Proposal for containers: Add 'pop' function to Data.Map

Carter Schonwald carter.schonwald at gmail.com
Sun Dec 6 21:37:15 UTC 2020 

Well said Andreas, I emphatically agree with your latter point:

We need to ground proposal need / utility with statistical information
about actual code out in the world.

On Sun, Dec 6, 2020 at 1:38 PM Andreas Abel <andreas.abel at ifi.lmu.de> wrote:

> I have reservations against this proposal:
>
> 1. `pop` seems to be below the Fairbairn threshold.
>
>  > pop :: Int -> IntMap a -> (Maybe a, IntMap a)
>  > pop = updateLookupWithKey (\_ _ -> Nothing)
>
> It specializes the Swiss knife `updateLookupWithKey` without involving
> any other map-specific function.
>
> 2. Addition of `pop` would require a similar function in all other
> standard collections, to keep their interfaces in sync.
>
> 3. Is there sufficient evidence that `pop` is a strongly user-demanded
> feature?  One way to quantify this is to look on hackage for the pattern
> `updateLookupWithKey (\_ _ -> Nothing)` (and variants) and maybe even on
> how often this pattern has been given a name.
>
> (The latter would be a more scientific way to develop our libraries in
> general.  Instead of randomly adding functionality based on
> non-representative discussions on this list, we could scan hackage for
> common patterns---especially, mine popular utility libraries.  Harvest
> the intelligence of the masses...)
>
> While I would not recommend to add `pop`, I would recommend to enhance
> the documentation of `updateLookupWithKey` to include `pop` as a use
> case.  This would also ensure the discoverability of the present solution.
>
> I seem to recall that we have given this recommendation to similar
> proposals in the past ("similar" in terms of the Fairbairn threshold).
>
> Cheers,
> Andreas
>
> On 2020-12-06 19:04, Oleg Grenrus wrote:
> > NonEmptySeq could be. NonEmptySet almost (it could have extract).
> >
> > - Oleg
> >
> > On 6.12.2020 20.02, David Feuer wrote:
> >> None of these types are comonads, so it doesn't seem *that* confusing.
> >>
> >> On Sun, Dec 6, 2020, 12:59 PM Bob Ippolito <bob at redivi.com
> >> <mailto:bob at redivi.com>> wrote:
> >>
> >>     `extract` overlaps with the Comonad typeclass (extract, duplicate,
> >>     extend)
> >>
> >>     On Sun, Dec 6, 2020 at 9:34 AM Tom Ellis
> >>     <tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk
> >>     <mailto:tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk>> wrote:
> >>
> >>         I really like `extract`!
> >>
> >>         On Sun, Dec 06, 2020 at 05:29:40PM +0000, Alexey Kuleshevich
> >>         wrote:
> >>         > `extract` is the name that I would suggest. I also agree
> >>         with David
> >>         > that `pop` is a bit too confusing in a non-stack/queue data
> >>         > structure. If all languages named it pop, then it would be
> good
> >>         > argument for the name, but we don't need to copy python. The
> >>         only
> >>         > thing that pyhon has that I wish we had in Haskell is the
> >>         popularity
> >>         > ;)
> >>         >
> >>         > ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
> >>         > On Sunday, December 6, 2020 8:16 PM, Carter Schonwald
> >>         <carter.schonwald at gmail.com
> >>         <mailto:carter.schonwald at gmail.com>> wrote:
> >>         >
> >>         > > LookupThenRemove seems like a more descriptive name.
> >>         Though I guess I can see why pop has appeal.
> >>         > >
> >>         > > On Sun, Dec 6, 2020 at 11:44 AM David Feuer
> >>         <david.feuer at gmail.com <mailto:david.feuer at gmail.com>> wrote:
> >>         > >
> >>         > >> I suggest you add a version for Data.Sequence combining
> >>         lookup with deleteAt. I wanted that for something fairly
> recently.
> >>         > >>
> >>         > >> On Sun, Dec 6, 2020, 11:41 AM Martijn Bastiaan via
> >>         Libraries <libraries at haskell.org
> >>         <mailto:libraries at haskell.org>> wrote:
> >>         > >>
> >>         > >>> Yeah, Python's `pop` made me call it `pop`. I had hoped
> >>         to find other
> >>         > >>> examples, but Java, Rust, and Ruby don't seem to offer
> >>         `pop`-like
> >>         > >>> functions for their (hash)maps.
> >>         > >>>
> >>         > >>> On 12/6/20 5:29 PM, Tom Ellis wrote:
> >>         > >>>> On Sun, Dec 06, 2020 at 11:25:33AM -0500, David Feuer
> >>         wrote:
> >>         > >>>>> The name pop makes me think of a stack. Is this use of
> >>         the word common?
> >>         > >>>> Python uses that name, which is why I'm familiar with it:
> >>         > >>>>
> >>         > >>>>>>> d = {'a': 1, 'b': 2}
> >>         > >>>>>>> d.pop('b')
> >>         > >>>> 2
> >>         > >>>>>>> d
> >>         > >>>> {'a': 1}
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