Is it possible to nest guards in Haskell?

Question

Haskell newbie here, trying to write code to parse math expressions. Code:

isDigit :: Char -> Bool
isDigit c = c >= \'0\' && c <= \'9\'

pars         


        

Answer 1

No, but you can use cases if you'd like:

parseNumber :: String -> Maybe (String, String)
parseNumber [] = Just ("", "")
parseNumber (h:ls)
    | isDigit h =
         case () of
           () | p == Nothing -> Just([h], ls)
              | otherwise -> Just (h:fst d, snd d) -- Ends in a digit
    | h == '.' =
         case () of
           () | p == Nothing -> Nothing
              | not ('.' `elem` (snd d)) -> Just (h:(fst d), snd d)
    | otherwise = Nothing
    where 
        p      = parseNumber ls
        Just d = parseNumber ls

Alternatively, multiway if works in a similar manner (if True | p1 -> b ; | p2 -> c).