Proposal: add Dict.intersectBoth

[mgold]

I recently found myself writing this code:

let commonKeys = Dict.intersect dict1 dict2 |> Dict.keys
    unsafeGet k d = Dict.get k d |> (\(Just x) -> x)
in List.map (\k -> (unsafeGet k dict1, unsafeGet k dict2)) commonKeys

This requires the unsafe (\(Just x) -> x) operation, even though I know it's safe because I'm only looking at keys in the intersection. Although, I was using the union at first, and that blew up in a runtime error, so this is a real runtime error that we can avoid by offering this API in Dict:

intersectBoth : Dict comparable v -> Dict.comparable w -> Dict.comparable (v,w)

The current intersect function keeps only the values from the first dictionary, even though we know they're also in the second one (because it's the intersection). It's pretty useless in my case:

intersect : Dict comparable v -> Dict comparable v -> Dict comparable v

A more extreme change would be to replace it with the function I've called intersectBoth, since you can get back the old behavior with Dict.map (always fst). But if you don't want to do that, adding another function shouldn't be too bad. Because the representation of dictionaries is opaque, this can't be done in a third party library.

[evancz]

I am a fan. I'd want it to be something like this:

intersectWith : (a -> b -> c) -> Dict comparable a -> Dict comparable b -> Dict comparable c

So you could do intersect = intersectWith always and intersectBoth = intersectWith (,). I'm not certain about switching the meaning of intersect but I'm doing some core stuff now, so I'll think about this more.

[evancz]

I am a fan. I'd want it to be something like this:

intersectWith : (a -> b -> c) -> Dict comparable a -> Dict comparable b -> Dict comparable c

So you could do intersect = intersectWith always and intersectBoth = intersectWith (,). I'm not certain about switching the meaning of intersect but I'm doing some core stuff now, so I'll think about this more.

[mgold]

I can live with intersectWith. Thanks.

[mgold]

I can live with intersectWith. Thanks.

[mgold]

I don't think the commit referenced above was ever released, but it's covered by Dict.merge in 4.0.1.

[mgold]

I don't think the commit referenced above was ever released, but it's covered by Dict.merge in 4.0.1.