[Haskell-cafe] safe ways how to get head/last of Seq (or Foldable in general)

[Petr Pudlák]

Yes, that'd be my suggestion.

čt 26. 2. 2015 v 16:03 odesílatel David Feuer <david.feuer at gmail.com>
napsal:

> So are you suggesting that we add these to Data.Foldable?
>
> headMay = foldr (\x _ -> Just x) Nothing
> lastMay = foldl (\_ x -> Just x) Nothing
> On Feb 26, 2015 9:18 AM, "Petr Pudlák" <petr.mvd at gmail.com> wrote:
>
>> Hi,
>>
>> today I was a bit surprised that apparently there is no easy way how to
>> safely get the head element of `Seq` in a point-free way. Of course there
>> is `viewl`, but it seems the data type has no folding function (something
>> like 'foldViewL :: b -> (a -> Seq a -> b) -> b`.
>>
>> Is there any existing function like `Seq a -> Maybe a` to safely retrieve
>> the head (or last) element?
>>
>> If not, I'd suggest to add
>>
>>     headMaybe :: (Foldable t) => t a -> Maybe a
>>     headMaybe = getFirst . foldMap (First . Just)
>>
>> and similarly lastMaybe to Data.Foldable.
>>
>>   Thanks,
>>   Petr
>>
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