Use of `maybe` combinator.

chan.hs

import Control.Applicative
import Control.Concurrent.Chan (Chan, newChan)
import qualified Data.Map as M
import Control.Concurrent.STM (atomically, TVar, readTVar, writeTVar)
import Control.Monad.Trans
 
type Id = Integer
 
getOrCreateChannel :: (MonadIO m) => Id -> TVar (M.Map Id (Chan a))  -> m (Chan a)
getOrCreateChannel cid t = liftIO $ do
    nchan <- newChan
    atomically $ do
        chans <- readTVar t
        maybe (nchan <$ writeTVar t (M.insert cid nchan chans))
              return
              (M.lookup cid chans)