Rust by Example

1.2.2.1 Testcase: List

Implementing fmt::Display for a structure where the elements must each be handled sequentially is tricky. The problem is that each write! generates a fmt::Result. Proper handling of this requires dealing with all the results. Rust provides the try! macro for exactly this purpose.

Using try! on write! looks like this:

// Try `write!` to see if it errors. If it errors, return
// the error. Otherwise continue.
try!(write!(f, "{}", value));

With try! available, implementing fmt::Display for a Vec is straightforward:

use std::fmt; // Import the `fmt` module.

// Define a structure named `List` containing a `Vec`.
struct List(Vec<i32>);

impl fmt::Display for List {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        // Dereference `self` and create a reference to `vec`
        // via destructuring.
        let List(ref vec) = *self;

        try!(write!(f, "["));

        // Iterate over `vec` in `v` while enumerating the iteration
        // count in `count`.
        for (count, v) in vec.iter().enumerate() {
            // For every element except the first, add a comma
            // before calling `write!`. Use `try!` to return on errors.
            if count != 0 { try!(write!(f, ", ")); }
            try!(write!(f, "{}", v));
        }

        // Close the opened bracket and return a fmt::Result value
        write!(f, "]")
    }
}

fn main() {
    let v = List(vec![1, 2, 3]);
    println!("{}", v);
}

Activity

Try changing the program so that the index of each element in the vector is also printed. The new output should look like this:

[0: 1, 1: 2, 2: 3]

See also

for, ref, Result, struct, try!, and vec!