tuple-like_struct_impl_FnOnce.rs

use std::io;

#[derive(Debug)]
/// tuple-like struct
struct Foo(i32);

fn num(n: i32) -> io::Result<i32> {
    Ok(n)
}

/// tuple-like struct `Foo` implements `FnOnce(i32) -> Self`
fn f<F: FnOnce(i32) -> Foo> (g: F, n: i32) -> Foo {
    g(n)
}

fn main() {
    println!("{:?}", num(10).map(Foo));
    // => Ok(Foo(10))
    println!("{:?}", f(Foo, 20));
    // => Foo(20)
}