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

Put them in separated functions.

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


parseNumber :: String -> Maybe (String, String)
parseNumber [] = Just ("", "")
parseNumber (h:ls)
    | isDigit h  = f_p (h:ls)            
    | h == '.'   = temp (h: ls)         
    | otherwise  = Nothing       -- Not a number, stop looking!


f_p :: String -> Maybe (String, String)
f_p (h:ls)
    | parseNumber ls == Nothing  = Just([h], ls)         -- Digit found <<< ERROR!!
    | otherwise                  = Just (h:fst d, snd d) -- Ends in a digit
    where 
        Just d = parseNumber ls -- Float version of p. Not used if p is Nothing


temp :: String -> Maybe (String, String)
temp (h:ls)
    | parseNumber ls == Nothing  = Nothing                 -- Ends in a point
    | not ('.' `elem` (snd d))   = Just (h:(fst d), snd d) -- We don't want multiple dots
    where 
        Just d = parseNumber ls -- Float version of p. Not used if p is Nothing

Have to admit I did not tested this code.