getting "the value of the associated type `Output` (from trait `FnOnce`) must be specified&quot-CodePudding

I have a function that takes 3 arguments namely a, b and a function that takes them and return the value that function produed.

fn from_func<T>(a: i32, b: i32, func: Fn) -> i32 {
    func(a, b)
}

but when im calling it :

fn main() {
    let my_func = |a: i32, b: i32| a   b;
    println!("{:?}", from_func(15, 20, my_func));
}

I'm getting

error[E0191]: the value of the associated type `Output` (from trait `FnOnce`) must be specified
 --> src\main.rs:5:34
  |
5 | fn from_func<T>(a:i32,b:i32,func:Fn)->i32{
  |                                  ^^ help: specify the associated type: `Fn<Output = Type>`

I tried using where keyword and it worked

fn from_func<T>(a: i32, b: i32, func: T) -> i32
where
    T: Fn(i32, i32) -> i32
{
    func(a,b)
}

but is there any other way of doing this?

CodePudding user response：

but is there any other way of doing this?

You must constrain the type variable to provide the arguments and return type of the function. You can do that with a where clause, which is probably best because it's on another line so makes the signature less cluttered.

But you could do this:

fn from_func<F: Fn(i32, i32) -> i32>(a: i32, b: i32, func: F) -> i32 {
    func(a, b)
}

Or, avoid a named type variable by using an impl trait, like this:

fn from_func(a: i32, b: i32, func: impl Fn(i32, i32) -> i32) -> i32 {
    func(a, b)
}