How's the `pallet::extra_constants` different from `pallet::constant`. How to choose?

Question

I'm trying to understand the #[pallet::extra_constants] and how it's different from #[pallet::constant].

And appropriate scenario for the same.

Answer 1

https://paritytech.github.io/substrate/master/frame_support/attr.pallet.html

// pallet_a.rs
#[pallet::config]
pub trait Config: frame_system::Config {
    #[pallet::constant]
    type Foo: Get<u32>;
}


#[pallet::extra_constants]
impl<T: Config> Pallet<T> {
    pub fn bar() -> u32 {
        0
    }
}

As you can see, the constant can only be defined in the Config trait. You must define a type first and then mark it as constant.

extra_constants can be defined without that limitation. But this constants can not be configured in runtime.

// runtime.rs
// You can not configure the `extra_constants` here.
impl pallet_a::Config for Runtime {
    type Foo = ConstU32<10>;
}