How to initialize Lazy with a closure?

[arucil]

I have the following code

fn main() {
  use once_cell::unsync::Lazy;
  let i = 1;
  let x: Lazy<i32> = Lazy::new(|| i);
}

Rustc reports that a fn pointer was expected, but a closure is found. I guess it is because the type parameter T of Lazy is defaulted to fn() -> T. But I don't know any way to explicitly specify the closure type parameter. So how can I make this code working?

[matklad]

Use Lazy<i32, _>, or omit the type altogether.

On Saturday, 18 January 2020, plodsoft notifications@github.com wrote:

I have the following code

fn main() { use once_cell::unsync::Lazy; let i = 1; let x: Lazy = Lazy::new(|| i); }

Rustc reports that a fn pointer was expected, but a closure is found. I guess it is because the type parameter T of Lazy is defaulted to fn() -> T. But I don't know any way to explicitly specify the closure type parameter. So how can I make this code working?

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[arucil]

Use Lazy<i32, _>, or omit the type altogether. … On Saturday, 18 January 2020, plodsoft @.***> wrote: I have the following code fn main() { use once_cell::unsync::Lazy; let i = 1; let x: Lazy = Lazy::new(|| i); } Rustc reports that a fn pointer was expected, but a closure is found. I guess it is because the type parameter T of Lazy is defaulted to fn() -> T. But I don't know any way to explicitly specify the closure type parameter. So how can I make this code working? — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#90?email_source=notifications&email_token=AANB3M3DU76HYDEDP3NPTDDQ6KYHRA5CNFSM4KIRVHB2YY3PNVWWK3TUL52HS4DFUVEXG43VMWVGG33NNVSW45C7NFSM4IHC2OMQ>, or unsubscribe https://github.com/notifications/unsubscribe-auth/AANB3M4JT3WU6KXKAG3X6S3Q6KYHRANCNFSM4KIRVHBQ .

Lazy<i32, _> works for me, thank you!

[pemistahl]

The trick using _ does not work with global and function type definitions. Why is it prohibited to use closures most of the time? Are there technical reasons? This restriction makes this library useless to me, unfortunately. Languages such as Kotlin allow that.

[matklad]

See https://github.com/matklad/rfcs/blob/std-lazy/text/0000-standard-lazy-types.md#default-type-parameter-on-lazy.

The reason is that:

• Rust doesn't allow type inference in global context
• Closures have non-namable types

Using OnceCell directly should always work, as it doesn't require naming the type, see this example:

https://github.com/matklad/once_cell/blob/0b65b1523453a0cfbdeeb756a77e0b85e03ea7bf/examples/lazy_static.rs#L14-L24

[pemistahl]

Thanks for the quick response @matklad. I'll make another attempt using OnceCell then.