Bug: preserveActionOrder is not prioritising execution of assign actions

[mieszko4]

Description

I've initially created #3382 to show how preserveActionOrder=true is confusing because it only preserve the order of context and not the order of execution of actions.
However, I am not sure anymore what is the intention of preserveActionOrder because it mentions the order of execution as described in (https://github.com/statelyai/xstate/blob/1d1c90c4be57e606975564114b7f23d951ad5ccb/packages/core/CHANGELOG.md#minor-changes-9):

There is a new .preserveActionOrder (default: false) setting in the machine configuration that preserves the order of actions when set to true. Normally, actions are executed in order except for assign(...) actions, which are prioritized and executed first. When .preserveActionOrder is set to true, assign(...) actions will not be prioritized, and will instead run in order. As a result, actions will capture the intermediate context values instead of the resulting context value from all assign(...) actions.

Hence I am creating this issue to show the problem.

// Based on example from docs (https://xstate.js.org/docs/guides/context.html#action-order)
const counterMachine = createMachine<ToggleContext, ToggleEvent>({
  preserveActionOrder: true, // Ensures that assign actions are called in order
  initial: "active",
  // ...
  context: { count: 0 },
  states: {
    active: {
      on: {
        INC_TWICE: {
          actions: [
            (context) => console.log(`Before: ${context.count}`), // "Before: 0"
            assign((context) => {
              console.log(`First assign`);
              return { ...context, count: context.count + 1 };
            }), // count === 1
            assign((context) => {
              console.log(`Second assign`);
              return { ...context, count: context.count + 1 };
            }), // count === 2
            (context) => console.log(`After: ${context.count}`) // "After: 2"
          ]
        }
      }
    }
  }
});

interpret(counterMachine).start().send({ type: "INC_TWICE" });

Expected result

"Before: 0"
"First assign"
"Second assign"
"After: 2"

Actual result

"First assign"
"Second assign"
"Before: 0"
"After: 2"

Reproduction

https://codesandbox.io/s/stupefied-torvalds-mjjhi0

Additional context

xstate@4.31.0

[davidkpiano]

Assign actions should be pure. We should mention this in the docs.

[mieszko4]

Assign actions should be pure. We should mention this in the docs.

Thank you for clarifying that! For me should be pure does not sound strong enough :) I think it would be better to mention must be pure in the docs.

[Andarist]

I'm working on another flag that makes this behavior even more predictable (PR) and with that PR you would actually get your expected result:

"Before: 0"
"First assign"
"Second assign"
"After: 2"