concatFoldl.hs

-- an abstract helper for 'modify-last-or-add-new' pattern
concatFoldl :: (a -> b -> [b]) -> [a] -> b -> [b]
concatFoldl f (x:xs) e  | null ys = error "Unexpected empty list"
			| null xs = f x e
			| otherwise = concatFoldl f xs (head ys) ++ (tail ys)
	where ys = f x e
concatFoldl f [] e = []

-- test
data Command = Inc | Push Int

runCommand :: Command -> Int -> [Int]
runCommand (Inc) i = [i + 1]
runCommand (Push d) i = [d, i]

commands = [Inc, Inc, Push 4, Inc, Push 3, Inc, Push 0]

main = print $ concatFoldl runCommand commands 0 -- [0,4,5,2]