Haskell Parsing Recursion and Maybe

Question

I'm trying to write a parser in Haskell.

This parser take a string (example: "abc def") in parameter and return a Maybe (String, String).

Maybe (String, String)

First String get characters while it's number or letter.

Second String get the rest

In this example, I want to return Maybe ("abc", " def").

parseString :: String -> Maybe (String, String)
parseString "" = Nothing
parseString expr = case isString expr of
                Just (char, rest) -> fmap (char:) (parseString rest)
                Nothing -> Just ("", expr)

isString return :

Maybe (Char, String) -> Char = first character, String = rest / Nothing if isn't a letter or digit.

The problem, I can not return the rest of my String in the maybe.

Joseph Sible-Reinstate Monica · Accepted Answer · 2020-01-08 00:28:16Z

2

The issue seems to be in

fmap (char:) (parseString rest)

Now, (char:) is a function String -> String, so fmap (char:) becomes Maybe String -> Maybe String (or its generalization to another functor). However, parseString rest is not a Maybe String, it is a Maybe (String, String).

So, we need to adapt (char:) to work on the first component of that pair. I'd try

fmap (\(res,rest2) -> (char:res, rest2)) (parseString rest)

(By importing first from Data.Bifunctor or Control.Arrow, that can be written as fmap (first (char:)) (parseString rest), but that's not that important.)

edited Jan 8, 2020 at 0:28

Joseph Sible-Reinstate Monica

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answered Jan 7, 2020 at 8:38

chi

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4 Comments

Evgeny Over a year ago

What is char: if I may? I do not understand the colon.

chi Over a year ago

@Evgeny (expression symbol) is a section, a shorthand for (\x -> expression symbol x), so (char:) means (\x -> char : x), the function which prepends char to the input string x.

Evgeny Over a year ago

Oh, the : is from list contructor, a kind of point free prepending at head of list? Thanks for explaining to me, @chi, it looked like a mystery.

chi Over a year ago

@Evgeny Yes, it is one of the list constructors (the other one being [] for the empty list).

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