[Haskell-beginners] Palindromic solution??

[Alan Cameron]

>Date: Mon, 16 Feb 2009 16:32:23 +0100
>From: Miguel Pignatelli <miguel.pignatelli at uv.es>
>Subject: [Haskell-beginners] Indentation of local functions
>To: beginners at haskell.org
>Message-ID: <90C28A9E-A67D-47CA-8416-5F1C5C27A1F9 at uv.es>
>Content-Type: text/plain; charset="us-ascii"
>
>Hi all,
>
>This is my first post in this forum, I'm pretty new to Haskell (although I
>have some previous experience in functional programming with OCaml)
>
>I'm trying to write the typical function that determines if a list is a
>palindrome.
>the typical answer would be something like:
>
>isPalindrome xs = xs == (reverse xs)
>
>But I find this pretty inefficient (duplication of the list and double of
>needed comparisons).
>So I tried my own version using just indexes:
>
>isPalindrome xs =
>   	isPalindrome' 0 (length xs)
>   	where isPalindrome' i j =
>             if i == j   -- line 43
>             then True
>             else
>             	if (xs !! i) == (xs !! (j-1))
>             	then isPalindrome' (i+1) (j-1)
>             	else False
>
>But, when trying to load this in ghci it throws the following error:
>
>xxx.hs:43:12: parse error (possibly incorrect indentation) Failed, modules
>loaded: none.
>(Line 43 is marked in the code)
>
>I seems that the definition of isPalindrome' must be in one line. So, this
>works as expected:
>
>isPalindrome xs =
>   	isPalindrome' 0 (length xs)
>   	  where isPalindrome' i j = if i == j then True else if (xs !! i) ==
(xs !! (j-1)) then isPalindrome' (i+1) (j-1) else False
>
>Is there any way to make the local definition of isPalindrome' more
>readable?
>
>Any help in understanding this would be appreciated
>
>Thanks in advance,
>
I have found one solution to your problem

isPalindrome xs =
    isPalindrome' 0 (length xs)
        where
            isPalindrome' i j =
               if i == j   -- line 43
               then True
               else
             	   if (xs !! i) == (xs !! (j-1))
             	   then isPalindrome' (i+1) (j-1)
             	   else False

This loads without error but poses a second problem it generates an index
too large exception.




Alan Cameron