[Haskell-cafe] Feeding a monad into itself

[Jeff Clites]

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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