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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In the case of both Maybe and [], an action that succeeds once will always succeed. Parsers, other state transformers, and IO don't work like that, however. I'm not sure where else these apply. On Sep 19, 2015 1:39 PM, "Tom Ellis" < tom-lists-haskell-cafe-2013@jaguarpaw.co.uk> wrote:

On Sat, Sep 19, 2015 at 06:13:18PM +0100, Tom Ellis wrote:

...

On Sat, Sep 19, 2015 at 01:25:00AM -0400, 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 [...]

An Alternative allows list-like behaviour when it comes to *construction*, but offers no operation analogous to pattern-matching and therefore doesn't allow you to look inside it or do any sort of *destruction*.

I tried foldrAlt and it looped forever, as I expected. I also expect the others to loop forever.

I now understand this can work for some Alternatives. The operational reading is

1. Try to read an a 2. If it fails, return the b 3. If it succeeds, update the b 4. Go to 1

However, since it doesn't work even for an Alternative as simple as Maybe it's worth thinking about what class of Alternatives we really want it to apply to.

Tom _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe