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title: "Transitions"

Transitions

Transitions are the most important parts of a state machine, since it embodies the major logic of the application.

Basic Transitions

The basic elements of a transition is the target state and the actions to execute.

{:states {:s1
          {:on {:event1 {:target :s2
                         :actions some-action-fn}}}}

If the current state is :s1 and event :event1 happens, the new state would be :s2 and the action function some-action-fn would be executed.

Some syntax sugars:

{:states {:s1
          {:on {:event1 {:target  :s2
                         :actions [action-fn1 action-fn2]} ;; (1)
                :event2 :s3}}}}                            ;; (2)

(1) The actions could be a vector of multiple action functions to execute.

(2) If there is no actions, the transition could be simplified to be a single keyword.

Please note that event names could be any keywords, with one exception: keywords namespace "fsm" is considered reserved for clj-statecharts's internal use, so do not use event names like :fsm/foo in your application code.

Internal & External Transitions {#internal-external-transitions}

Before reading this section, make sure you have read the [Identifying States]({{< relref "/docs/identifying-states.md" >}}) about the absolute and relative syntax of representing a state.

A state could transition to one of its child states.

  • If the transition triggers the entry/exit actions on the parent state, it's called an external transition.
  • Otherwise, it's called an internal transition.

Here is an example of an internal & external transition:

{:states {:s1 {:initial :s1.1
               :entry entry1
               :exit exit1
               :on {:event1_1.2_internal [:. :s1.2]      ;; (1)
                    :event1_1.2_external [:> :s1 :s1.2]} ;; (2)
               :states {:s1.1 {}
                        :s1.2 {}}}}}

State :s1 has two child states :s1.1 and :s1.2.

(1) When event event1_1.2_internal happens, the new state would be :s1.2, but the entry/exit actions of state :s1 would not be executed.

(2) When event event1_1.2_external happens, the new state would also be :s1.2, but the entry/exit actions of state :s1 would be executed.

Self Transitions {#self-transitions}

A state could transition to itself, this is called a self-transition.

For instance, a calculator FSM could have a state :operand1 representing the state that the user is typing the first operand. So the :input-digit event should keep the state machine in that state (but using actions to update the value of the operand on each input), instead of transitioning to a new state.

A self-transition could also be either internal or external. To specify an internal self-transition, simply omit the :target key in the transition map.

For instance:

{:states {:s1 {:entry entry1
               :exit  exit1
               :on    {:event1_1_internal {:actions some-action} ;; (1)
                       :event1_1_external :s1                    ;; (2)
                       }}}}

(1) For event :event1_1_internal, the target state is not specified (or nil). In this case, the entry/exit actions of :s1 would not be executed.

(2) For event :event1_1_external, the target state is explicitly provided as itself, so the entry/exit actions of :s1 would be executed.

EventLess Transitions

Quoting XState doc:

An eventless transition is a transition that is always taken when the machine is in the state where it is defined, and when its guards evaluates to true. They are always checked when the state is first entered, before handling any other events.

Eventless transitions are defined on the :always key of the state node:

Given the following state machine:

{:states {:s1 {:entry entry1
               :exit exit1
               :on {:e12 :s2
                    :actions action12}}
          :s2 {:entry entry2
               :exit exit2
               :always [{:guard guard23
                         :target :s3
                         :actions action23}
                        {:guard guard24
                         :target :s4
                         :actions action23}]
               :on {:e23 :s3}}
          :s3 {:entry entry3}
          :s4 {}}}

Assume current state is :s1, and event :e12 happens.

  • The target state is :s2. Because :s2 has eventless transitions defined, it would immediately evaluates the guard function guard23.
  • If guard23 returns a truthy value, the machine would transition to state :s3. In this case these actions would be executed one by one:
    • exit1
    • action12
    • entry2
    • exit2
    • action23
    • entry3
  • Otherwise guard24 would be evaluated. If guard24 returns truthy, the machine would transition to :s4 (and similarly a list of actions would be executed). Otherwise it would stay in :s2.

The event args that accompanies :e12 would be passed to every action that is executed.

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