rust-by-practice/en/src/formatted-output/debug-display.md

# Debug and Display
All types which want to be printable must implement the `std::fmt` formatting trait: `std::fmt::Debug` or `std::fmt::Display`. 

Automatic implementations are only provided for types such as in the `std` library. All others have to be manually implemented.

## Debug
The implementation of `Debug` is very straightfoward: All types can `derive` the `std::fmt::Debug` implementation. This is not true for `std::fmt::Display` which must be manually implemented.

`{:?}` must be used to print out the type which has implemented the `Debug` trait.

```rust
// This structure cannot be printed either with `fmt::Display` or
// with `fmt::Debug`.
struct UnPrintable(i32);

// To make this struct printable with `fmt::Debug`, we can derive the automatic implementations provided by Rust
#[derive(Debug)]
struct DebugPrintable(i32);
```

1. 🌟
```rust,editable

/* Fill in the blanks and Fix the errors */
struct Structure(i32);

fn main() {
    // Types in std and Rust have implemented the fmt::Debug trait
    println!("__ months in a year.", 12);

    println!("Now __ will print!", Structure(3));
}
```

2. 🌟🌟 So `fmt::Debug` definitely makes one type printable, but sacrifices some elegance. Maybe we can get more elegant by replacing `{:?}` with something else( but not `{}` !) 
```rust,editable
#[derive(Debug)]
struct Person {
    name: String,
    age: u8
}

fn main() {
    let person = Person { name:  "Sunface".to_string(), age: 18 };

    /* Make it output: 
    Person {
        name: "Sunface",
        age: 18,
    }
    */
    println!("{:?}", person);
}
```

3. 🌟🌟 We can also manually implement `Debug` trait for our types
```rust,editable

#[derive(Debug)]
struct Structure(i32);

#[derive(Debug)]
struct Deep(Structure);


fn main() {    
    // The problem with `derive` is there is no control over how
    // the results look. What if I want this to just show a `7`?

    /* Make it print: Now 7 will print! */
    println!("Now {:?} will print!", Deep(Structure(7)));
}
```

## Display
Yeah, `Debug` is simple and easy to use. But sometimes we want to customize the output appearance of our type. This is where `Display` really shines.

Unlike `Debug`, there is no way to derive the implementation of the `Display` trait, we have to manually implement it.

Anotherthing to note: the placefolder for `Display` is `{}` not `{:?}`.

4. 🌟🌟
```rust,editable

/* Make it work*/
use std::fmt;

struct Point2D {
    x: f64,
    y: f64,
}

impl fmt::Display for Point2D {
    /* Implement.. */
}

impl fmt::Debug for Point2D {
    /* Implement.. */
}

fn main() {
    let point = Point2D { x: 3.3, y: 7.2 };
    assert_eq!(format!("{}",point), "Display: 3.3 + 7.2i");
    assert_eq!(format!("{:?}",point), "Debug: Complex { real: 3.3, imag: 7.2 }");
    
    println!("Success!")
}
```


### `?` operator

Implementing `fmt::Display` for a structure whose elements must be handled separately is triky. The problem is each `write!` generates a `fmt::Result` which must be handled in the same place.

Fortunately, Rust provides the `?` operator to help us eliminate some unnecessary codes for deaing with `fmt::Result`.

5. 🌟🌟
```rust,editable

/* Make it work */
use std::fmt; 

struct List(Vec<i32>);

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

        write!(f, "[")?;

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

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

fn main() {
    let v = List(vec![1, 2, 3]);
    assert_eq!(format!("{}",v), "[0: 1, 1: 2, 2: 3]");
    println!("Success!")
}
```