为数字类型创建类型别名时关联类型不明确

Question

我正在编写一个几何库并正在处理表示长度的值。我已经为表示长度的类型编写了一个类型别名，目前设置为 f64 但将来可能会更改为 f32 或其他可能的数字类型（可能取决于精度和 space 要求):

type Length = f64;

这个库中的所有函数和结构都将按照这种类型编写，例如这个函数：

fn circumference(radius: Length) -> Length {
    Length::consts::PI * radius * radius
}

但是，上述函数会导致以下编译器错误：

   Compiling playground v0.0.1 (/playground)
error[E0223]: ambiguous associated type
 --> src/lib.rs:4:5
  |
4 |     Length::consts::PI * radius * radius
  |     ^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<f64 as Trait>::consts`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0223`.
error: could not compile `playground`

To learn more, run the command again with --verbose.

but doesn't give any elaboration about the Trait I should use — presumably I need to write <Length as Trait>::consts::PI for some appropriately chosen Trait, but it doesn't tell me what this trait should是。

Playground link

应该是什么特质？

Answer 1

您无法访问 std::f64::consts::PI through the type alias Length. That PI is a module constant belonging to the consts submodule of std::f64. It isn't associated with the f64 原始类型。

要访问模块常量，您需要使用其完全限定名称 (::std::f64::consts::PI) 或在其路径中导入其中一个模块并使用部分限定名称。例如，

use std::f64;
fn circumference(radius: Length) -> Length {
    f64::consts::PI * radius * radius
}

// or

use std::f64::consts as foo;
fn circumference(radius: Length) -> Length {
    foo::PI * radius * radius
}

为避免混淆 Length 与其当前定义，您始终可以定义一个局部常量，如

const LENGTH_PI: Length = f64::consts::PI as Length;

这样，以后Length的类型发生变化时，只需替换f64::consts::PI一次即可。

为数字类型创建类型别名时关联类型不明确

Ambiguous associated type when making a type alias for a numeric type

traits

rust

type-alias

associated-types