Debug

All types which want to use core::fmt formatting traits require an implementation to be printable. Automatic implementations are only provided for types such as in the core library. All others must be manually implemented somehow.

The fmt::Debug trait makes this very straightforward. All types can derive (automatically create) the fmt::Debug implementation. This is not true for fmt::Display which must be manually implemented.

// This structure cannot be printed with `fmt::Display` or
// with `fmt::Debug` by default.
struct UnPrintable {
    value: i32
}

// The `derive` attribute automatically creates the implementation
// required to make this `struct` printable with `fmt::Debug`.
#[derive(Debug)]
struct DebugPrintable {
    value: i32
}

All core library types are automatically printable with {:?} too:

// This structure cannot be printed with `fmt::Display` or
// with `fmt::Debug` by default.
#[derive(Drop)]
struct UnPrintable {
    value: felt252,
}

// The `derive` attribute automatically creates the implementation
// required to make this `struct` printable with `fmt::Debug`.
#[derive(Drop, Debug)]
struct DebugPrintable {
    value: felt252,
}

// Derive the `fmt::Debug` implementation for `Structure`.
#[derive(Drop, Debug)]
struct Structure {
    value: felt252,
}

// Put a `Structure` inside of the structure `Deep`. Make it printable
// also.
#[derive(Drop, Debug)]
struct Deep {
    inner: Structure,
}

fn main() {
    // Printing with `{:?}` is similar to with `{}`.
    println!("{:?} months in a year.", 12);
    let christian: ByteArray = "Christian";
    let slater: ByteArray = "Slater";
    let object: ByteArray = "actor's";
    println!("{1:?} {0:?} is the {2:?} name.", slater, christian, object);

    // `Structure` is printable!
    println!("Now {:?} will print!", Structure { value: 3 });

    // The problem with `derive` is there is no control over how
    // the results look. What if I want this to just show a `7`?
    println!("Now {:?} will print!", Deep { inner: Structure { value: 7 } });
}

So fmt::Debug definitely makes this printable but sacrifices some elegance.

Activities

• Try removing the Debug derive from one of the structs and see what error you get
• Add a new field to the Person struct and see how it appears in the debug output
• Try implementing Display for one of the structs to control its output format

See also:

derive, core::fmt, and struct