foo.hs

nan = 0/0
silenceNaN d = if isNaN d then 0 else d

cumSumWithMissing :: [Double] -> [Double]
cumSumWithMissing ds = let
  ret = scanl1 (+) $ silenceNaN <$> ds
  go x y = if isNaN x then x else y
  in zipWith go ds ret

foo :: [Double] -> [Double]
foo ds = let
  go x y = if isNaN x then x else y
  in zipWith go ds $ silenceNaN <$> ds

main = do
  print $ foo [1,2,nan,3]
  print $ cumSumWithMissing [1,2,nan,3]

output.txt

$ runghc foo.hs
[1.0,2.0,NaN,3.0]
[1.0,3.0,NaN,6.0]