Haskell Merge Sort

This is an implementation of Mergesort using higher order functions,guards,where and recursion.

However getting an error from compiler 6:26: parse error on input ‘=’

 mergeSort :: ([a] -> [a] -> [a]) -> [a] -> [a]
    mergeSort merge xs
        | length xs < 2 = xs
        | otherwise = merge (mergeSort merge first) (mergeSort merge second)
        where first = take half xs 
              second = drop half xs 
               half = (length xs) `div` 2

I can’t see whats wrong? or rather I don’t understand the compiler.