Rename operation::chain to then

... and make `focus_*` operations generic over
the output type.

core/src/widget/operation.rs

@@ -295,7 +295,7 @@ where
 
 /// Chains the output of an [`Operation`] with the provided function to
 /// build a new [`Operation`].
-pub fn chain<A, B, O>(
+pub fn then<A, B, O>(
  operation: impl Operation<A> + 'static,
  f: fn(A) -> O,
 ) -> impl Operation<B>
@@ -361,7 +361,7 @@ where
  Outcome::Chain(Box::new((self.next)(value)))
  }
  Outcome::Chain(operation) => {
- Outcome::Chain(Box::new(chain(operation, self.next)))
+ Outcome::Chain(Box::new(then(operation, self.next)))
  }
  }
  }

core/src/widget/operation/focusable.rs

@@ -94,7 +94,10 @@ pub fn count() -> impl Operation<Count> {
 /// Produces an [`Operation`] that searches for the current focused widget, and
 /// - if found, focuses the previous focusable widget.
 /// - if not found, focuses the last focusable widget.
-pub fn focus_previous() -> impl Operation {
+pub fn focus_previous<T>() -> impl Operation<T>
+where
+ T: Send + 'static,
+{
  struct FocusPrevious {
  count: Count,
  current: usize,
@@ -128,13 +131,16 @@ pub fn focus_previous() -> impl Operation {
  }
  }
 
- operation::chain(count(), |count| FocusPrevious { count, current: 0 })
+ operation::then(count(), |count| FocusPrevious { count, current: 0 })
 }
 
 /// Produces an [`Operation`] that searches for the current focused widget, and
 /// - if found, focuses the next focusable widget.
 /// - if not found, focuses the first focusable widget.
-pub fn focus_next() -> impl Operation {
+pub fn focus_next<T>() -> impl Operation<T>
+where
+ T: Send + 'static,
+{
  struct FocusNext {
  count: Count,
  current: usize,
@@ -162,7 +168,7 @@ pub fn focus_next() -> impl Operation {
  }
  }
 
- operation::chain(count(), |count| FocusNext { count, current: 0 })
+ operation::then(count(), |count| FocusNext { count, current: 0 })
 }
 
 /// Produces an [`Operation`] that searches for the current focused widget