Re: [Haskell-cafe] MonadPlus and Alternative folds

Sorry, you are right, I was thinking about folding `Alt` as a `Monoid`, but what you need is different. On Sat, Sep 19, 2015 at 10:37 AM, David Feuer wrote:

Alt doesn't have a Foldable instance; these "folds" don't have quite the right types for that. I don't understand how the reducers relate to what I'm asking.

On Sat, Sep 19, 2015 at 3:22 AM, Danny Navarro wrote:

...

Not sure about performance, but you could wrap the `Alternative` with an `Alt`[1] and use the `Foldable` instance.

There is also `Alternative` folding in the `reducers` package.

[1]: https://hackage.haskell.org/package/base-4.8.1.0/docs/Data-Monoid.html#t:Alt [2]: https://hackage.haskell.org/package/reducers-3.12.1/docs/Data-Semigroup-Alte...

On Sat, Sep 19, 2015 at 7:25 AM, David Feuer wrote:

...

It kind of seems like there should be fold-like things to match some and many.

foldrAlt :: Alternative f => (a -> b -> b) -> b -> f a -> f b foldrAlt c n m = foldr1Alt c n m <|> pure n

foldr1Alt :: Alternative f => (a -> b -> b) -> b -> f a -> f b foldr1Alt c n m = c <$> m <*> foldrAlt c n m

foldlMP' :: MonadPlus f => (b -> a -> b) -> b -> f a -> f b foldlMP' f b m = foldl1MP' f b m <|> pure b

foldl1MP' :: MonadPlus f => (b -> a -> b) -> b -> f a -> f b foldl1MP' f b m = m >>= \r -> let !b' = f b r in foldlMP' f b' m

--These look a bit iffy foldlAlt :: Alternative f => (b -> a -> b) -> b -> f a -> f b foldlAlt f b m = ($ b) <$> foldrAlt (\x r acc -> r (f acc x)) id m

foldl1Alt :: Alternative f => (b -> a -> b) -> b -> f a -> f b foldl1Alt f b m = ($ b) <$> foldr1Alt (\x r acc -> r (f acc x)) id m

Do these or similar exist somewhere? If not, should they exist somewhere? I'm particularly interested in whether either of these will be fast, or will admit a fast implementation, for attoparsec parsers.

Thanks,

David Feuer

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-- Danny Navarro | http://dannynavarro.net

-- Danny Navarro | http://dannynavarro.net