[Haskell-cafe] Feeding a monad into itself

[Kim-Ee Yeoh]

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 at 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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