Using Generic Types in Custom Data in a Substrate Pallet

When developing a custom state transition logic with substrate, you’ll often need to create a custom data structure to handle information about the blockchain state or to temporarily store data before it is processed. This guide will walk you through a common problem faced when trying to implement a custom data type that leverages substrate rich type system, as well as an in-depth explanation of relevant concepts.

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Reproducing errors

For the sake of this guide, we’ve created a custom blockchain and built a new pallet. The blockchain serves as an archive of books and has a pallet that allows users to upload a summary of a book and retrieve a book if it is already archived. Currently, the only functionality in this pallet is the archive_book method which ensures all relevant information about a book is stored on the blockchain.

Environment and project setup.

To follow along with this tutorial, ensure that you have the Rust toolchain installed

• Visit substrate official documentation page for the installation processes.
• Clone the project repository.

git clone https://github.com/cenwadike/book-archive-pallet.git

• Navigate into the project’s directory.

cd book-archive-pallet

• Run the command below to compile the node.

cargo build --release

While attempting to compile the node above, you’ll encounter an error similar to the one below:

error[E0412]: cannot find type `AccountId` in this scope
 --> src/lib.rs:72:15
 |
72 |         BookSummary<AccountId, BlockNumber>,
|                     ^^^^^^^^^ not found in this scope

and

error[E0412]: cannot find type `BlockNumber` in this scope
--> src/lib.rs:72:26
 |
72   |         BookSummary<AccountId, BlockNumber>,
 |                                ^^^^^^^^^^^

Solving the error

The pallet has a unique data type (ie, BookSummary ) which was defined like so:

#[derive(Clone, Encode, Decode, Default, Eq, PartialEq, RuntimeDebug, TypeInfo)]
        pub struct BookSummary<AccountId, BlockNumber> {
                title: Vec<u8>,          // title of book
                author: Vec<u8>,         // author of book
                url: Vec<u8>,            // web url to off-chain storage
                archiver: AccountId,     // account id of archiver
                timestamp: BlockNumber,  // time when book was archived
        }

BookSummary has five components:

• title: The title of a book that is archived, stored as a vector of u8.
• author: The author of a book that is archived, stored as a vector of u8.
• url: The web url of a book that is archived, stored as a vector of u8.
• archiver: The user who archived a book, stored as a vector of AccountId.
• timestamp: The block height when a user archived a book, stored as BlockNumber.

BoookSummary uses two FRAME generic types:

1. AccountId: which is used to record who archived a book.
2. BlockNumber: which is used to record the time when a book was archived. The function body does the following:

The error we’re getting above originates from line 72:

#[pallet::storage]
pub(super) type ArchiveStore<T: Config> = StorageMap<
        _,
        Blake2_128Concat,
        T::Hash,
        BookSummary<AccountId, BlockNumber>,
        OptionQuery,
    >;

Rust compiler could not know where the type AccountId and BlockNumber within the scope of the custom data type ArchiveStore. It requires that AccountId and BlockNumber types are brought into the scope of our custom struct.

Because AccountId and BlockNumber are defined by default in our pallet Config trait, we can use them within the scope of our custom data type.

To resolve the error, replace

#[pallet::storage]
pub(super) type ArchiveStore<T: Config> = StorageMap<
        _,
        Blake2_128Concat,
        T::Hash,
        BookSummary<T::AccountId, T::BlockNumber>,
        OptionQuery,
    >;

Take note of T::AccountId and T::BlockNumber which replace AccountId and BlockNumber respectively.

• Re-compile the node

cargo build --release

Going In-depth

Inspecting the FRAME System Config trait

The System pallet defines the core data types used in a Substrate runtime. It also provides several utility functions and types for other pallets including our custom pallet.

The FRAME system Config is a system configuration trait that can be implemented for any pallet by a runtime. This means that a pallet can define a specific configuration that summarises all relevant substrate primitive data types required to fully construct the pallet and interact with its state and functions.

Any runtime that is constructed with a pallet configuration and implements a concrete Config trait of a pallet can utilize all the state and functions of that pallet.

To learn more about the types available in FRAME System Config visit the documentation.

A look at complementary errors you may encounter

Most of the common errors associated with using custom types are related to absence trait implemtation.

1. MaxEncodedLen is not implemented for Vec<u8>

A common error when using Vec<u8> is shown below:

trait `MaxEncodedLen` is not implemented for `Vec<u8>`

This error originates because the Rust compiler does not know the maximum length of the Vec<u8> at compile time.

A simple fix is to ensure that your Pallet struct has this #[pallet::without_storage_info] implemented like so:

#[pallet::pallet]
#[pallet::without_storage_info]
pub struct Pallet<T>(_);

2. trait bound TypeInfo is not satisfied; TypeInfo is not implemented for custom_struct.

Note that TypeInfo could be any other trait that is implemented using a derived macro.

The fix is frequently to add the derived macros to your custom struct like so:

#[derive(Clone, Encode, Decode, Default, Eq, PartialEq, RuntimeDebug, TypeInfo)]
pub struct BookSummary<AccountId, BlockNumber> {
    title: Vec<u8>,          // title of book
    author: Vec<u8>,         // author of book
    url: Vec<u8>,            // web url to off-chain storage
    archiver: AccountId,     // account id of archiver
    timestamp: BlockNumber,  // time when book was archived
}

3. OptionQuery vs ValueQuery vs ResultQuery

• OptionQuery should be your preferred when defining custom data without a default implementation. A query from such data, will either Some value if storage contains a value or None if there's no value in storage.

• ValueQuery always returns a value from storage or causes the program to panic loudly. You can use ValueQuery to return the default value if you have configured a specific default for a storage item or return the value configured with the OnEmpty generic.

• ResultQuery queries a result value from storage and returns an error if there's no value in storage.

Summary

In this guide, you:

• Encountered a compilation error in the template due to not Config type being out of the scope of a custom data type.
• To fix the error, you replaced the problematic line of code with the appropriate line that include the generic type parameter <T>.

Also, you learned that:

• All Substrate pallets must define trait T which defines a Pallet configuration.
• This trait must implement FRAME system Config trait.
• A concrete Pallet trait Config is implemented on a runtime.
• A custom struct must implement all relevant traits.

To learn more about the concepts discussed in this guide, here are some resources that we recommend:

• FRAME System
• Runtime storage structures

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