equilibriumIndices :: [Int] -> [Int]
equilibriumIndices xs =
  foldr
    (\(x, y, i) a ->
        (if x == y
           then i : a
           else a))
    []
    (zip3
       (scanl1 (+) xs) -- Sums from the left
       (scanr1 (+) xs) -- Sums from the right
       [0 ..] -- Indices
     )

-- TEST -----------------------------------------------------------------------
main :: IO ()
main =
  mapM_
    print
    (equilibriumIndices <$>
     [ [-7, 1, 5, 2, -4, 3, 0]
     , [2, 4, 6]
     , [2, 9, 2]
     , [1, -1, 1, -1, 1, -1, 1]
     , [1]
     , []
     ])