[Haskell-cafe] Feeding a monad into itself

[Jeff Clites]

Another interesting thought: although all of those give you a way to obtain a zero, I don't think any give you a way to test whether something is that zero. Hmm.

JEff

> On Jul 20, 2017, at 8:17 PM, Jake <jake.waksbaum at gmail.com> wrote:
> 
> 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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