wip: distribute layout operator parsing

output/src/either.rs

@@ -1,15 +1,38 @@
 use crate::*;
-
 /// Show one item if a condition is true and another if the condition is false
-pub struct Either<A, B>(pub bool, pub A, pub B);
-
-impl<E: Output, A: Render<E>, B: Render<E>> Content<E> for Either<A, B> {
+pub struct Either<E, A, B>(pub PhantomData<E>, pub bool, pub A, pub B);
+impl<E, A, B> Either<E, A, B> {
+    pub fn new (c: bool, a: A, b: B) -> Self {
+        Self(Default::default(), c, a, b)
+    }
+}
+impl<'a, T, E, A, B> TryFromEdn<'a, T> for Either<E, A, B>
+where
+    T: EdnProvide<'a, bool> + EdnProvide<'a, B> + EdnProvide<'a, A> + 'a,
+    E: Output,
+    A: Render<E> + 'a,
+    B: Render<E> + 'a,
+{
+    fn try_from_edn (state: &'a T, head: &EdnItem<&str>, tail: &'a [EdnItem<&str>]) -> Option<Self> {
+        use EdnItem::*;
+        if let (Key("either"), [condition, content, alternative]) = (head, tail) {
+            Some(Self::new(
+                state.get(condition).expect("either: no condition"),
+                state.get(content).expect("either: no content"),
+                state.get(alternative).expect("either: no alternative")
+            ))
+        } else {
+            None
+        }
+    }
+}
+impl<E: Output, A: Render<E>, B: Render<E>> Content<E> for Either<E, A, B> {
     fn layout (&self, to: E::Area) -> E::Area {
-        let Self(cond, a, b) = self;
+        let Self(_, cond, a, b) = self;
         if *cond { a.layout(to) } else { b.layout(to) }
     }
     fn render (&self, to: &mut E) {
-        let Self(cond, a, b) = self;
+        let Self(_, cond, a, b) = self;
         if *cond { a.render(to) } else { b.render(to) }
     }
 }