The specification is ambiguous. I believe that assumption was the intent of the author because my solution passes all the tests, and there are no counterexamples given in the specification.

This seems to assume that there are no characters after any closing parenthesis. The specification only mentions the last closing parenthesis.

I learned so much from this solution!
I believe it should be higher on best practices, since it's tail recursive (see Haskell's wiki), unlike the most common one.

You're welcome.

Thanks for this. I am newish to Haskell and was trying to formulate a solution in terms of Data.Set but missing something in my implementation. This helps.