Loose coupling solves the problem of coupling to a specific implementation. When loosely coupling to another pallet, you add an associated type to the pallet's configuration trait and ensure the supplied type implements the necessary behavior by specifying a trait bound.

pub trait Trait: system::Trait { // --snip--

/// A type that will supply a set of members to check access control against
type MembershipSource: AccountSet<AccountId = Self::AccountId>;

}

Many pallets throughout the ecosystem are coupled to a token through the Currency trait.

Having this associated type means that the loosely coupled variant of the check-membership pallet can be installed in any runtime that can supply it with a set of accounts to use as an access control list. The code for the AccountSet trait lives in traits/account-set/src/lib.rs directory and is quite short.

pub trait AccountSet { type AccountId;

fn accounts() -> BTreeSet<Self::AccountId>;

}

We also see the loose coupling in the pallet's Cargo.toml file, where account-set is listed.

account-set = { path = '../../traits/account-set', default-features = false }

To actually get the set of members, we use the accounts method supplied by the trait.

// Get the members from the vec-set pallet let members = T::MembershipSource::accounts();