[英]“Ambiguous associated type” when returning a generic lambda
我有這個代碼(設計和純實驗)
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
}
它返回一個函數/ lambda,它接受兩個操作數並返回一個結果(在這種情況下是加法,減法,除法和乘法運算符)
可以像這樣調用:
let add: Option<Box<Fn(i32, i32) -> i32>> = math_op("+");
println!("Add {}", add.unwrap()(10, 2));
我真的想制作一個通用版本,到目前為止我...
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
}
但是當我構建時,我得到了這些錯誤:
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`
我理解這是因為編譯器無法從我實現的各種特征中確定T :: Output的類型。 是否有其他方式來編寫它以使其工作?
您需要Add
, Sub
, Mul
和Div
的輸出類型相同。 您可以通過添加另一個類型參數並將每個特征的Output
約束為此類型參數來強制執行此操作。
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
}
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