Could MonadPlus (with mzero) or Alternative (with empty) provide the termination condition, if this pattern turned out to be more generally useful?

Joshua Grosso

On Thu, Jul 20, 2017 at 7:16 PM, Jeff Clites <jclites@mac.com> wrote:

Also this pattern does seem Maybe-specific, in that for the general Monad case there's not an obvious termination condition.

JEff

On Jul 20, 2017, at 11:03 AM, Rein Henrichs <rein.henrichs@gmail.com> wrote:

This is about as good as you can do. The monad-loops package provides a variety of similar combinators, generally implemented in similar ways, but not this specific combinator. The downside of this combinator is that it is partial: it will run forever without producing anything if f never gives a Nothing.

On Thu, Jul 20, 2017 at 10:27 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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