output: add Memo

output/src/ops/memo.rs

@@ -0,0 +1,30 @@
+use crate::*;
+use std::sync::{Arc, RwLock};
+
+#[derive(Debug, Default)] pub struct Memo<T, U> {
+    pub value: T,
+    pub view: Arc<RwLock<U>>
+}
+
+impl<T: PartialEq, U> Memo<T, U> {
+    pub fn new (value: T, view: U) -> Self {
+        Self { value, view: Arc::new(view.into()) }
+    }
+    pub fn update <R> (
+        &mut self,
+        newval: T,
+        render: impl Fn(&mut U, &T, &T)->R
+    ) -> Option<R> {
+        if newval != self.value {
+            let result = render(&mut*self.view.write().unwrap(), &newval, &self.value);
+            self.value = newval;
+            return Some(result);
+        }
+        None
+    }
+}
+
+/// Clear a pre-allocated buffer, then write into it.
+#[macro_export] macro_rules! rewrite {
+    ($buf:ident, $($rest:tt)*) => { |$buf,_,_|{ $buf.clear(); write!($buf, $($rest)*) } }
+}