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Using other pallets

We successfully constructed a function that registers a user in the previous section. Within that function, we used the T: Config generic to access two noteworthy traits:

  • Currency - i.e., T::Currency
  • Randomness - i.e., T::Randomness

Softly coupling pallets via Config​

Both of these allowed us to check user balances with Currency and a source of randomness with Randomness. Taking a closer look, these are both in our pallet's configuration. The act of including a pallet's functionality within the configuration is called soft coupling:

/// Configure the pallet by specifying the parameters and types it depends on.
#[pallet::config]
pub trait Config: frame_system::Config {
    /// Using the pallet_balances exposed 'Currency' trait to fetch user balance info
    type Currency: ReservableCurrency<Self::AccountId> + LockableCurrency<Self::AccountId, Moment = Self::BlockNumber>;
    /// Randomness!
    type Randomness: Randomness<Self::Hash, Self::BlockNumber>;
}

Each of these traits interfaces with other pallets. ReservableCurrency and LockableCurrency are both traits of the pallet_balances and give us the ability to do things such as lock balance, check balance, and whatever other methods the trait exposes.

Randomness is a more straightforward example and also accomplishes the same concept. Any pallet that implements the Randomness trait can now be used within our pallet's configuration, just as the ReservableCurrency and LockableCurrency traits can also be utilized with any pallet that exposes these as interfaces.

Calling the Balances and Randomness pallet within register​

Because both traits are part of our Config, we can access these types using the generic T. This is possible due to how the Pallet struct must take some generic T which implements Config as a trait:

/// The extrinsics, or dispatchable functions, for this pallet.
#[pallet::call]
impl<T: Config> Pallet<T> { ... }

We can now access any type within our configuration, per T: Config.

Type breakdown: BalanceOf<T>​

At the top of the file, you notice a type alias that uses the Currency trait as well:

type BalanceOf<T> =
    <<T as Config>::Currency as Currency<<T as frame_system::Config>::AccountId>>::Balance;

This type of declaration defines our minimum lockable amount for a user. In short, it correctly states what balance should look like, as per the currency system (in this case, whatever Currency is) in place and used in the rest of the chain.