[Haskell-cafe] Feeding a monad into itself

[Joshua Grosso]

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 at 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 at 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 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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