Proposal: composition util for Maps

[Elliot Cameron]

+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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