Question

What is wrong with rs definition in first where section?

palindrome :: [a] -> [a]  palindrome xs = con xs rs     where con a b = rev (rev a []) b         rs = rev xs                        -- here         where rev [] rs = rs             rev (x:xs) rs = rev xs (x:rs) 

I’m just learning Haskell but its syntax rules confuse me. The error message is

[1 of 1] Compiling Main             ( pelindrome.hs, interpreted )  pelindrome.hs:5:8: parse error on input `rs' 

Answer 1

Your indentation was wrong and i think you can only have one where in there (i could be very well wrong. I’m not a haskell guy). There was also a argument missing for the call to rev (an empty list):

palindrome :: [a] -> [a] palindrome xs = con xs rs     where con a b = rev (rev a []) b           rs = rev xs []                       -- here           rev [] rs = rs           rev (x:xs) rs = rev xs (x:rs)  main = print (palindrome 'hello') 

Prints out:

'helloolleh' 

I’m going to try to understand it now. Anyway, have fun!

Edit: Makes perfect sense to me now. I think that’s the right version. For Haskell indentation rules, read Haskell Indentation