How do I define trait bounds on an associated type?

Question

I want to write a function that accepts Iterator of type that has ToString trait.

What I have in mind:

fn parse<T: Iterator /* ?T::Item : ToString? */>(mut args: T) -> Result<String, String> {
    match args.next() {
        Some(x) => x.to_string(),
        None => String::from("Missing parameter"),
    }
}

Calgary

Answer 1

Yes, you can do that with a where clause:

fn parse<T: Iterator>(mut args: T) -> Result<String, String>
where 
    <T as Iterator>::Item: ToString,
{
   // ....
}

Or, since it's unambiguous which Item is meant here, the bound can just be:

where T::Item: ToString

Redden

Thanks! How is the <T as Iterator>::Item part called? I was not able to google it, since I was probably missing the correct terminology. – Calgary

That part doesn't have a name by itself (I don't think), but T::Item: ToString is called a bound, or a trait bound. You might also see it called a constraint. – Redden

Answer 2

You can use the Item = syntax:

fn parse<I: ToString, T: Iterator<Item = I>>(mut args: T) -> Result<String, String>

That allows you to simplify this further with the impl syntax:

fn parse<T: Iterator<Item = impl ToString>>(mut args: T) -> Result<String, String>

and finally:

fn parse(mut args: impl Iterator<Item = impl ToString>) -> Result<String, String>

I would consider this a more readable alternative.

Timelag