Question

While working through Real World Haskell, I tried to complete the palindrome exercise using the following code solution:

palin :: [a] -> [a]
palin list = list ++ rev list
    where rev list
           | null list = []
           | otherwise = rev (tail list) ++ (head list)

Which raised a “cannot construct an infinite type error. However, simply replacing the parenthesis around the head list with square brackets, and it works correctly, as demonstrated in the following example:

palin :: [a] -> [a]
palin list = list ++ rev list
    where rev list
           | null list = []
           | otherwise = rev (tail list) ++ [head list]

I don’t really understand why it matters, nor do I understand what does the “cannot construct infinite type a = [a]” error means. Could someone explain this?

Answer 1

In the last line, you are trying to append a non-list to a list. head list gives the first item of the list, which is type a. When you try to use ++ to append, you can’t append something that isn’t a list to a list. By appending [head list], you are appending a list of 1 item to the other list. The [] construct the single item list in this case.