therefore it does not matter if it has been mutated or not.