[Haskell-beginners] Type definition doesn't match with number of arguments

Hi, Please help me understund this code: http://hackage.haskell.org/packages/archive/daemons/0.1.2/doc/html/src/Contr... The type of commandSender is: (Serialize a, Serialize b) => a -> Handler (Maybe b) But definition looks like this: commandSender command reader writer = do ... As I understund, command has type a and this function returns Handler (Maybe b). reader and writer doesn't fit here. What's going on? Emanuel