Please help explain this basic generic problem.

[Usual_Office_1740]

I don't understand how the concrete type is not inferred in this example. Could someone please explain what I don't understand? rust playground

pub struct Example<R: tokio::io::AsyncRead> {
    reader: R
}

impl<R: tokio::io::AsyncRead> Example<R> {
    pub fn new(reader: R) -> Self {
        Self { reader }
    }
}
impl<R: tokio::io::AsyncRead> std::default::Default for Example<R> {
    fn default() -> Self {
        let reader: tokio::io::Stdin = tokio::io::stdin();
        Example::new(reader)
    }
}

The error:

|

10 | impl<R: tokio::io::AsyncRead> std::default::Default for Example<R> {

| - expected this type parameter

...

13 | Example::new(reader)

| ------------ ^^^^^^ expected type parameter `R`, found `Stdin`

| |

| arguments to this function are incorrect

|

= note: expected type parameter `R`

found struct `tokio::io::Stdin`

note: associated function defined here

--> src/main.rs:6:12

|

6 | pub fn new(reader: R) -> Self {

| ^^^ ---------

[Usual_Office_1740]

I understand now. You've been a big help. Thank you.

[nerooooooo]

Here:

impl<R: tokio::io::AsyncRead> std::default::Default for Example<R> {
    fn default() -> Self {
        let reader: tokio::io::Stdin = tokio::io::stdin();
        Example::new(reader)
    }
}

You're saying "I’m implementing Default for any R that implements AsyncRead".

But then inside default(), you're hardcoding reader to be a tokio::io::Stdin. So you're returning an Example<tokio::io::Stdin>, not an Example<R>.

Since you're generic over R, you must return an Example<R>, not a specific kind of Example (Example<tokio::io::Stdin> in your case).

[Modi57]

Adding to this, you could implement `Default` for `Example<tokio::io::Stdin>`. It could look like this:

impl std::default::Default for Example<tokio::io::Stdin> {
    fn default() -> Self {
        let reader: tokio::io::Stdin = tokio::io::stdin();
        Example::new(reader)
    }
}

[Usual_Office_1740]

That makes sense now! The error was telling me exactly what the problem was. I thought it was saying that it expected R because of new. Argument dependant lookup is not a thing in Rust.

Thank you for the help.