groupBy with transform

[David Feldman]

It would be nice to have a version of groupBy that also maps the values. This seems like a common scenario when taking apart and reassembling dictionaries, as below.

invert : Dict comparable (List comparable1) -> Dict comparable1 (List comparable)
invert dict =
    let
        f ( x, ys ) =
            List.map (\y -> ( x, y )) ys

        g _ xys =
            List.map Tuple.first xys
    in
        dict
            |> Dict.toList
            |> List.concatMap f
            |> Dict.Extra.groupBy Tuple.second
            |> Dict.map g

Come to think of it, inverting dictionaries is also pretty common.

[Robin Heggelund Hansen]

If I understand your example correctly, this would only save you one line of code (the last .map)? If so, I don't see the point, to be honest.

When it comes to inverting dictionaries, you mean making the keys into values or vice versa? I could see the need for such a function.

[David Feldman]

I guess it feels inefficient to first build the dictionary, then go into each value (a list) and map all the sub-values. Earlier I was trying to speed up a slow dictionary manipulation, which I've since given up on.

Functions to invert dictionaries in general would be useful, too.