BChan: add writeBChanNonBlocking, raise STM lower bound to 2.4.3

brick.cabal

@@ -101,7 +101,7 @@ library
                        config-ini,
                        vector,
                        contravariant,
-                       stm >= 2.4,
+                       stm >= 2.4.3,
                        text,
                        text-zipper >= 0.7.1,
                        template-haskell,

src/Brick/BChan.hs

@@ -2,6 +2,7 @@ module Brick.BChan
   ( BChan
   , newBChan
   , writeBChan
+  , writeBChanNonBlocking
   , readBChan
   , readBChan2
   )
@@ -26,6 +27,16 @@ newBChan size = atomically $ BChan <$> newTBQueue (fromIntegral size)
 writeBChan :: BChan a -> a -> IO ()
 writeBChan (BChan q) a = atomically $ writeTBQueue q a
 
+-- | Attempts to write a value to a @BChan@. If the channel has room,
+-- the value is written and this returns 'True'. Otherwise this returns
+-- 'False' and returns immediately.
+writeBChanNonBlocking :: BChan a -> a -> IO Bool
+writeBChanNonBlocking (BChan q) a = atomically $ do
+    f <- isFullTBQueue q
+    if f
+       then return False
+       else writeTBQueue q a >> return True
+
 -- | Reads the next value from the @BChan@; blocks if necessary.
 readBChan :: BChan a -> IO a
 readBChan (BChan q) = atomically $ readTBQueue q