def itWithStop(init, stop, *options): if init is not None and not stop(init): yield(init) for f in options: newV

def itWithStop(init, stop, *options): if init is not None and not stop(init): yield(init) for f in options: newVal = f(init) yield from itWithStop(newVal, stop, *options) for s in itWithStop("", lambda x: len(x) > 2, lambda x: x + "a", lambda x: x + "b", lambda x: x + "c"): print(s) yields the combinations of 0 - 2 length strings with a, b, and c. Python has a number of ways to achieve this depending on exactly how you want to pass the arguments; multiple functions, optional arguments, etc. How nice the final call looks is more about your local language's closures look. The main point here is that this will happily iterate on things that don't "exist". module Tmp where iter :: forall a. (a -> Bool) -> [a -> a] -> a -> [a] iter p opts x = if p x then x:concatMap (iter p opts) (opts <*> [x]) else [] ghci> :l tmp.hs [1 of 1] Compiling Tmp ( tmp.hs, interpreted ) Ok, one module loaded. ghci> iter (\x -> length x < 3) [(++ "a"), (++ "b"), (++ "c")] "" ["","a","aa","ab","ac","b","ba","bb","bc", "c","ca","cb","cc"] (Since things are lazy in Haskell, functions that return lists effectively are iterators. There's probably something in the standard library somewhere for (opts <*> [x]) to avoid the wrapping x in an unnecessary list, but my Haskell is rusty.)