Writing parsers by hand this way can be fun (and might be required for the highest performance ones, maybe?), but for robustness and ease of development you are generally better off using a parser combinator library.
The majority of production compilers use hand rolled parsers, ostensibly for better error reporting and panic synch.
Admittedly, INI is quite a simple format, hence I mention this as an anecdote.
I've got 10 full time senior engineers on a project heading in to its 15th year. We rewrite even extremely low level code like std::vector or malloc to make sure it matches our requirements.
UNIX was written by a couple of dudes.
In this case, this was a project at $EMPLOYER in an existing codebase with colleagues who have never seen Haskell code, using Haskell would've been a major error in judgement.
Haskell is a great language. It can even be a great language for beginners, especially if there's some senior help on hand.
But it's a terrible language to foist upon an unsuspecting and even unwilling victim.