Re: [Haskell-cafe] Feeding a monad into itself

Another possibility is to use Either for an early exit. iterateM :: Monad m => (a -> m a) -> a -> m b iterateM f a = f a >>= iterateM f exitOnNothing :: (a -> Maybe a) -> a -> Either a a exitOnNothing f a = maybe (Left a) Right (f a) Now, your untilNothing is iterateM . exitOnNothing, but you can easily extend it to use IO or whatever pattern you like. On Thu, Jul 20, 2017 at 11:17 PM, Jake 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 wrote:

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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 wrote:

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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 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 wrote:

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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? _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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-- Dave Menendez http://www.eyrie.org/~zednenem/