Introduce the Get trait and type_parameter. (#2565)

substrate/srml/support/src/traits.rs

@@ -22,6 +22,45 @@ use crate::runtime_primitives::traits::{
 	MaybeSerializeDebug, SimpleArithmetic
 };
 
+/// New trait for querying a single fixed value from a type.
+pub trait Get<T> {
+	/// Return a constant value.
+	fn get() -> T;
+}
+
+/// Macro for easily creating a new implementation of the `Get` trait. Use similarly to
+/// how you would declare a `const`:
+///
+/// ```no_compile
+/// parameter_types! {
+///   pub const Argument: u64 = 42;
+/// }
+/// trait Config {
+///   type Parameter: Get<u64>;
+/// }
+/// struct Runtime;
+/// impl Config for Runtime {
+///   type Parameter = Argument;
+/// }
+/// ```
+#[macro_export]
+macro_rules! parameter_types {
+	(pub const $name:ident: $type:ty = $value:expr; $( $rest:tt )*) => (
+		pub struct $name;
+		parameter_types!{IMPL $name , $type , $value}
+		parameter_types!{ $( $rest )* }
+	);
+	(const $name:ident: $type:ty = $value:expr; $( $rest:tt )*) => (
+		struct $name;
+		parameter_types!{IMPL $name , $type , $value}
+		parameter_types!{ $( $rest )* }
+	);
+	() => ();
+	(IMPL $name:ident , $type:ty , $value:expr) => {
+		impl $crate::traits::Get<$type> for $name { fn get() -> $type { $value } }
+	}
+}
+
 /// The account with the given id was killed.
 pub trait OnFreeBalanceZero<AccountId> {
 	/// The account was the given id was killed.