haskell get specific elements of a String code example

Example: haskell get specific elements of a String

onlyParenthesis :: String -> String
onlyParenthesis [] = []
onlyParenthesis (x:xs)
                 | x /= '(' && x /= ')' && x /= '[' && x /= ']' && x /= '{' && x /= '}'= onlyParenthesis xs
                 | otherwise = x : onlyParenthesis xs