How do I implement a palindrome function in Haskell?

Question

the task is to implement a function palindrome (:: String -> Bool) which checks whether the input is a palindrome.

import Data.List

palindrome :: String -> Bool 
palindrome str
    |str == revstring = True
    |otherwise = False

revstring :: String -> String
revstring str = reverse str

I guess I made a mistake in my signature, but I don't know exactly where.

Answer 1

str == revstring

On the left of the equality, we have a string ( str ). On the right, a function ( revstring ). This can't be right. We can fix it using instead

str == revstring str

However, at that point, we can simply call reverse . There is no need to define revstring at all.

We could then have

palindrome :: String -> Bool 
palindrome str
    |str == reverse str = True
    |otherwise = False

but this is still sub-optimal. The comparison str == reverse str already produces the Bool value we need to return. There is no need to use guards to check that value, we can simply return it.

palindrome :: String -> Bool 
palindrome str = str == reverse str