f.hs

data Config = Config
  { cfgUser :: Maybe String
  , cfgHost :: String
  , cfgPort :: Int
  }
 

word :: Parser String
word = ...

number :: Parser Int
number = ...

assign :: Parser a -> String -> Parser a
assign = ...

assignMaybe :: Parser a -> String -> Parser (Maybe a)
assignMaybe = ...

config :: Parser Config
config = 
  Config <$> assignMaybe word "user"
         <*> assign word "host"
         <*> assign number "port"