Discussion: fold the Alts

[David Feuer]

You make a good point, yes.

On Sat, Nov 8, 2014 at 2:46 PM, Edward Kmett <ekmett at gmail.com> wrote:

> We don't try to provide every single function that could map to any monoid
> in Data.Monoid in Data.Foldable.
>
> *e.g.* by the same token we also don't have versions that take a function
> and then compute a sum or product.
>
> Global namespace in a library like base is a fairly precious resource,
> because any name you take is taken from all users who use unqualified
> imports.
>
> The assumption is rather implicit that if you need something more exotic
> you can roll it yourself from the parts available to you.
>
> -Edward
>
> On Sat, Nov 8, 2014 at 2:35 PM, David Feuer <david.feuer at gmail.com> wrote:
>
>> 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
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> Libraries mailing list
>>>>> Libraries at haskell.org
>>>>> http://www.haskell.org/mailman/listinfo/libraries
>>>>>
>>>>>
>>>>
>>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/libraries/attachments/20141108/0877f47a/attachment-0001.html>