[Haskell-cafe] MonadPlus and Alternative folds

[David Feuer]

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 <j at dannynavarro.net> 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-Alternative.html
>
> On Sat, Sep 19, 2015 at 7:25 AM, David Feuer <david.feuer at gmail.com> 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