a.rs

// https://play.rust-lang.org/?gist=cce473b90dd798d61725d50295026f33&version=stable&backtrace=0
fn main() {
    let vec = vec![2, 3, 4, 5];
    // let res: Vec<Result<i32, String>> = vec.iter()
    let res: Result<Vec<i32>, String> = vec.iter()
        .map(|a| if a % 2 == 0 {
            println!("a:{}", a);
            Ok(a * 3)
        } else {
            Err(format!("Error! {} is odd value!", a))
        })
        .collect();
    println!("{:?}", res);
}

b.rs

// https://is.gd/3bSQ38
fn incr(a:i32) -> i32 { a+1 }
fn apply<F>(t: i32, f: F) -> i32 where F: FnOnce(i32) -> i32 {
    f(t)
}
fn main() {
    // map can take both closure and function pointer 
    // fn map<U, F>(self, f: F) -> Option<U> where F: FnOnce(T) -> U
    let f = |a:i32| a+1;
    assert_eq!(Some(2), Some(1).map(f));
    assert_eq!(Some(2), Some(1).map(incr));
    
    let f = |a:i32| a+1;
    assert_eq!(51, apply(50, f));
    apply(50, incr);
}