Rust by Example

fmt::Debug hardly looks compact and clean, so it is often advantageous to customize the output appearance. This is done by manually implementing fmt::Display, which uses the {} print marker. Implementing it looks like this:

#![allow(unused)]

fn main() {

// Import (via `use`) the `fmt` module to make it available.

use std::fmt;

// Define a structure for which `fmt::Display` will be implemented. This is

// a tuple struct named `Structure` that contains an `i32`.

struct Structure(i32);

// To use the `{}` marker, the trait `fmt::Display` must be implemented

// manually for the type.

impl fmt::Display for Structure {

// This trait requires `fmt` with this exact signature.

fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {

// Write strictly the first element into the supplied output

// stream: `f`. Returns `fmt::Result` which indicates whether the

// operation succeeded or failed. Note that `write!` uses syntax which

// is very similar to `println!`.

write!(f, "{}", self.0)

}

}

}

fmt::Display may be cleaner than fmt::Debug but this presents a problem for the std library. How should ambiguous types be displayed? For example, if the std library implemented a single style for all Vec, what style should it be? Would it be either of these two?

• Vec: /:/etc:/home/username:/bin (split on :)

• Vec: 1,2,3 (split on ,)

No, because there is no ideal style for all types and the std library doesn't presume to dictate one. fmt::Display is not implemented for Vec or for any other generic containers. fmt::Debug must then be used for these generic cases.

This is not a problem though because for any new container type which is not generic,fmt::Display can be implemented.

use std::fmt; // Import `fmt`

// A structure holding two numbers. `Debug` will be derived so the results can

// be contrasted with `Display`.

#[derive(Debug)]

struct MinMax(i64, i64);

// Implement `Display` for `MinMax`.

impl fmt::Display for MinMax {

fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {

// Use `self.number` to refer to each positional data point.

write!(f, "({}, {})", self.0, self.1)

}

}

// Define a structure where the fields are nameable for comparison.

#[derive(Debug)]

struct Point2D {

x: f64,

y: f64,

}

// Similarly, implement `Display` for `Point2D`

impl fmt::Display for Point2D {

fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {

// Customize so only `x` and `y` are denoted.

write!(f, "x: {}, y: {}", self.x, self.y)

}

}

fn main() {

let minmax = MinMax(0, 14);

println!("Compare structures:");

println!("Display: {}", minmax);

println!("Debug: {:?}", minmax);

let big_range =   MinMax(-300, 300);

let small_range = MinMax(-3, 3);

println!("The big range is {big} and the small is {small}",

small = small_range,

big = big_range);

let point = Point2D { x: 3.3, y: 7.2 };

println!("Compare points:");

println!("Display: {}", point);

println!("Debug: {:?}", point);

// Error. Both `Debug` and `Display` were implemented, but `{:b}`

// requires `fmt::Binary` to be implemented. This will not work.

// println!("What does Point2D look like in binary: {:b}?", point);

}

הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה

XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

So, fmt::Display has been implemented but fmt::Binary has not, and therefore cannot be used. std::fmt has many such traits and each requires its own implementation. This is detailed further in std::fmt.