Discussion: fold the Alts

[David Feuer]

Er, actually, no. This is not quite right.

asum :: (Foldable t, Alternative f) => t (f a) -> f a
firstAlt :: (Foldable t, Alternative f) => (a -> f b) -> t a -> f b

If the container is a Functor, asum is good enough. In other cases, it
doesn't help any.

On Sat, Nov 8, 2014 at 2:27 PM, David Feuer <david.feuer at gmail.com> wrote:

> Ah, yes, it's lovely how I can look at a function one day and then forget
> it exists the very next. Carry on!
>
> On Sat, Nov 8, 2014 at 2:22 PM, Edward Kmett <ekmett at gmail.com> wrote:
>
>> Um, msum / asum?
>>
>> On Sat, Nov 8, 2014 at 2:06 PM, David Feuer <david.feuer at gmail.com>
>> wrote:
>>
>>> We've had First and Last in Data.Monoid for some time, and we now have
>>> Alt as well. Unfortunately, we don't have nice convenient functions for
>>> using them, the way we have sum and product to use Sum and Product. So
>>> we're stuck with funny-looking constructions like listToMaybe . mapMaybe f
>>> for lists, and nothing good for other containers.
>>>
>>> My thought (the names firstAlt and lastAlt would absolutely have to be
>>> changed to accommodate non-catch Alternatives, but I'm not inspired at
>>> present):
>>>
>>> firstAlt, lastAlt :: (Foldable f, Alternative m) => (a -> m b) -> f a ->
>>> m b
>>>
>>> firstAlt f = getAlt #. foldMap (Alt #. f)
>>>
>>> lastAlt f = getAlt #. getDual #. foldMap (Dual #. Alt #. f)
>>>
>>> where
>>> (#.) :: Coercible b c => (b -> c) -> (a -> b) -> (a -> c)
>>> f #. g = coerce g
>>>
>>> This specializes to Maybe very nicely:
>>>
>>> firstMaybe :: Foldable f => (a -> Maybe b) -> f a -> Maybe b
>>> firstMaybe = firstAlt
>>>
>>> lastMaybe :: Foldable f => (a -> Maybe b) -> f a -> Maybe b
>>> lastMaybe = lastAlt
>>>
>>>
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>>>
>>
>
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