I was afraid that there would be phrases like "TREE FIDDY" or "THREE FIFTY", and wrote unnecessary command s.toLowerCase().

i did something similar but i find it not very functional

Q: Does this have a problem with empty input?

but it's beautiful of list monad ;)

Fixed

You're right, parse is only a partial function.

*Main> validParentheses "(h)"
*** Exception: [...] Non-exhaustive patterns in case

I don't think it can be done in O(n). Your own solution isn't either, it's O(n^2) just like this one. I think it can be done in O(n log n) or O(n*w) ( where w = bit width of n ), but no better.

People will vote lots of bad oneliners Clever and even Best Practice. (!!) indeed is a bad idea here, as it is almost always.

Confused as to why this solution is considered best practice and clever. It's a fairly simple solution that is at least O(n^2) for a problem that can be solved in O(n). The (!!) operator is also fairly inefficient.

Great solution, but it does not work with this limitation: "input may contain any valid ASCII characters"

Well, gosh. I understand how fix works but I don't know how to make these functions applicable, as the way I'd usually define these would involve calling secondary functions recursively. But with fix, as far as I can see, you can only recur on the primary, main function.

I'll sleep on it; after all, this is Haskell—there's always a trick!

EDIT: I should point out that I have solutions, just they are not in O(n) time and so they fail the "Submit" test :/

The sample test for laziness is calling foldr instead of fixFoldr in Haskell