Simple Manual Lexer in Haskell

Question

As a beginner exercise, I made small manual lexer that recognizes three types of inputs:

• integers: /[-]?[0-9]+/
• strings, inside double quotes, with backslash escaping
• nil

using only Data.Char and Data.Maybe.

I would have wanted to do something that looks like this

parse s = tryParseInteger s
 OR tryParseString s
 OR tryParseNil

where each tryParse* would return a (Maybe MyToken) and a failing case (= Nothing) would continue to the next tryParse*. But I didn't find a clean way to do it.

So here it goes:

import Data.Char;
import Data.Maybe;
data MyToken = MyNil
 | MyNumber Int
 | MyString String
 deriving (Show)
tryParse :: String -> Maybe MyToken
tryParse "nil" = Just MyNil
tryParse (c : t)
 -- Ignoring white space and parse the tail
 | isSpace c = tryParse t
tryParse s = tryParseNumber s
tryParseNumber :: String -> Maybe MyToken
tryParseNumber s = case (parseNum s) of
 Just v -> Just $ MyNumber v
 Nothing -> tryParseString s
tryParseString :: String -> Maybe MyToken
tryParseString ('"':t) = fmap MyString (parseInsideString t)
tryParseString _ = Nothing
parseInsideString :: String -> Maybe String
parseInsideString ('\\':'"':cs) = fmap ('"':) (parseInsideString cs)
parseInsideString ('"':_) = Just ""
parseInsideString (c:cs) = fmap (c:) (parseInsideString cs)
parseInsideString _ = Nothing
parseNum :: String -> Maybe Int
parseNum ('-':xs) = fmap (\x -> -x) (parseNum xs)
parseNum cs@(c:_)
 | isDigit c = foldl step Nothing cs
 | otherwise = Nothing
 where step acc c
 | isDigit c = Just ((10 * fromMaybe 0 acc) + (digitToInt c))
 | otherwise = acc
main = print $ tryParse "\"abcd\\\"efgh\""

Answer 1

I would have wanted to do something that looks like this

parse s = tryParseInteger s
 OR tryParseString s
 OR tryParseNil

OR is <|>:

tryParse :: String -> Maybe MyToken
tryParse = tryParse' . dropWhile isSpace
 where
 tryParse' s
 = MyNumber <$> parseNumber s
 <|> tryParseString s
 <|> tryParseNil s
tryParseNil :: String -> Maybe MyToken
tryParseNil "nil" = Just MyNil
tryParseNil _ = Nothing