[Haskell-beginners] Reversing a list

[Jona Ekenberg]

Hello,

I'm currently going through the exercises in chapter 3 of Real World
Haskell. One of the exercises challenges me to create a function which
takes a list and makes a palindrome of it.

I tried to solve it this way:
palindrome :: [t] -> [t]
palindrome xs = xs ++ (reverse xs)
  where
    reverse []     = []
    reverse (x:xs) = (reverse xs) ++ x

But when I try to compile this I get this error:
Kapitel3.hs:14:32-33: error: …
    • Couldn't match type ‘t’ with ‘[t]’
      ‘t’ is a rigid type variable bound by
        the type signature for:
          palindrome :: forall t. [t] -> [t]
        at /Users/jona/programmering/haskell/boken/Kapitel3.hs:13:15
      Expected type: [[t]]
        Actual type: [t]
    • In the first argument of ‘reverse’, namely ‘xs’
      In the second argument of ‘(++)’, namely ‘(reverse xs)’
      In the expression: xs ++ (reverse xs)
    • Relevant bindings include
        xs :: [t]
          (bound at
/Users/jona/programmering/haskell/boken/Kapitel3.hs:14:12)
        palindrome :: [t] -> [t]
          (bound at
/Users/jona/programmering/haskell/boken/Kapitel3.hs:14:1)
Compilation failed.

It looks like I have used a function that want a list of lists, but I don't
understand where.
Also, is there some way to declare the type of my reverse-function in this
case?

Kind regards,
Jona Ekenberg
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