Here's one solution:
isPalindrome xs =
   isPalindrome' 0 (length xs)
   where isPalindrome' i j = if i == j then True else check i j
                 check i j = if (xs !! i) == (xs !! (j-1)) then recurse i j else False
                 recurse i j = isPalindrome' (i+1) (j-1)


On Mon, Feb 16, 2009 at 10:32 AM, Miguel Pignatelli <miguel.pignatelli@uv.es> wrote:
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,

M;


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