What a parser did is to tansfer a "plain text" representation of something to a "structured" representation. In this case, the plain text is a String representing regular expression, and the the structured thing is the data structure that has been defined for you.

Random test failures make me find sorts of bugs in parsers I had wrote earlier. Nice kata!

I write this by analog the imperative algorithm, but I don't understand this in Monad. What should I think of monads as?

Interesting kata with instructive hints! Thank you.

sorry for this gross code v^v

Thanks, it's accually the only solution I can figure out when before checked the solusions. LOL