support multipass layout, barely

src/layout/cond.rs

@@ -0,0 +1,46 @@
+use crate::*;
+
+fn_kw_layout!(kw_when |state, output, expr| {
+    Ok(matches!(expr.head()?, Some("when")).then(||{
+        when(state.namespace(expr.tail().head())?.unwrap(),
+            draw(move|output: &mut O|{state.interpret(output, &expr.tail().tail().head())})
+        ).draw(output)
+    }).transpose()?.flatten())
+});
+
+/// Only render when condition is true.
+///
+/// ```
+/// # use tengri::*;
+/// # fn test () -> impl Draw<Tui> {
+///     when(true, "Yes")
+/// # }
+/// ```
+pub const fn when <T: Screen> (condition: bool, item: impl Draw<T>) -> impl Draw<T> {
+    draw(move|to: &mut T|if condition { item.draw(to) } else { Ok(Default::default()) })
+}
+
+fn_kw_layout!(kw_either |state, output, expr| {
+    Ok(matches!(expr.head()?, Some("either")).then(||{
+        either(state.namespace(expr.tail().head()?)?.unwrap(),
+            draw(move|output: &mut O|{
+                state.interpret(output, &expr.tail().tail().head()?)
+            }),
+            draw(move|output: &mut O|{
+                state.interpret(output, &expr.tail().tail().tail().head()?)
+            }),
+        ).draw(output)
+    }).transpose()?.flatten())
+});
+
+/// Render one thing if a condition is true and another false.
+///
+/// ```
+/// # use tengri::*;
+/// # fn test () -> impl Draw<Tui> {
+///     either(true, "Yes", "No")
+/// # }
+/// ```
+pub const fn either <T: Screen> (condition: bool, a: impl Draw<T>, b: impl Draw<T>) -> impl Draw<T> {
+    draw(move|to: &mut T|if condition { a.draw(to) } else { b.draw(to) })
+}