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);
}