Rust 中的空泛型参数用于 Option。

huangapple
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英文:

Rust empty explicit generic argument for Option<impl Fn(i64)>

问题

The code you provided is in Rust, and it seems like you're encountering type inference issues when trying to call the apply(None) function. Here's a possible solution:

  1. fn main() {
  2.     let a = apply(Some(|i| i + 1));
  4.     let b = apply(None::<fn(i64) -> i64>);
  6.     println!("a: {}; b: {}", a, b);
  7. }
  9. fn apply(f: Option<impl Fn(i64) -> i64>) -> i64 {
  10.     let i = 0_i64;
  11.     match f {
  12.         Some(f) => f(i),
  13.         None => i,
  14.     }
  15. }

In the revised code, I specified the generic type parameter for None as None::<fn(i64) -> i64>, which should help the Rust compiler infer the type correctly.

英文:

I have Option&lt;impl Fn(i64) -&gt; i64&gt; as an argument to a function apply() and I want to pass None value. apply(None). How do I write the apply(None) function call correctly so it compiles?

  1. fn main(){
  2. 	let a = apply(Some(|i| i + 1));
  3. 	
  4. 	let b = apply(None);
  5. 	let b2 = apply(None::&lt;dyn Fn(i64) -&gt; i64&gt;);
  6. 	
  7. 	println!(&quot;a: {0}; b: {1}&quot;, a, b)
  8. }
  10. fn apply(f: Option&lt;impl Fn(i64) -&gt; i64&gt;) -&gt; i64 {
  11. 	let i = 0_i64;
  12. 	match f {
  13. 		Some(f) =&gt; f(i),
  14. 		None =&gt; i,
  15. 	}
  16. }

compile error:

  1. Compiling playground v0.0.1 (/playground)
  2. error[E0282]: type annotations needed
  3.  --&gt; src/main.rs:4:16
  4.   |
  5. 4 |     let b = apply(None);
  6.   |                   ^^^^ cannot infer type of the type parameter `T` declared on the enum `Option`
  7.   |
  8. help: consider specifying the generic argument
  9.   |
  10. 4 |     let b = apply(None::&lt;T&gt;);
  11.   |                       +++++
  13. error[E0283]: type annotations needed
  14.   --&gt; src/main.rs:4:16
  15.    |
  16. 4  |     let b = apply(None);
  17.    |             ----- ^^^^ cannot infer type of the type parameter `T` declared on the enum `Option`
  18.    |             |
  19.    |             required by a bound introduced by this call
  20.    |
  21.    = note: multiple `impl`s satisfying `_: Fn&lt;(i64,)&gt;` found in the following crates: `alloc`, `core`:
  22.            - impl&lt;A, F&gt; Fn&lt;A&gt; for &amp;F
  23.              where A: Tuple, F: Fn&lt;A&gt;, F: ?Sized;
  24.            - impl&lt;Args, F, A&gt; Fn&lt;Args&gt; for Box&lt;F, A&gt;
  25.              where Args: Tuple, F: Fn&lt;Args&gt;, A: Allocator, F: ?Sized;
  26. note: required by a bound in `apply`
  27.   --&gt; src/main.rs:10:25
  28.    |
  29. 10 | fn apply(f: Option&lt;impl Fn(i64) -&gt; i64&gt;) -&gt; i64 {
  30.    |                         ^^^^^^^^^^^^^^ required by this bound in `apply`
  31. help: consider specifying the generic argument
  32.    |
  33. 4  |     let b = apply(None::&lt;T&gt;);
  34.    |                       +++++
  36. Some errors have detailed explanations: E0282, E0283.
  37. For more information about an error, try `rustc --explain E0282`.

I tried let b2 = apply(None::&lt;dyn Fn(i64) -&gt; i64&gt;); but it still doesn't compile the errors are

  1.  Compiling playground v0.0.1 (/playground)
  2. error[E0277]: the size for values of type `dyn Fn(i64) -&gt; i64` cannot be known at compilation time
  3.  --&gt; src/main.rs:5:20
  4.   |
  5. 5 |     let b2 = apply(None::&lt;dyn Fn(i64) -&gt; i64&gt;);
  6.   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn&#39;t have a size known at compile-time
  7.   |
  8.   = help: the trait `Sized` is not implemented for `dyn Fn(i64) -&gt; i64`
  9. note: required by a bound in `None`
  10.  --&gt; /rustc/eb26296b556cef10fb713a38f3d16b9886080f26/library/core/src/option.rs:567:5
  12. For more information about this error, try `rustc --explain E0277`.
  13. error: could not compile `playground` (bin &quot;playground&quot;) due to previous error

答案1

得分: 3

The type dyn Fn(i64) -> i64 is unsized, meaning you can't store it in an enum like Option.

But a trait object reference is sized, so you can use &dyn instead:

  1. apply(None::<&dyn Fn(i64) -> i64>);

However, it's probably better to use a function pointer type instead:

  1. apply(None::<fn(i64) -> i64>);

playground

英文:

The type dyn Fn(i64) -&gt; i64 is unsized, meaning you can't store it in an enum like Option.

But a trait object reference is sized, so you can use &amp;dyn instead:

  1. apply(None::&lt;&amp;dyn Fn(i64) -&gt; i64&gt;);

However, it's probably better to use a function pointer type instead:

  1. apply(None::&lt;fn(i64) -&gt; i64&gt;);

playground

huangapple
  • 本文由 发表于 2023年8月4日 22:06:52
  • 转载请务必保留本文链接：https://go.coder-hub.com/76836692.html
  • closures
  • generics
  • rust
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