Loosely Coupling a Pallet

Goal

Learn how to use a function from another pallet.

Use Cases

Reuse a specific type from another pallet.

Overview

Loose coupling is a technique that enables re-using logic from another pallet inside a pallet. In this guide we show the simple pattern of using a type from an outside pallet in our working pallet, by using trait bounds in our pallet's configuration trait. We will loosely couple a pallet to make use of the Currency trait from frame_support.

Steps

1. Configure your workspace

In the Cargo.toml file of the pallet in your working directory, make sure you specify the pallet you want to couple to accordingly:

[dependencies]
frame-support = { default-features = false, git = 'https://github.com/paritytech/substrate.git', tag = 'monthly-2021-08', version = '4.0.0-dev' }

# -- snip 

[features]
default = ['std']
std = [
    'frame-support/std',
# -- snip 
]

2. Import the trait

We want to use the Currency trait so that we can give our pallet access to the its methods.

Import the trait in your pallet:

use frame_support::traits::Currency;

3. Create a type for your pallet's `Config` trait

In your configuration trait, create a type that is bound by the type you want to expose to your pallet
(in this-pallet/src/lib.rs):

pub trait Config: frame_system::Config {
    // --snip--

    /// A type that is accessing our loosely coupled pallet `my-pallet`
    type LocalCurrency: Currency<Self::AccountId>;
}

4. Use the type

Use the method that the type of your loosely coupled pallet provides (in this-pallet/src/lib.rs):

// Use the getter from `my-pallet`
let total_balance = T::LocalCurrency::total_issuance();

In the above snippet, we're using total_issuance that the Currency trait exposes from frame_support.

5. Provide the implementation in runtime configuration

In our runtime configuration, usually runtime/src/lib.rs, we specify the LocalCurrency to be Balances, which is defined inside construct_runtime! macro and has a type of pallet_balances that implements the Currency trait.

impl my_pallet::Config for Runtime {
    type LocalCurrency = Balances;
}

construct_runtime! (
  pub enum Runtime where
    Block = Block,
    NodeBlock = opaque::Block,
    UncheckedExtrinsic = UncheckedExtrinsic
  {
    Balances: pallet_balances::{Pallet, Call, Storage, Config<T>, Event<T>},
  }
)

Examples

try_origin from the EnsureOrigin trait in FRAME's Democracy pallet
the use of WeightInfo in all FRAME pallets, such as the the Identity pallet and its use of its specific weighting methods
the KeyOwnerProofSystem trait used in pallet_granpa

Resources

Pallet Coupling
A how-to guide on Tightly Coupling two pallets