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+{-# LANGUAGE RecordWildCards #-}
+module Xmobar.Plugins.QueueReader
+ (QueueReader (..)
+ ) where
+
+import Xmobar.Run.Exec (Exec (..))
+
+import Control.Monad (forever)
+import qualified Control.Concurrent.STM as STM
+
+-- | A 'QueueReader' displays data from an 'TQueue a' where
+-- the data items 'a' are rendered by a user supplied function.
+--
+-- Like the 'HandleReader' plugin this is only useful if you are
+-- running @xmobar@ from other Haskell code. You should create a
+-- new @TQueue a@ and pass it to this plugin.
+--
+-- @
+-- main :: IO
+-- main = do
+-- q <- STM.newQueueIO @String
+-- bar <- forkIO $ xmobar conf
+-- { commands = Run (QueueReader q id "Queue") : commands conf }
+-- STM.atomically $ STM.writeTQueue q "Some Message"
+-- @
+data QueueReader a
+ = QueueReader
+ { qQueue :: STM.TQueue a
+ , qShowItem :: a -> String
+ , qName :: String
+ }
+
+-- | This cannot be read back.
+instance Show (QueueReader a) where
+ -- | Only show the name/alias for the queue reader.
+ show q = "QueueReader " <> qName q
+
+-- | WARNING: This read instance will throw an exception if used! It is
+-- only implemented, because it is required by 'Xmobar.Run` in 'Xmobar.commands'.
+instance Read (QueueReader a) where
+ -- | Throws an 'error'!
+ readsPrec = error "QueueReader: instance is a stub"
+
+-- | Async queue/channel reading.
+instance Exec (QueueReader a) where
+ -- | Read from queue as data arrives.
+ start QueueReader{..} cb =
+ forever (STM.atomically (qShowItem <$> STM.readTQueue qQueue) >>= cb)
+
+ alias = qName