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:
fn main() {
let a = apply(Some(|i| i + 1));
let b = apply(None::<fn(i64) -> i64>);
println!("a: {}; b: {}", a, b);
}
fn apply(f: Option<impl Fn(i64) -> i64>) -> i64 {
let i = 0_i64;
match f {
Some(f) => f(i),
None => i,
}
}
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<impl Fn(i64) -> i64> 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?
fn main(){
let a = apply(Some(|i| i + 1));
let b = apply(None);
let b2 = apply(None::<dyn Fn(i64) -> i64>);
println!("a: {0}; b: {1}", a, b)
}
fn apply(f: Option<impl Fn(i64) -> i64>) -> i64 {
let i = 0_i64;
match f {
Some(f) => f(i),
None => i,
}
}
compile error:
Compiling playground v0.0.1 (/playground)
error[E0282]: type annotations needed
--> src/main.rs:4:16
|
4 | let b = apply(None);
| ^^^^ cannot infer type of the type parameter `T` declared on the enum `Option`
|
help: consider specifying the generic argument
|
4 | let b = apply(None::<T>);
| +++++
error[E0283]: type annotations needed
--> src/main.rs:4:16
|
4 | let b = apply(None);
| ----- ^^^^ cannot infer type of the type parameter `T` declared on the enum `Option`
| |
| required by a bound introduced by this call
|
= note: multiple `impl`s satisfying `_: Fn<(i64,)>` found in the following crates: `alloc`, `core`:
- impl<A, F> Fn<A> for &F
where A: Tuple, F: Fn<A>, F: ?Sized;
- impl<Args, F, A> Fn<Args> for Box<F, A>
where Args: Tuple, F: Fn<Args>, A: Allocator, F: ?Sized;
note: required by a bound in `apply`
--> src/main.rs:10:25
|
10 | fn apply(f: Option<impl Fn(i64) -> i64>) -> i64 {
| ^^^^^^^^^^^^^^ required by this bound in `apply`
help: consider specifying the generic argument
|
4 | let b = apply(None::<T>);
| +++++
Some errors have detailed explanations: E0282, E0283.
For more information about an error, try `rustc --explain E0282`.
I tried let b2 = apply(None::<dyn Fn(i64) -> i64>); but it still doesn't compile the errors are
Compiling playground v0.0.1 (/playground)
error[E0277]: the size for values of type `dyn Fn(i64) -> i64` cannot be known at compilation time
--> src/main.rs:5:20
|
5 | let b2 = apply(None::<dyn Fn(i64) -> i64>);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
|
= help: the trait `Sized` is not implemented for `dyn Fn(i64) -> i64`
note: required by a bound in `None`
--> /rustc/eb26296b556cef10fb713a38f3d16b9886080f26/library/core/src/option.rs:567:5
For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` (bin "playground") 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:
apply(None::<&dyn Fn(i64) -> i64>);
However, it's probably better to use a function pointer type instead:
apply(None::<fn(i64) -> i64>);
英文:
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:
apply(None::<&dyn Fn(i64) -> i64>);
However, it's probably better to use a function pointer type instead:
apply(None::<fn(i64) -> i64>);
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