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

Clearly you have not read the last paragraph of the description.

Anyways, added the lazy check to the sample tests too.