From 20e809668b3624ed87cf1255f2fae003622edcc0 Mon Sep 17 00:00:00 2001
From: Gavin Wood <gavin@parity.io>
Date: Mon, 13 May 2019 20:58:57 +0200
Subject: [PATCH] Introduce the `Get` trait and type_parameter. (#2565)

---
 substrate/srml/support/src/lib.rs    |  6 ++---
 substrate/srml/support/src/traits.rs | 39 ++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+), 3 deletions(-)

diff --git a/substrate/srml/support/src/lib.rs b/substrate/srml/support/src/lib.rs
index 9d1bbff73a..cdaec09632 100644
--- a/substrate/srml/support/src/lib.rs
+++ b/substrate/srml/support/src/lib.rs
@@ -61,7 +61,7 @@ pub use self::storage::{StorageList, StorageValue, StorageMap, EnumerableStorage
 pub use self::hashable::Hashable;
 pub use self::dispatch::{Parameter, Dispatchable, Callable, IsSubType};
 pub use self::double_map::StorageDoubleMapWithHasher;
-pub use runtime_io::print;
+pub use runtime_io::{print, storage_root};
 
 #[doc(inline)]
 pub use srml_support_procedural::decl_storage;
@@ -96,9 +96,9 @@ macro_rules! ensure {
 #[cfg(feature = "std")]
 macro_rules! assert_noop {
 	( $x:expr , $y:expr ) => {
-		let h = runtime_io::storage_root();
+		let h = $crate::storage_root();
 		$crate::assert_err!($x, $y);
-		assert_eq!(h, runtime_io::storage_root());
+		assert_eq!(h, $crate::storage_root());
 	}
 }
 
diff --git a/substrate/srml/support/src/traits.rs b/substrate/srml/support/src/traits.rs
index 67b1f973ce..a8db5a946e 100644
--- a/substrate/srml/support/src/traits.rs
+++ b/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.