Module polkadot_sdk_docs::reference_docs::frame_tokens

Learn about the token-related logic in FRAME and how to apply it to your use case.

FRAME Tokens

This reference doc serves as a high-level overview of the token-related logic in FRAME, and how to properly apply it to your use case.

On completion of reading this doc, you should have a good understanding of:

• The distinction between token traits and trait implementations in FRAME, and why this distinction is helpful.
• Token-related traits available in FRAME.
• Token-related trait implementations in FRAME.
• How to choose the right trait or trait implementation for your use case.
• Where to go next.

Getting Started

The most ubiquitous way to add a token to a FRAME runtime is pallet_balances. Read more about pallets here.

You may then write custom pallets that interact with pallet_balances. The fastest way to get started with that is by tightly coupling your custom pallet to pallet_balances.

However, to keep pallets flexible and modular, it is often preferred to loosely couple.

To achieve loose coupling, we separate token logic into traits and trait implementations.

Traits and Trait Implementations

Broadly speaking, token logic in FRAME can be divided into two categories: traits and trait implementations.

Traits define common interfaces that types of tokens should implement. For example, the fungible::Inspect trait specifies an interface for inspecting token state such as the total issuance of the token, the balance of individual accounts, etc.

Trait implementations are concrete implementations of these traits. For example, one of the many traits pallet_balances implements is fungible::Inspect¹. It provides the concrete way of inspecting the total issuance, balance of accounts, etc. There can be many implementations of the same traits.

The distinction between traits and trait implementations is helpful because it allows pallets and other logic to be generic over their dependencies, avoiding tight coupling.

To illustrate this with an example let’s consider pallet_preimage. This pallet takes a deposit in exchange for storing a preimage for later use. A naive implementation of the pallet may use pallet_balances in a tightly coupled manner, directly calling methods on the pallet to reserve and unreserve deposits. This approach works well, until someone has a use case requiring that an asset from a different pallet such as pallet_assets is used for the deposit. Rather than tightly coupling pallet_preimage to pallet_balances, pallet_assets, and every other token-handling pallet, a user could possibly specify that pallet_preimage does not specify a concrete pallet as a dependency, but instead accepts any dependency which implements the currency::ReservableCurrency trait, namely via its Config::Currency associated type. This allows pallet_preimage to support any arbitrary pallet implementing this trait, without needing any knowledge of what those pallets may be or requiring changes to support new pallets which may be written in the future.

Read more about coupling, and the benefits of loose coupling here.

Fungible Token Traits in FRAME

The fungible crate contains the latest set of FRAME fungible token traits, and is recommended to use for all new logic requiring a fungible token. See the crate documentation for more info about these fungible traits.

fungibles provides very similar functionality to fungible, except it supports managing multiple tokens.

You may notice the trait Currency with similar functionality is also used in the codebase, however this trait is deprecated and existing logic is in the process of being migrated to fungible (tracking issue).

Fungible Token Trait Implementations in FRAME

pallet_balances implements fungible, and is the most commonly used fungible implementation in FRAME. Most of the time, it’s used for managing the native token of the blockchain network it’s used in.

pallet_assets implements fungibles, and is another popular fungible token implementation. It supports the creation and management of multiple assets in a single crate, making it a good choice when a network requires more assets in addition to its native token.

Non-Fungible Tokens in FRAME

pallet_nfts is recommended to use for all NFT use cases in FRAME. See the crate documentation for more info about this pallet.

pallet_uniques is deprecated and should not be used.

What Next?

• If you are interested in implementing a single fungible token, continue reading the fungible and pallet_balances docs.
• If you are interested in implementing a set of fungible tokens, continue reading the fungibles trait and pallet_assets docs.
• If you are interested in implementing an NFT, continue reading the pallet_nfts docs.

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1. Rust Advanced Tip: The knowledge that pallet_balances implements fungible::Inspect is not some arcane knowledge that you have to know by heart or memorize. One can simply look at the list of the implementors of any trait in the Rust Doc to find all implementors (e.g. Mutate trait implementors), or use the rust-analyzer’s Implementations action. ↩