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 ? operator for exactly this purpose.

Using ? on write! looks like this:

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

Alternatively, you can also use the try! macro, which works the same way. This is a bit more verbose and no longer recommended, but you may still see it in older Rust code. Using try! looks like this:

1. try!(write!(f, "{}", value));

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

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

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

6. impl fmt::Display for List {
7. fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
8. // Extract the value using tuple indexing
9. // and create a reference to `vec`.
10. let vec = &self.0;

12. write!(f, "[")?;

14. // Iterate over `vec` in `v` while enumerating the iteration
15. // count in `count`.
16. for (count, v) in vec.iter().enumerate() {
17. // For every element except the first, add a comma.
18. // Use the ? operator, or try!, to return on errors.
19. if count != 0 { write!(f, ", ")?; }
20. write!(f, "{}", v)?;
21. }

23. // Close the opened bracket and return a fmt::Result value
24. write!(f, "]")
25. }
26. }

28. fn main() {
29. let v = List(vec![1, 2, 3]);
30. println!("{}", v);
31. }

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:

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

See also

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