diff --git a/Control/Concurrent/STM/TBQueue.hs b/Control/Concurrent/STM/TBQueue.hs
index 79ffc43..0e53d4b 100644
--- a/Control/Concurrent/STM/TBQueue.hs
+++ b/Control/Concurrent/STM/TBQueue.hs
@@ -39,6 +39,7 @@ module Control.Concurrent.STM.TBQueue (
         tryPeekTBQueue,
         writeTBQueue,
         unGetTBQueue,
+        lengthTBQueue,
         isEmptyTBQueue,
         isFullTBQueue,
   ) where
@@ -52,14 +53,15 @@ import GHC.Conc
 --
 -- @since 2.4
 data TBQueue a
-   = TBQueue _UPK_(TVar Int)  -- CR: read capacity
-             _UPK_(TVar [a])  -- R:  elements waiting to be read
-             _UPK_(TVar Int)  -- CW: write capacity
-             _UPK_(TVar [a])  -- W:  elements written (head is most recent)
+   = TBQueue _UPK_(TVar Int)  -- CR:  read capacity
+             _UPK_(TVar [a])  -- R:   elements waiting to be read
+             _UPK_(TVar Int)  -- CW:  write capacity
+             _UPK_(TVar [a])  -- W:   elements written (head is most recent)
+             _UPK_(Int)       -- CAP: initial capacity
   deriving Typeable
 
 instance Eq (TBQueue a) where
-  TBQueue a _ _ _ == TBQueue b _ _ _ = a == b
+  TBQueue a _ _ _ _ == TBQueue b _ _ _ _ = a == b
 
 -- Total channel capacity remaining is CR + CW. Reads only need to
 -- access CR, writes usually need to access only CW but sometimes need
@@ -83,7 +85,7 @@ newTBQueue size = do
   write <- newTVar []
   rsize <- newTVar 0
   wsize <- newTVar size
-  return (TBQueue rsize read wsize write)
+  return (TBQueue rsize read wsize write size)
 
 -- |@IO@ version of 'newTBQueue'.  This is useful for creating top-level
 -- 'TBQueue's using 'System.IO.Unsafe.unsafePerformIO', because using
@@ -95,11 +97,11 @@ newTBQueueIO size = do
   write <- newTVarIO []
   rsize <- newTVarIO 0
   wsize <- newTVarIO size
-  return (TBQueue rsize read wsize write)
+  return (TBQueue rsize read wsize write size)
 
 -- |Write a value to a 'TBQueue'; blocks if the queue is full.
 writeTBQueue :: TBQueue a -> a -> STM ()
-writeTBQueue (TBQueue rsize _read wsize write) a = do
+writeTBQueue (TBQueue rsize _read wsize write _size) a = do
   w <- readTVar wsize
   if (w /= 0)
      then do writeTVar wsize $! w - 1
@@ -114,7 +116,7 @@ writeTBQueue (TBQueue rsize _read wsize write) a = do
 
 -- |Read the next value from the 'TBQueue'.
 readTBQueue :: TBQueue a -> STM a
-readTBQueue (TBQueue rsize read _wsize write) = do
+readTBQueue (TBQueue rsize read _wsize write _size) = do
   xs <- readTVar read
   r <- readTVar rsize
   writeTVar rsize $! r + 1
@@ -143,16 +145,17 @@ tryReadTBQueue c = fmap Just (readTBQueue c) `orElse` return Nothing
 --
 -- @since 2.4.5
 flushTBQueue :: TBQueue a -> STM [a]
-flushTBQueue (TBQueue rsize read wsize write) = do
+flushTBQueue (TBQueue rsize read wsize write size) = do
   xs <- readTVar read
   ys <- readTVar write
-  r <- readTVar rsize
-  w <- readTVar wsize
-  writeTVar read []
-  writeTVar write []
-  writeTVar rsize 0
-  writeTVar wsize (r + w)
-  return (xs ++ reverse ys)
+  if null xs && null ys
+    then return []
+    else do
+      writeTVar read []
+      writeTVar write []
+      writeTVar rsize 0
+      writeTVar wsize size
+      return (xs ++ reverse ys)
 
 -- | Get the next value from the @TBQueue@ without removing it,
 -- retrying if the channel is empty.
@@ -176,7 +179,7 @@ tryPeekTBQueue c = do
 -- |Put a data item back onto a channel, where it will be the next item read.
 -- Blocks if the queue is full.
 unGetTBQueue :: TBQueue a -> a -> STM ()
-unGetTBQueue (TBQueue rsize read wsize _write) a = do
+unGetTBQueue (TBQueue rsize read wsize _write _size) a = do
   r <- readTVar rsize
   if (r > 0)
      then do writeTVar rsize $! r - 1
@@ -188,9 +191,18 @@ unGetTBQueue (TBQueue rsize read wsize _write) a = do
   xs <- readTVar read
   writeTVar read (a:xs)
 
+-- |Return the length of a 'TBQueue'.
+--
+-- @Since FIXME
+lengthTBQueue :: TBQueue a -> STM Int
+lengthTBQueue (TBQueue rsize _read wsize _write size) = do
+  r <- readTVar rsize
+  w <- readTVar wsize
+  return $! size - r - w
+
 -- |Returns 'True' if the supplied 'TBQueue' is empty.
 isEmptyTBQueue :: TBQueue a -> STM Bool
-isEmptyTBQueue (TBQueue _rsize read _wsize write) = do
+isEmptyTBQueue (TBQueue _rsize read _wsize write _size) = do
   xs <- readTVar read
   case xs of
     (_:_) -> return False
@@ -203,7 +215,7 @@ isEmptyTBQueue (TBQueue _rsize read _wsize write) = do
 --
 -- @since 2.4.3
 isFullTBQueue :: TBQueue a -> STM Bool
-isFullTBQueue (TBQueue rsize _read wsize _write) = do
+isFullTBQueue (TBQueue rsize _read wsize _write _size) = do
   w <- readTVar wsize
   if (w > 0)
      then return False