refactor midi module

layout/src/ops.rs

@@ -1,23 +1,25 @@
 use crate::*;
 
-use std::sync::RwLock;
-
 impl<E: Engine> Layout<E> for E {}
 
 pub trait Layout<E: Engine> {
     /// Content `item` when `cond` is true.
-    fn when <A: Content<E>> (cond: bool, item: A) -> When<E, A> {
+    fn when <A> (cond: bool, item: A) -> When<E, A> where
+        A: Content<E>
+    {
         When(cond, item, Default::default())
     }
     /// Content `item` if `cond` is true, otherwise render `other`.
-    fn either <A: Content<E>, B: Content<E>> (cond: bool, a: A, b: B)
-        -> Either<E, A, B>
+    fn either <A, B> (cond: bool, a: A, b: B) -> Either<E, A, B> where
+        A: Content<E>,
+        B: Content<E>,
     {
         Either(cond, a, b, Default::default())
     }
     /// Maps an [Option<T>] through a callback `F`
-    fn opt <A, F: Fn(A)->R, R: Content<E>> (option: Option<A>, cb: F)
-        -> Opt<E, A, F, R>
+    fn opt <A, F, R> (option: Option<A>, cb: F) -> Opt<E, A, F, R> where
+        F: Fn(A) -> R,
+        R: Content<E>
     {
         Opt(option, cb, Default::default())
     }