Proposal: composition util for Maps

[George Wilson]

+1, and I like the name "compose".

On Mon, 8 Jul 2019 at 06:42, Alexandre Rodrigues <alexandreR_B at outlook.com>
wrote:

> I can recall having had a need for this function a few times, so that’s
> cause for a +1 from me.
>
> “compose” is not a bad name.
>
>
> ------------------------------
> *From:* Libraries <libraries-bounces at haskell.org> on behalf of Elliot
> Cameron <eacameron at gmail.com>
> *Sent:* Sunday, July 7, 2019 8:28:31 PM
> *To:* Tom Murphy
> *Cc:* Haskell Libraries
> *Subject:* Re: Proposal: composition util for Maps
>
> +1 for "compose" and inclusion
>
> On Sun, Jul 7, 2019, 2:17 PM <amindfv at gmail.com> wrote:
>
>> +1 from me on the addition and the name "compose"
>>
>> Tom
>>
>> El 7 jul 2019, a las 13:49, David Feuer <david.feuer at gmail.com> escribió:
>>
>> For what it's worth, I'm fairly confident that implementation is optimal,
>> though I don't know how to even try to prove it. The only situation I see
>> where we could obviously do better is when ab maps many keys to the same
>> value and bc is very large compared to ab. In that case, it would make
>> sense to store a "cache" of lookups into bc. But the constant factors for
>> such an arrangement would be terrible, and there would be no benefit
>> whatsoever in the general case. So I think the simple thing is the right
>> thing here. As for names, I like compose. I support the proposal for the
>> reason Alexandre gives: it feels like a fundamental operation.
>>
>> On Sun, Jul 7, 2019, 1:22 PM Alexandre Esteves <
>> alexandre.fmp.esteves at gmail.com> wrote:
>>
>>> In https://github.com/haskell/containers/issues/647 I proposed the
>>> following util:
>>>
>>> composition :: Ord b => Map b c -> Map a b -> Map a c
>>> composition bc ab = flip mapMaybe ab $ flip lookup bc
>>>
>>> Which has O(|ab| * log |bc|) performance. It's not particularly hard to
>>> write, but it does feel like a primitive-ish operation, and it's not
>>> obvious to me whether there's a faster implementation.
>>>
>>> Other name suggestions
>>> - (.)
>>> - chain
>>> - compose
>>>
>>>
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