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Explain usage of
<T: Config> in FRAME storage + Update parachain pa…
…llet template (paritytech#4941) Explains one of the annoying parts of FRAME storage that we have seen multiple times in PBA everyone gets stuck on. I have not updated the other two templates for now, and only reflected it in the parachain template. That can happen in a follow-up. - [x] Update possible answers in SE about the same topic. --------- Co-authored-by: Serban Iorga <[email protected]> Co-authored-by: command-bot <>
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,199 @@ | ||
| //! <section class="info"> | ||
| //! In all examples, a few lines of boilerplate have been hidden from each snippet for conciseness. | ||
| //! </section> | ||
| //! | ||
| //! Let's begin by starting to store a `NewType` in a storage item: | ||
| //! | ||
| //! ```compile_fail | ||
| //! #[frame::pallet] | ||
| //! pub mod pallet { | ||
| //! # use frame::prelude::*; | ||
| //! # #[pallet::config] | ||
| //! # pub trait Config: frame_system::Config {} | ||
| //! # #[pallet::pallet] | ||
| //! # pub struct Pallet<T>(_); | ||
| //! pub struct NewType(u32); | ||
| // | ||
| //! #[pallet::storage] | ||
| //! pub type Something<T> = StorageValue<_, NewType>; | ||
| //! } | ||
| //! ``` | ||
| //! | ||
| //! This raises a number of compiler errors, like: | ||
| //! ```text | ||
| //! the trait `MaxEncodedLen` is not implemented for `NewType`, which is required by | ||
| //! `frame::prelude::StorageValue<_GeneratedPrefixForStorageSomething<T>, NewType>: | ||
| //! StorageInfoTrait` | ||
| //! ``` | ||
| //! | ||
| //! This implies the following set of traits that need to be derived for a type to be stored in | ||
| //! `frame` storage: | ||
| //! ```rust | ||
| //! #[frame::pallet] | ||
| //! pub mod pallet { | ||
| //! # use frame::prelude::*; | ||
| //! # #[pallet::config] | ||
| //! # pub trait Config: frame_system::Config {} | ||
| //! # #[pallet::pallet] | ||
| //! # pub struct Pallet<T>(_); | ||
| //! #[derive(codec::Encode, codec::Decode, codec::MaxEncodedLen, scale_info::TypeInfo)] | ||
| //! pub struct NewType(u32); | ||
| //! | ||
| //! #[pallet::storage] | ||
| //! pub type Something<T> = StorageValue<_, NewType>; | ||
| //! } | ||
| //! ``` | ||
| //! | ||
| //! Next, let's look at how this will differ if we are to store a type that is derived from `T` in | ||
| //! storage, such as [`frame::prelude::BlockNumberFor`]: | ||
| //! ```compile_fail | ||
| //! #[frame::pallet] | ||
| //! pub mod pallet { | ||
| //! # use frame::prelude::*; | ||
| //! # #[pallet::config] | ||
| //! # pub trait Config: frame_system::Config {} | ||
| //! # #[pallet::pallet] | ||
| //! # pub struct Pallet<T>(_); | ||
| //! #[derive(codec::Encode, codec::Decode, codec::MaxEncodedLen, scale_info::TypeInfo)] | ||
| //! pub struct NewType<T: Config>(BlockNumberFor<T>); | ||
| //! | ||
| //! #[pallet::storage] | ||
| //! pub type Something<T: Config> = StorageValue<_, NewType<T>>; | ||
| //! } | ||
| //! ``` | ||
| //! | ||
| //! Surprisingly, this will also raise a number of errors, like: | ||
| //! ```text | ||
| //! the trait `TypeInfo` is not implemented for `T`, which is required | ||
| //! by`frame_support::pallet_prelude::StorageValue<pallet_2::_GeneratedPrefixForStorageSomething<T>, | ||
| //! pallet_2::NewType<T>>:StorageEntryMetadataBuilder | ||
| //! ``` | ||
| //! | ||
| //! Why is that? The underlying reason is that the `TypeInfo` `derive` macro will only work for | ||
| //! `NewType` if all of `NewType`'s generics also implement `TypeInfo`. This is not the case for `T` | ||
| //! in the example above. | ||
| //! | ||
| //! If you expand an instance of the derive, you will find something along the lines of: | ||
| //! `impl<T> TypeInfo for NewType<T> where T: TypeInfo { ... }`. This is the reason why the | ||
| //! `TypeInfo` trait is required for `T`. | ||
| //! | ||
| //! To fix this, we need to add a `#[scale_info(skip_type_params(T))]` | ||
| //! attribute to `NewType`. This additional macro will instruct the `derive` to skip the bound on | ||
| //! `T`. | ||
| //! ```rust | ||
| //! #[frame::pallet] | ||
| //! pub mod pallet { | ||
| //! # use frame::prelude::*; | ||
| //! # #[pallet::config] | ||
| //! # pub trait Config: frame_system::Config {} | ||
| //! # #[pallet::pallet] | ||
| //! # pub struct Pallet<T>(_); | ||
| //! #[derive(codec::Encode, codec::Decode, codec::MaxEncodedLen, scale_info::TypeInfo)] | ||
| //! #[scale_info(skip_type_params(T))] | ||
| //! pub struct NewType<T: Config>(BlockNumberFor<T>); | ||
| //! | ||
| //! #[pallet::storage] | ||
| //! pub type Something<T: Config> = StorageValue<_, NewType<T>>; | ||
| //! } | ||
| //! ``` | ||
| //! | ||
| //! Next, let's say we wish to store `NewType` as [`frame::prelude::ValueQuery`], which means it | ||
| //! must also implement `Default`. This should be as simple as adding `derive(Default)` to it, | ||
| //! right? | ||
| //! ```compile_fail | ||
| //! #[frame::pallet] | ||
| //! pub mod pallet { | ||
| //! # use frame::prelude::*; | ||
| //! # #[pallet::config] | ||
| //! # pub trait Config: frame_system::Config {} | ||
| //! # #[pallet::pallet] | ||
| //! # pub struct Pallet<T>(_); | ||
| //! #[derive(codec::Encode, codec::Decode, codec::MaxEncodedLen, scale_info::TypeInfo, Default)] | ||
| //! #[scale_info(skip_type_params(T))] | ||
| //! pub struct NewType<T: Config>(BlockNumberFor<T>); | ||
| //! | ||
| //! #[pallet::storage] | ||
| //! pub type Something<T: Config> = StorageValue<_, NewType<T>, ValueQuery>; | ||
| //! } | ||
| //! ``` | ||
| //! | ||
| //! Under the hood, the expansion of the `derive(Default)` will suffer from the same restriction as | ||
| //! before: it will only work if `T: Default`, and `T` is not `Default`. Note that this is an | ||
| //! expected issue: `T` is merely a wrapper of many other types, such as `BlockNumberFor<T>`. | ||
| //! `BlockNumberFor<T>` should indeed implement `Default`, but `T` implementing `Default` is rather | ||
| //! meaningless. | ||
| //! | ||
| //! To fix this, frame provides a set of macros that are analogous to normal rust derive macros, but | ||
| //! work nicely on top of structs that are generic over `T: Config`. These macros are: | ||
| //! | ||
| //! - [`frame::prelude::DefaultNoBound`] | ||
| //! - [`frame::prelude::DebugNoBound`] | ||
| //! - [`frame::prelude::PartialEqNoBound`] | ||
| //! - [`frame::prelude::EqNoBound`] | ||
| //! - [`frame::prelude::CloneNoBound`] | ||
| //! - [`frame::prelude::PartialOrdNoBound`] | ||
| //! - [`frame::prelude::OrdNoBound`] | ||
| //! | ||
| //! The above traits are almost certainly needed for your tests: To print your type, assert equality | ||
| //! or clone it. | ||
| //! | ||
| //! We can fix the following example by using [`frame::prelude::DefaultNoBound`]. | ||
| //! ```rust | ||
| //! #[frame::pallet] | ||
| //! pub mod pallet { | ||
| //! # use frame::prelude::*; | ||
| //! # #[pallet::config] | ||
| //! # pub trait Config: frame_system::Config {} | ||
| //! # #[pallet::pallet] | ||
| //! # pub struct Pallet<T>(_); | ||
| //! #[derive( | ||
| //! codec::Encode, | ||
| //! codec::Decode, | ||
| //! codec::MaxEncodedLen, | ||
| //! scale_info::TypeInfo, | ||
| //! DefaultNoBound | ||
| //! )] | ||
| //! #[scale_info(skip_type_params(T))] | ||
| //! pub struct NewType<T:Config>(BlockNumberFor<T>); | ||
| //! | ||
| //! #[pallet::storage] | ||
| //! pub type Something<T: Config> = StorageValue<_, NewType<T>, ValueQuery>; | ||
| //! } | ||
| //! ``` | ||
| //! | ||
| //! Finally, if a custom type that is provided through `Config` is to be stored in the storage, it | ||
| //! is subject to the same trait requirements. The following does not work: | ||
| //! ```compile_fail | ||
| //! #[frame::pallet] | ||
| //! pub mod pallet { | ||
| //! use frame::prelude::*; | ||
| //! #[pallet::config] | ||
| //! pub trait Config: frame_system::Config { | ||
| //! type CustomType; | ||
| //! } | ||
| //! #[pallet::pallet] | ||
| //! pub struct Pallet<T>(_); | ||
| //! #[pallet::storage] | ||
| //! pub type Something<T: Config> = StorageValue<_, T::CustomType>; | ||
| //! } | ||
| //! ``` | ||
| //! | ||
| //! But adding the right trait bounds will fix it. | ||
| //! ```rust | ||
| //! #[frame::pallet] | ||
| //! pub mod pallet { | ||
| //! use frame::prelude::*; | ||
| //! #[pallet::config] | ||
| //! pub trait Config: frame_system::Config { | ||
| //! type CustomType: codec::FullCodec | ||
| //! + codec::MaxEncodedLen | ||
| //! + scale_info::TypeInfo | ||
| //! + Debug | ||
| //! + Default; | ||
| //! } | ||
| //! #[pallet::pallet] | ||
| //! pub struct Pallet<T>(_); | ||
| //! #[pallet::storage] | ||
| //! pub type Something<T: Config> = StorageValue<_, T::CustomType>; | ||
| //! } | ||
| //! ``` |
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