when use nonisolated with stored properties of actor?

I am not sure when use nonisolated with stored properties of actor.

P.s In this blog Douglas Gregor asking same question "So... just always require nonisolated let for synchronous access?". But I don't understand his answer.

Who can explain please ?

Edit:

Clarification, as it is written in the blog, using nonisolated helps to get rid of problems in the future if the property becomes mutable, but then it turns out that any immutable property must be noniolsated, because no one knows how their program will change over time

Solution

You said:

as it is written in the blog, using nonisolated helps to get rid of problems in the future if the property becomes mutable

The nonisolated let does not solve this problem. (One can argue that it contributes to this problem, requiring that if you need/want to enjoy synchronous access within the module, that you also make an explicit contract to that effect to external modules, which may or may not be desired.)

If you use nonisolated let, and later change it to a mutable var, this will require refactoring of code currently accessing this property synchronously to now do so asynchronously. This proposed change to the language is suggesting a potential mechanism to somewhat mitigate this.

Specifically, it proposed that in the absence of the nonisolated qualifier on a constant, that the module still enjoy the benefits of synchronous (non-isolated) access. But it is also proposing that outside this module, that it would remain isolated (which, while you would not enjoy that synchronous interface, would mean that a subsequent introduction of a mutable state would not necessarily break its interface).

You observed:

In this blog Douglas Gregor asking same question “So... just always require nonisolated let for synchronous access?”

He is not advocating that position. It is merely a strawman, an acknowledgment of the state of affairs at that point in time. In that section, he outlined a few reasons why marking all constants with nonisolated let may not be prudent.

That is why he is proposing (in the next section) that a simple let allow synchronous access within the module, but not across modules.

This is the behavior we currently see in Swift.

You ask:

do I need to use nonisolated every time with let?

You should only use nonisolated let where both of the following are true:

you have a legitimate requirement for synchronous, non-isolated, access; and
where you are reasonably confident that you are unlikely to ever make it mutable and/or, should you need to make this change in the future, that you (and any other developers using this type) are willing to sign up for the refactoring exercise that may entail.

As a general matter, we want to avoid code-breaking interface changes as an implementation evolves. So, where necessary, certainly feel free to make your constants nonisolated, but otherwise, you probably want to refrain from doing so.

You ask:

I am not sure when use nonisolated with stored properties of actor.

It should be noted that we do not need to use nonisolated qualifier within a module anymore to enjoy synchronous, cross-actor access. As SE-0306 says:

… a cross-actor reference to immutable state is allowed from anywhere in the same module as the actor is defined because, once initialized, that state can never be modified (either from inside the actor or outside it), so there are no data races by definition.

So, you now only really need to use nonisolated let where you want to allow synchronous access to other modules and you are signing up for the implications of that contract.