Non-allocating pearson's correlation

Correlation.hs

correlation :: T.Text -> T.Text -> DataFrame -> Maybe Double
correlation first second df = do
  (UnboxedColumn (f :: VU.Vector a)) <- getColumn first df
  (UnboxedColumn (s :: VU.Vector b)) <- getColumn second df
  Refl <- testEquality (typeRep @a) (typeRep @Double)
  Refl <- testEquality (typeRep @b) (typeRep @Double)
  let n = VG.length f
  let
    go (-1) acc = acc
    go i (mX :: Double, mY :: Double) = go (i - 1) (mX + f VU.! i, mY + s VU.! i)
  let (mX, mY) = go (n - 1) (0, 0)
  let
    go' (-1) acc = acc
    go' i (cov, varX, varY) = go' (i - 1) (cov + (x' * y'), varX +  (x' * x'), varY + (y' * y'))
      where x' = f VU.! i - mX
            y' = s VU.! i - mY
  let (cov, varX, varY) = go' (n - 1) (0, 0, 0)
  return $ if n == 0 then 0 else (cov / fromIntegral n) / sqrt ((varX / fromIntegral n) * (varY / fromIntegral n))