Macro frame_support::parameter_types

macro_rules! parameter_types {
    (
		$( #[ $attr:meta ] )*
		$vis:vis const $name:ident: $type:ty = $value:expr;
		$( $rest:tt )*
	) => { ... };
    (
		$( #[ $attr:meta ] )*
		$vis:vis $name:ident: $type:ty = $value:expr;
		$( $rest:tt )*
	) => { ... };
    (
		$( #[ $attr:meta ] )*
		$vis:vis storage $name:ident: $type:ty = $value:expr;
		$( $rest:tt )*
	) => { ... };
    () => { ... };
    (IMPL_CONST $name:ident, $type:ty, $value:expr) => { ... };
    (IMPL $name:ident, $type:ty, $value:expr) => { ... };
    (IMPL_STORAGE $name:ident, $type:ty, $value:expr) => { ... };
}

Create new implementations of the Get trait.

The so-called parameter type can be created in three different ways:

• Using const to create a parameter type that provides a const getter. It is required that the value is const.

• Declare the parameter type without const to have more freedom when creating the value.

• Using storage to create a storage parameter type. This type is special as it tries to load the value from the storage under a fixed key. If the value could not be found in the storage, the given default value will be returned. It is required that the value implements Encode and Decode. The key for looking up the value in the storage is built using the following formular:

  twox_128(":" ++ NAME ++ ":") where NAME is the name that is passed as type name.

Examples

// This function cannot be used in a const context.
fn non_const_expression() -> u64 { 99 }

const FIXED_VALUE: u64 = 10;
parameter_types! {
   pub const Argument: u64 = 42 + FIXED_VALUE;
   /// Visibility of the type is optional
   OtherArgument: u64 = non_const_expression();
   pub storage StorageArgument: u64 = 5;
}

trait Config {
   type Parameter: Get<u64>;
   type OtherParameter: Get<u64>;
   type StorageParameter: Get<u64>;
}

struct Runtime;
impl Config for Runtime {
   type Parameter = Argument;
   type OtherParameter = OtherArgument;
   type StorageParameter = StorageArgument;
}

Invalid example:

// This function cannot be used in a const context.
fn non_const_expression() -> u64 { 99 }

parameter_types! {
   pub const Argument: u64 = non_const_expression();
}