diff --git a/lib/allocator/src/freelist.rs b/lib/allocator/src/freelist.rs
index ed63acb..304ae2d 100644
--- a/lib/allocator/src/freelist.rs
+++ b/lib/allocator/src/freelist.rs
@@ -119,7 +119,7 @@ impl FreeList {
     pub fn reserve(&mut self, start: PhysAddr, size: usize) {
         // NOTE: only support inclusive range
         if let Some((region, _)) =
-            self.alloc_node(|region| (region.start_addr() <= start && start < region.end_addr()).some(|| (), ()))
+            self.alloc_node(|region| (region.start_addr() <= start && start < region.end_addr()).then_ok((), ()))
         {
             let region_start = region.start_addr();
             let region_end = region.end_addr();
diff --git a/lib/utils/src/then.rs b/lib/utils/src/then.rs
index c51cfd7..a02e842 100644
--- a/lib/utils/src/then.rs
+++ b/lib/utils/src/then.rs
@@ -1,22 +1,42 @@
 pub trait Then {
-    fn and<T, E, F: FnOnce() -> Result<T, E>>(self, f: F, err: E) -> Result<T, E>;
-    fn some<T, E, F: FnOnce() -> T>(self, f: F, err: E) -> Result<T, E>;
+    fn then_ok<T, E>(self, ok: T, err: E) -> Result<T, E>;
+    fn then_err<T, E>(self, ok: T, err: E) -> Result<T, E>;
+    fn else_ok<T, E>(self, ok: T, err: E) -> Result<T, E>;
+    fn else_err<T, E>(self, ok: T, err: E) -> Result<T, E>;
 }
 
 impl Then for bool {
     #[inline]
-    fn and<T, E, F: FnOnce() -> Result<T, E>>(self, f: F, err: E) -> Result<T, E> {
+    fn then_ok<T, E>(self, ok: T, err: E) -> Result<T, E> {
         if self {
-            f()
+            Ok(ok)
         } else {
             Err(err)
         }
     }
 
     #[inline]
-    fn some<T, E, F: FnOnce() -> T>(self, f: F, err: E) -> Result<T, E> {
+    fn then_err<T, E>(self, ok: T, err: E) -> Result<T, E> {
+        if !self {
+            Ok(ok)
+        } else {
+            Err(err)
+        }
+    }
+
+    #[inline]
+    fn else_ok<T, E>(self, ok: T, err: E) -> Result<T, E> {
+        if !self {
+            Ok(ok)
+        } else {
+            Err(err)
+        }
+    }
+
+    #[inline]
+    fn else_err<T, E>(self, ok: T, err: E) -> Result<T, E> {
         if self {
-            Ok(f())
+            Ok(ok)
         } else {
             Err(err)
         }