import("std/op").

takeWhileS(f, "") -> "".
takeWhileS(f, x::xs) -> do
	if f(x) == true then x :: takeWhileS(f, xs)
	else ""
end.

takeUntilS(f, xs) -> takeWhileS(\x -> op\not(f(x)), xs).

dropWhileS(f, "") -> "".
dropWhileS(f, x::xs) -> do
	if f(x) == true then dropWhileS(f, xs)
	else x :: xs
end.

dropS(0, x) -> x.
dropS(n, "") -> "".
dropS(n, _::xs) -> dropS(n-1, xs).

takeS(0, _) -> "".
takeS(n, "") -> "".
takeS(n, x::xs) -> x :: takeS(n-1, xs).

concat([]) -> "".
concat(x :: xs) -> x + concat(xs).