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

It would be enlightening to see some examples of how pop is intended to be used. After reading Philip's response I suspect that users might rather want

State (Map k v) (Maybe v)

and that the Maybe should therefore be inside the tuple. Am Mo., 7. Dez. 2020 um 03:07 Uhr schrieb David Feuer :

That depends on whether you want

State (Map k v) (Maybe v)

or

StateT (Map k v) Maybe v

Both ideas are useful, but for a data structure library I believe the latter is more often what you want, and what more users will expect. The approach you suggest is redundant, and makes the type less informative. You have to read the documentation to know what map gets returned if the key isn't present.

On Sun, Dec 6, 2020, 7:25 PM Keith wrote:

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Think of it as 'pop' again. Your state is the Map. You return Maybe the deleted value and a new state of the Map with the value (maybe) deleted. It makes no sense for your new state to be an empty Map (Nothing) when the key wasn't in the map. — Sent from my phone with K-9 Mail.

On December 6, 2020 11:30:15 PM UTC, David Feuer wrote:

...

It feels very awkward to me.

On Sun, Dec 6, 2020, 5:59 PM Philip Hazelden wrote:

...

Hm. I note that with `minView` and `uncons`, moving the Maybe to the inside doesn't increase the space of return values. That is, any input that gives `Nothing` now would give `(Nothing, empty)` if Maybe was on the inside. That's not true of this function; if the key isn't found, that doesn't tell us what is in the map. Of course it tells us that the second part of the return value is the same as the input map, but that feels not super-close to "the second part of the return value is this one specific value".

And, my sense is that having Maybe on the inside would be more often what people would want to use. Like... if you don't care what the map-without-this-element looks like, you'd just use lookup. So clearly the caller of this function is going to use it in some cases. Maybe-on-the-outside is fine if you'll use it "only if the element was already there", but seems likely awkward otherwise.

So I'd argue for keeping the Maybe on the inside.

On Sun, Dec 6, 2020 at 7:42 PM Simon Jakobi via Libraries wrote:

...

Regarding the type signature:

pop :: Ord k => k -> Map k a -> (Maybe a, Map k a)

I think it might be better to be consistent with similar functions like

minView :: Map k a -> Maybe (a, Map k a)

and

uncons :: [a] -> Maybe (a, [a])

Therefore the type should be

pop :: Ord k => k -> Map k a -> Maybe (a, Map k a)

---

I like the "pop" name though – I think the analogy to stacks is pretty obvious.

---

Like Andreas Abel I believe, that if there is a good, simple implementation, it might be a good first step to simply document the implementation as an example.

If there's much demand for exporting the function or if a fast implementation is more complex, we can still enhance the API later on.

Am So., 6. Dez. 2020 um 17:20 Uhr schrieb Martijn Bastiaan via Libraries :

...

Hi all,

Proposal:

* Add `pop` and `popWithDefault` to `Data.Map` and `Data.IntMap`. * See https://github.com/haskell/containers/pull/757 for exact definition

Why:

* They're useful functions I expected to be in `Data.Map` and `Data.IntMap`. (This might be influenced by the fact that they're defined on Python's `dict`.)

* Their implementations (~ `updateLookupWithKey (\_ _ -> Nothing)`) are harder to parse than a simple `pop`, which should help Haskell codebases become a bit cleaner :).

* Their implementations are a bit non-obvious. My first instinct was to write `(Map.lookup ..., Map.delete ...)`, which would have done two traversals. Having "properly" implemented functions in the lib would prevent people from writing their own suboptimal ones.

Details and implementation:

* https://github.com/haskell/containers/pull/757

Kind regards, Martijn Bastiaan _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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