Immutable Component Config (SRC-107) (#107)

* feat: add Immutable Component Config

* feat: set 107 as the SNIP number

* feat: apply review updates

SNIPS/snip-107.md

@@ -0,0 +1,183 @@
+---
+snip: 107
+title: Immutable Component Config
+authors: Eric Nordelo <[email protected]>
+discussions-to: https://community.starknet.io/t/immutable-component-config/114434
+status: Draft
+type: Standards Track
+category: SRC
+created: 2024-08-15
+---
+
+## Simple Summary
+
+A standard method to configure components by setting constants from the implementing contract, avoiding
+unnecessary storage access if these constants were to be saved in storage.
+
+## Abstract
+
+This standard proposes a mechanism to allow setting components’ configurable constants in the implementing contracts
+without needing to save them in storage. Component providers can leverage this standard to make them configurable while
+keeping the constants hardcoded in the bytecode, removing the extra storage reads required if storage were to be used for
+this matter.
+
+## Rationale
+
+Library and protocol developers often want to make components configurable over a set of parameters to leverage
+flexibility for the final users. In Solidity, immutable variables and/or virtual functions are often used to provide this
+kind of flexibility, since they are hardcoded into the bytecode, not requiring storage reads for accessing the value
+later, even when they can be initialized in construction time. Cairo doesn’t implement these immutable or virtual
+mechanisms, but this SNIP proposes a standard to allow users of the component to set these bytecode constants in the
+implementing contracts.
+
+## Specification
+
+The keywords "**MUST**", "**MUST NOT**", "**REQUIRED**", "**SHALL**", "**SHALL NOT**", "**SHOULD**", "**SHOULD NOT**", "**RECOMMENDED**", "**MAY**", and "**OPTIONAL**" in this document are to be interpreted as described in [RFC 2119](https://www.ietf.org/rfc/rfc2119.txt).
+
+### `ImmutableConfig` trait
+
+Components that use a set of one-time configurable constants, should provide an `ImmutableConfig` trait inside the
+component module, and this trait **MUST** contain only associated constants, and an **OPTIONAL** single function
+named `validate`, which specification is described in the last section.
+
+In the following example, we can see how a component provides the `ImmutableConfig` trait and requires an implementation
+of it to be in scope for the specification of the embeddable functions, accessing the constant value
+`Immutable::FEE_DENOMINATOR` even when it hasn't been defined yet.
+
+```cairo
+#[starknet::component]
+pub mod MyERC2981Component {
+
+    #[storage]
+    struct Storage {
+        (...)
+    }
+
+    /// Constants expected to be defined at the contract level used to configure the component
+    /// behavior.
+    ///
+    /// - `FEE_DENOMINATOR`: The denominator with which to interpret the fee.
+    pub trait ImmutableConfig {
+        const FEE_DENOMINATOR: u256;
+    }
+
+    #[embeddable_as(ERC2981Impl)]
+    impl ERC2981<
+        TContractState,
+        +HasComponent<TContractState>,
+        impl Immutable: ImmutableConfig,
+        +Drop<TContractState>,
+    > of IERC2981<ComponentState<TContractState>> {
+        fn royalty_info(
+            self: @ComponentState<TContractState>, token_id: u256, sale_price: u256
+        ) -> (ContractAddress, u256) {
+            (...)
+
+            let royalty_amount = sale_price
+                * royalty_fraction
+                / Immutable::FEE_DENOMINATOR;
+
+            (receiver, royalty_amount)
+        }
+    }
+
+    (...)
+}
+```
+
+Contracts implementing the component **MUST** provide an implementation of `ImmutableConfig` specifying the values for each configurable constant.
+
+```cairo
+#[starknet::contract]
+pub mod MyContract {
+    use super::MyERC2981Component::ImmutableConfig;
+    use super::MyERC2981Component;
+
+    component!(path: MyERC2981Component, storage: erc2981, event: ERC2981Event);
+
+    #[abi(embed_v0)]
+    impl ERC2981Impl = MyERC2981Component::ERC2981Impl<ContractState>;
+
+    impl ERC2981Config of MyERC2981Component::ImmutableConfig {
+        const DEFAULT_FEE_DENOMINATOR: u256 = 10_000;
+    }
+
+    (...)
+}
+```
+
+### `DefaultConfig` implementation
+
+Sometimes even while we want some constants to be configurable, we want to provide default values for them, that can be
+used if the implementing contract doesn’t need to modify the default values. For this, a default implementation named
+`DefaultConfig` **MAY** be provided, and **MUST** be a sibling of the component itself (direct child of the parent module).
+
+Example:
+
+
+```cairo
+#[starknet::component]
+pub mod MyERC2981Component {
+
+    #[storage]
+    struct Storage {
+        (...)
+    }
+
+    /// Constants expected to be defined at the contract level used to configure the component
+    /// behavior.
+    ///
+    /// - `FEE_DENOMINATOR`: The denominator with which to interpret the fee.
+    pub trait ImmutableConfig {
+        const FEE_DENOMINATOR: u256;
+    }
+
+    (...)
+}
+
+/// Default implementation of MyERC2981Component Immutable Config.
+///
+/// The default fee denominator is set to 10_000.
+pub impl DefaultConfig of MyERC2981Component::ImmutableConfig {
+    const FEE_DENOMINATOR: u256 = 10_000;
+}
+```
+
+Contracts implementing the component **MAY** use the `DefaultConfig` by bringing it into scope:
+
+```cairo
+#[starknet::contract]
+pub mod MyContract {
+    use super::{MyERC2981Component, DefaultConfig};
+
+    component!(path: MyERC2981Component, storage: erc2981, event: ERC2981Event);
+
+    #[abi(embed_v0)]
+    impl ERC2981Impl = MyERC2981Component::ERC2981Impl<ContractState>;
+
+    (...)
+}
+```
+
+Note that we are not defining the implementation, just importing the default one.
+
+### `validate` function
+
+Sometimes, the constants specified in the `ImmutableConfig` should be validated to ensure implementing contracts
+don't set incorrect values that may break the component behavior. To address this, an **OPTIONAL** `validate`
+function **SHOULD** be added to the `ImmutableConfig` trait with the following specification:
+
+1. The `ImmutableConfig` trait **MUST** provide a default implementation for the `validate` function asserting the
+correctness of the set of constants.
+2. The default implementation **MUST NOT** be overridden by the `ImmutableConfig` implementations used in the implementing contract.
+3. The `validate` function **MUST** panic if the values are incorrect and silently pass otherwise.
+4. The `validate` function **MUST** be called in the component's initializer function if there's one.
+5. If there's no initializer function in the component, the function **MUST** be called in the implementing contracts' constructor.
+
+## Security Considerations
+
+This SNIP has no impact at all in terms of security.
+
+## Copyright
+
+Copyright and related rights waived via [MIT](../LICENSE).