sumArr = scan (\acc x -> let !newAcc = acc + x in newAcc) 0
sumArr' = proc v -> do sumArr -< v

testData :: [Int]
testData = [1..1000000]

main = print $ L.last $ evalList sumArr' testData

Running time for main with sumArr (i.e. no proc notation) is 0.087 sec, while for sumArr' it is 3.2 seconds (and around 300mb memory usage), although sumArr' is just sumArr called within a proc block.