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
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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