beginner haskell programmer

Question

isPalindrome::(Eq a) => [a] -> Bool
isPalindrome [] = True
isPalindrome [x] = True
isPalindrome (x1:xs:x2:[]) 
    | x1 == x2 = isPalindrome xs
    |otherwise = False


[1 of 1] Compiling Main             ( myHas.hs, interpreted )

myHas.hs:37:27:
    Couldn't match expected type `[a]' against inferred type `a1'
      `a1' is a rigid type variable bound by
           the type signature for `isPalindrome' at myHas.hs:33:18
    In the first argument of `isPalindrome', namely `xs'
    In the expression: isPalindrome xs
    In the definition of `isPalindrome':
        isPalindrome (x1 : xs : x2 : [])
                       | x1 == x2 = isPalindrome xs
                       | otherwise = False

I'm a beginner haskell programmer and got no clue as to why I'm getting this error, any help please?

Answer 1

You treat xs like a list, but (x1:xs:x2:[]) assumes it is an element of your input list.

Note that (x1:xs:x2:[]) will match only lists with 3 elements, and x1, xs and x2 will be elements of type a.

So xs is of type a, but as you pass it to isPalindrome, we can only assume it must be a list of something, so the type system calls the type [a1].

The easiest way to encode what you want is:

isPalindrome::(Eq a) => [a] -> Bool
isPalindrome l = l == (reverse l)