Question

‘m a student, just starting working with Haskell and have problems with part of code. I does not understand how this code works. Can anyone explain my how it worked?

check :: String -> Bool
check = check' [] where
    check' []       []       = True
    check' _        []       = False
    check' x       ('(':xs) = check' ('(':x) xs
    check' ('(':x) (')':xs) = check' x xs
    check' _        (')':xs) = False
    check' x       (_  :xs) = check' x xs

Answer 1

This first parameter of check' function serves as a stack to count the opened brackets. Every time it encounters an opening bracket, it appends it to the stack and proceeds with the rest of the input line:

check' x ('(':xs) = check' ('(':x) xs

Then, when it encounters a closing bracket, it pops an opening bracket and proceeds:

check' ('(':x) (')':xs) = check' x xs

But if there is a closing bracket and no opening ones left in the stack, it fails:

check' _ (')':xs) = False

Also, if the line is over, and unclosed brackets remain, fail:

check' _ []       = False

By default, the stack is empty. The rest are the obvious boundary cases.