Lazily creating new value consuming old one

help
1

Hello,

I have some value of type A that cannot be cloned, and I have a function f(A) -> B that consumes said value to create a new value B. The A value is created during runtime and is not needed for anything else than creating the B value. But, I have multiple threads that need to see the B value, and the B value needs to be calculated lazily when it is first needed. More precisely, I will be having an Arc<B> that can be cloned to share the B value between threads.

How can I do this?

It seems like a use case for LazyLock or OnceLock, but I could not make it work.

I think I know a solution using RwLock (or Mutex, but I think RwLock would be more efficient) and some contained containing an enum, but I'm wondering if there is an easier solution.

Thanks!

2

Could you describe your struggles with LazyLock in more detail? Because the closure you pass to LazyLock::new can capture your A instance. You could then share the LazyLock with Arc between your threads. Example.

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3

Interesting. I ran into an error message that told me that the closure cannot capture, but from your example, it looks like it can.

4

Be aware of the second generic parameter F on LazyLock. It defaults to fn() -> T, which can't capture. So if you don't let type inference figure out the type for F (i.e. if you were to let ll: LazyLock<B> = ...), then you get the error that function pointers can't capture their environment.

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5

Yes, that was exactly my problem! Thanks for the explanation!

6

So what do I do if my LazyLock is part of a struct and I need to specify its type?

7 
struct C<F>(Arc<LazyLock<B,F>>);

something like this?

8

You could propagate F as a generic parameter to the wrapping struct or use dynamic dispatch, i.e. with Box<dyn FnOnce() -> B>.

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9

Awesome, thanks!

1 Like