Add a `lookupKey` function to aid with sharing / interning

[sjakobi]

I noticed that haskell-language-server is putting a lot of effort into reducing memory usage by replacing certain keys with the keys that are already stored in a map. There is an explanation here, and another instance here: haskell/haskell-language-server#4759 ( a bit embarrassing for me ;) )

I wonder if we could make this much simpler if we'd offer a function

lookupKey :: Hashable k => k -> HashMap k v -> Maybe k

[fendor]

I think to simplify the code, it'd need to be:

lookupKey :: Hashable k => k -> HashSet k -> Maybe k

as the complexity stems from having to convert HashSet k -> HashMap k k and then doing the lookup.

However, I like the idea :)

[sjakobi]

So, since HashSet.toMap simply removes the newtype wrapper around the inner HashMap k (), it wouldn't be hard to use the same lookupKey function on sets too.

But we can add a HashSet version anyway.

The reason why I proposed lookupKey to operate on maps, is because I think it would allow you to get rid of KnownTargets.normalisingMap.

[fendor]

Yes, it looks like it would get rid of that as well!

[sjakobi]

I've realized that the HashSet version shouldn't be named lookupKey: Sets don't have keys, but elements or members.

So, how about these options?

• HS.lookup
• HS.lookupElement
• HS.lookupMember