equi function in haskell

equi.hs

import Data.List

-- | Find the index corresponding to the equilibrium point of a
-- list. This is where the sum of the values to the left of the
-- index equals the sum of the values to the right.
equi :: [Int] -> Maybe Int
equi xs = findIndex match $ scanl (+) 0 xs
  where match x = x * 2 == sum xs