Add GetByKey trait and get_by_key_type macro. (#304)

* Add GetByKey trait and get_by_key_type macro.

* Renaming. Add docs.

* Make clippy happy.

Author: shaunxw

traits/src/get_by_key.rs

@@ -0,0 +1,57 @@
+/// A trait for querying a value by a key.
+pub trait GetByKey<Key, Value> {
+	/// Return the value.
+	fn get(k: &Key) -> Value;
+}
+
+/// Create new implementations of the `GetByKey` trait.
+///
+/// The implementation is typically used like a map or set.
+///
+/// Example:
+/// ```ignore
+/// use primitives::CurrencyId;
+/// parameter_type_with_key! {
+///     pub Rates: |currency_id: CurrencyId| -> u32 {
+///         match currency_id {
+///             CurrencyId::DOT => 1,
+///             CurrencyId::KSM => 2,
+///             _ => 3,
+///         }
+///     }
+/// }
+/// ```
+#[macro_export]
+macro_rules! parameter_type_with_key {
+	(
+		pub $name:ident: |$k:ident: $key:ty| -> $value:ty $body:block;
+	) => {
+		pub struct $name;
+		impl $crate::get_by_key::GetByKey<$key, $value> for $name {
+			fn get($k: &$key) -> $value {
+				$body
+			}
+		}
+	};
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+
+	parameter_type_with_key! {
+		pub Test: |k: u32| -> u32 {
+			match k {
+				1 => 1,
+				_ => 2,
+			}
+		};
+	}
+
+	#[test]
+	fn get_by_key_should_work() {
+		assert_eq!(Test::get(&1), 1);
+		assert_eq!(Test::get(&2), 2);
+		assert_eq!(Test::get(&3), 2);
+	}
+}

traits/src/lib.rs

@@ -16,13 +16,15 @@ pub use currency::{
 	LockIdentifier, MultiCurrency, MultiCurrencyExtended, MultiLockableCurrency, MultiReservableCurrency, OnReceived,
 };
 pub use data_provider::{DataFeeder, DataProvider, DataProviderExtended};
+pub use get_by_key::GetByKey;
 pub use price::{DefaultPriceProvider, PriceProvider};
 pub use rewards::RewardHandler;
 
 pub mod arithmetic;
 pub mod auction;
 pub mod currency;
 pub mod data_provider;
+pub mod get_by_key;
 pub mod price;
 pub mod rewards;