simplify Loop slightly

cpp/src/arrow/util/future.h

@@ -408,10 +408,11 @@ class ARROW_MUST_USE_TYPE Future {
   ///
   /// Returns true if a callback was actually added and false if the callback failed
   /// to add because the future was marked complete.
-  template <typename OnComplete>
-  bool TryAddCallback(const std::function<OnComplete()>& callback_factory) const {
+  template <typename CallbackFactory>
+  bool TryAddCallback(const CallbackFactory& callback_factory) const {
     return impl_->TryAddCallback([this, &callback_factory]() {
-      return Callback<OnComplete>{WeakFuture<T>(*this), callback_factory()};
+      return Callback<detail::result_of_t<CallbackFactory()>>{WeakFuture<T>(*this),
+                                                              callback_factory()};
     });
   }
 
@@ -691,25 +692,22 @@ Future<BreakValueType> Loop(Iterate iterate) {
 
       auto control_fut = iterate();
       while (true) {
-        if (control_fut.is_finished()) {
-          // There's no need to AddCallback on a finished future; we can
-          // CheckForTermination now. This also avoids recursion and potential stack
-          // overflow.
-          if (CheckForTermination(control_fut.result())) return;
-
-          control_fut = iterate();
-        } else {
-          std::function<Callback()> callback_factory = [this]() { return *this; };
-          if (control_fut.TryAddCallback(callback_factory)) {
-            break;
-          }
-          // Else we tried to add a callback but someone had stolen in and marked the
-          // future finished so we can just resume iteration
+        if (control_fut.TryAddCallback([this]() { return *this; })) {
+          // Adding a callback succeeded; control_fut was not finished
+          // and we must wait to CheckForTermination.
+          return;
         }
+        // Adding a callback failed; control_fut was finished and we
+        // can CheckForTermination immediately. This also avoids recursion and potential
+        // stack overflow.
+        if (CheckForTermination(control_fut.result())) return;
+
+        control_fut = iterate();
       }
     }
 
     Iterate iterate;
+
     // If the future returned by control_fut is never completed then we will be hanging on
     // to break_fut forever even if the listener has given up listening on it.  Instead we
     // rely on the fact that a producer (the caller of Future<>::Make) is always