[英]Converting Rust macro types into expressions
I'd like to assume a given type implements some trait (eg Default
) with a method (eg default()
). 我想假设一个给定的类型用一个方法(例如
default()
)实现一些特征(例如Default
)。 I want to call that method and store its value into a local variable. 我想调用该方法并将其值存储到局部变量中。 Here is a general idea of it:
以下是它的一般概念:
macro_rules! get_default {
( $x:ty = $alias:ident ) => {
let $alias = $x::default();
};
}
fn main() {
// get_default!(i32 = z);
// println!("get_default! {:?} ", z);
println!("i32 default {:?} ", i32::default());
}
Playground link . 游乐场链接 。
When I try that I get an error: 当我尝试时,我收到一个错误:
error: expected expression, found `i32`
--> <anon>:3:22
|>
3 |> let $alias = $x::default();
|> ^^
I understand it's because it expects an expression, but I want to limit input to types only. 我理解这是因为它需要一个表达式,但我想将输入限制为仅仅类型。 Is there a way to turn
$x
from ty
to expr
, or a way to call a method on a type (even if it's potentially missing). 有没有办法将
$x
从ty
为expr
,或者是一种在类型上调用方法的方法(即使它可能缺失)。
You were almost there. 你快到了。 You can hint the expected default type to the compiler and then just use the universal function call syntax :
您可以向编译器提示预期的默认类型,然后只使用通用函数调用语法 :
macro_rules! get_default {
( $x:ty = $alias:ident ) => {
let $alias = <$x as Default>::default();
};
}
fn main() {
get_default!(i32 = z);
println!("get_default! {:?} ", z);
println!("i32 default {:?} ", i32::default());
}
( Playground link ) ( 游乐场链接 )
The key bit is this: 关键是这个:
let $alias = <$x as Default>::default();
This casts $x
to the Default
trait and then invokes the default()
method, as you needed. 这会将
$x
转换为Default
trait,然后根据需要调用default()
方法。
You can also use a shorthand when you don't need to disambiguate between traits: 当您不需要消除特征之间的歧义时,您也可以使用速记:
let $alias = <$x>::default();
( Playground link ) ( 游乐场链接 )
Using UFCS as shown above, you can disambiguate between traits that implement the same methods. 使用如上所示的UFCS,您可以消除实现相同方法的特征之间的歧义。 This is the ' angle-bracket form ' which is useful if the
default()
method is implemented in two traits. 这是' 角括号形式 ',如果
default()
方法在两个特征中实现,这将非常有用。
In this specific scenario, you can also use UFCS more specifically, like so: 在此特定方案中,您还可以更具体地使用UFCS,如下所示:
let $alias: $x = Default::default();
That alone provides enough information for Rust to infer the correct impl
. 仅这一点就为Rust提供了足够的信息来推断出正确的
impl
。
( Playground link ) ( 游乐场链接 )
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