Make `notNull` for any `Foldable`

[IvanRenison]

Hello, the notNull defined in Data.List.Extra, as a composition of not and null, is defined only for list, however, de null function works with any Foldable, so I think that it would make sense (and it would be very useful) for the notNull function to also work for any Foldable. Some other functions of Data.List.Extra also would make sense if there where for any Foldable, such as allSame and anySame.

[ndmitchell]

Note that there is https://hackage.haskell.org/package/extra-1.7.9/docs/Data-Foldable-Extra.html, and I'm happy to take patches porting things from List to Foldable with a more generic type. Things like notNull seem sensible.

[IvanRenison]

Yes, I know about that module, and I think that is the best place for the notNull function (the null function is in Data.Foldable). Also, I have noticed that some functions, such as sumOn', already have been defined for any Foldable in Data.Foldable.Extra, but the versions of Data.List.Extra are redefinitions, so if you tray to use them when importing both Data.List.Extra and Data.Foldable.Extra you get a Ambiguous occurrence error:

Prelude Data.List.Extra Data.Foldable.Extra> :t sumOn'

<interactive>:1:1: error:
    Ambiguous occurrence ‘sumOn'’
    It could refer to either ‘Data.Foldable.Extra.sumOn'’,
                             imported from ‘Data.Foldable.Extra’
                          or ‘Data.List.Extra.sumOn'’, imported from ‘Data.List.Extra’

Instead of that, Data.List.Extra could simple re-export the functions from Data.Foldable.Extra so that there are the same, such as Data.List re-exports null and other functions from Data.Foldable.

[ndmitchell]

If they weren't redefinitions, they wouldn't have a List type but a Foldable type. I think the most likely solution is for Foldable.Extra to define nearly everything that List.Extra does, and then you are unlikely to be able to need to combine them. There is a big cost in terms of comprehension to abstracting over Foldable, so the List utilities should ideally stay as lists.