insertLookup function ?

[jfaure]

Data.HashMap seems to lack one, although such a function is important for performance reasons.

[ollef]

It's not entirely obvious, but this can be implemented using alterF which was added in 0.2.9, by letting the functor f = (,) (Maybe v). See http://hackage.haskell.org/package/unordered-containers-0.2.10.0/docs/Data-HashMap-Lazy.html#v:alterF.

It would still be nice to have a special-purpose function for this though.

[sjakobi]

What does the implementation in terms of alterF look like?

I suspect that it might not cross the Fairbairn threshold – in that case I'd probably prefer to add it only as an example in the alterF docs, instead of adding a new export.

[ollef]

I believe it would be

insertLookup :: (Eq k, Hashable k) => k -> v -> HashMap k v -> (Maybe v, HashMap k v)
insertLookup k v = alterF (\old -> (old, Just v)) k

Data.Map has a withKey version of this function, which might be reason enough to add at least that.

[sjakobi]

Hm, consistency with containers is also nice. containers is huge though, and that also comes with costs with regard to discoverability and maintainability IMHO.

If there's a choice between making it easier to use alterF and adding more functions that can be implemented with alterF, I'd currently prefer doing the former.