The past few days I have been solving problems at this site called Project Euler.  This site contains many seemingly simple math programming problems.  I have been using Haskell to solve the problems on the site and in order to help solve one of the problems I wrote this bit of code to generate palindromes

   1: palindrome :: (Integral a) => a -> [Char] -> [String]
   2: palindrome n al =  concat  $  map (pal n) al
   3:     where 
   4:       pal :: (Integral a)=> a -> Char -> [String]
   5:       pal n x 
   6:           | n > 2 =  map (surround x) (palindrome (n-2) al)
   7:           | n > 1 = [[x,x]]
   8:           | otherwise = [[x]]
   9:           where 
  10:             surround :: Char -> String -> String
  11:             surround lt str = [lt] ++ str ++ [lt]

 

This code take a length(as an integer) and a list of characters and returns all possible palindromes that can be created.  For example:

palindrome 3 ['a'..'f']

will result in

["aaa","aba","aca","ada","aea","afa","bab","bbb","bcb","bdb",

"beb","bfb","cac","cbc","ccc","cdc","cec","cfc","dad","dbd",

"dcd","ddd","ded","dfd","eae","ebe","ece","ede","eee",

"efe","faf","fbf","fcf","fdf","fef","fff"] 

 

While I doubt this is the greatest implementation of a method which generates palindromes,  it was the first one I came up with and I am curious if anyone can do it in a very different (or better) way. So, if anyone reading this wants to show me a different way (in any language) feel free.