Another perfectly cromulent definition is:

untilNothing f = fromJust . last . takeWhile isJust . iterate (f =<<) . Just

This has 2 advantages:

1. It illustrates the haskellism that "A list is a loop is a list."

2. It composes much-beloved list combinators into a reasonable pipeline.

-- Kim-Ee

On Fri, Jul 21, 2017 at 12:26 AM, Jake <jake.waksbaum@gmail.com> wrote:

I have a function f :: a -> Maybe a that I want to feed into itself with some initial input until it returns Nothing.

untilNothing :: (a -> Maybe a) -> a -> a
untilNothing f x = case f x of
Just x' -> untilNothing f x'
Nothing -> x

Is there a better way to do this? I feel like there is something more general going on with Monads being fed into themselves, but maybe I'm wrong. Thoughts?

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