+1 from me on the addition and the name "compose" Tom

El 7 jul 2019, a las 13:49, David Feuer 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 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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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 on behalf of Elliot Cameron 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 mailto:amindfv@gmail.com> wrote: +1 from me on the addition and the name "compose" Tom El 7 jul 2019, a las 13:49, David Feuer mailto:david.feuer@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 mailto:alexandre.fmp.esteves@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 _______________________________________________ Libraries mailing list Libraries@haskell.orgmailto:Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries _______________________________________________ Libraries mailing list Libraries@haskell.orgmailto:Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries _______________________________________________ Libraries mailing list Libraries@haskell.orgmailto:Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

If someone wants to play around with the caching idea, this is (at least approximately) how I'd write it: data Res c a = Hit !(Maybe c) | Miss !(Maybe c) a deriving Functor compose bc ab = flip evalState Strict.empty $ traverseMaybeWithKey go ab where go _ b = do cache <- get case Strict.alterF (update b) b cache of Hit mc -> pure mc Miss mc cache' -> do put cache' pure mc update b Nothing = let mc = lookup b bc in Miss mc (Just mc) update _ (Just mc) = Hit mc On Sun, Jul 7, 2019, 1:49 PM David Feuer wrote:

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@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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