我有这个代码(人为的和纯粹的实验)
fn math_op(op: &str) -> Option<Box<Fn(i32, i32) -> i32>> {
let ret: Option<Box<Fn(i32, i32) -> i32>> = match op {
"+" => Some(Box::new(|a: i32, b: i32| -> i32 { a + b } )),
"-" => Some(Box::new(|a: i32, b: i32| -> i32 { a - b } )),
"*" => Some(Box::new(|a: i32, b: i32| -> i32 { a * b } )),
"/" => Some(Box::new(|a: i32, b: i32| -> i32 { a / b } )),
_ => None,
};
ret
}
返回一个函数/lambda,该函数接受两个操作数并返回结果(在本例中为加、减、除和乘操作符)
可以这样调用:
let add: Option<Box<Fn(i32, i32) -> i32>> = math_op("+");
println!("Add {}", add.unwrap()(10, 2));
我真的很想做一个通用的版本,到目前为止,我已经…
fn math_op_gen<T>(op: &str) -> Option<Box<Fn(T, T) -> T::Output>>
where T: std::ops::Add + std::ops::Sub + std::ops::Mul + std::ops::Div {
let ret: Option<Box<Fn(T, T) -> T::Output>> = match op {
"+" => Some(Box::new(|a, b| { a + b } )),
"-" => Some(Box::new(|a, b| { a - b } )),
"*" => Some(Box::new(|a, b| { a * b } )),
"/" => Some(Box::new(|a, b| { a / b } )),
_ => None,
};
ret
}
但是当我构建时,我得到这些错误:
error: ambiguous associated type `Output` in bounds of `T` [E0221]
note: associated type `T` could derive from `core::ops::Div`
note: associated type `T` could derive from `core::ops::Mul`
note: associated type `T` could derive from `core::ops::Sub`
note: associated type `T` could derive from `core::ops::Add`
我理解这是因为编译器无法从我实现的各种特征中确定T::Output的类型。有没有另一种写法让它工作?
需要Add
, Sub
, Mul
和Div
的输出类型相同。您可以通过添加另一个类型参数并将每个trait的Output
约束为该类型参数来执行此操作。
fn math_op_gen<T, R>(op: &str) -> Option<Box<Fn(T, T) -> R>>
where T: std::ops::Add<Output=R> +
std::ops::Sub<Output=R> +
std::ops::Mul<Output=R> +
std::ops::Div<Output=R> {
let ret: Option<Box<Fn(T, T) -> R>> = match op {
"+" => Some(Box::new(|a, b| { a + b } )),
"-" => Some(Box::new(|a, b| { a - b } )),
"*" => Some(Box::new(|a, b| { a * b } )),
"/" => Some(Box::new(|a, b| { a / b } )),
_ => None,
};
ret
}