[Haskell-cafe] Feeding a monad into itself

[Jake]

That's what I was thinking, but I couldn't figure out if I wanted
MonadPlus, Alternative or maybe Monoid because they all have zero elements.

On Thu, Jul 20, 2017, 21:52 Joshua Grosso <jgrosso1 at asu.edu> wrote:

> 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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