Question

I have the following code, and I think it is ugly:

loginCheck = do
  ml <- getPostParam "login" -- ml and mp :: Maybe ByteString
  mp <- getPostParam "password"
  if isJust ml && isJust mp
    then authAs (fromJust ml) (fromJust mp)
    else render "Msg" [("text", "Form incomplete")]

This code seems to be very imperative. Can I simplify it somehow?

Answer 1

As others have suggested, Applicative could be nice here, as well as MaybeT depending on the context. A third thing you might keep in mind is that a pattern match failure in a do block binding calls fail.

This is what I would do:

loginCheck = do
  ml <- getPostParam "login" 
  mp <- getPostParam "password"
  fromMaybe (render "Msg" [("text", "Form incomplete")]) $
            authAs <$> ml <*> mp

Or a solution with MaybeT, albeit one with a different return value (again more context might show this to be a good approach or not):

getPostParamT = MaybeT . getPostParam
loginCheckT = do
    ml <- getPostParamT "login" -- ml and mp :: Maybe ByteString
    mp <- getPostParamT "password"
    liftIO $ authAs ml mp
   <|> (liftIO $ render "Msg" [("text", "Form incomplete")] )

…actually the above is rather hokey now that I look at it