F# and Series, How to write a Series.choose?

Question

I have decided to write a Series.choose function, similar to the one we have on Seq and I wanted to know if you think it is the best way to write it, going back to a Seq as there is a function there.

let choose (f: 'K * 'V -> ('K * 'U) option) (e : Series<'K,'V>) =
        e |> Series.map (fun k v -> k, v)
          |> Series.values
          |> (Seq.choose f)
          |> Series.ofValues
          |> Series.map (fun _ v -> snd v)
          |> Series.indexWith (e |> Series.keys)

Thanks

Answer 1

mapAll is nearly what you need, it just has a slightly different signature (and in particular, can't change the keys). If you do need to change the keys, it becomes more difficult but still shouldn't require going via Seq. It also depends on how you want to deal with missing values in e.

Untested (because I don't have Visual Studio here), and assumes e has no missing values, but should give the idea:

let choose (f: 'K * 'V -> ('K * 'U) option) (e : Series<'K,'V>) =
    e |> Series.mapAll (fun k (Some v) -> f (k, v)) // Series<'K, ('K, 'U)> 
      |> Series.dropMissing // drops cases where f returned None
      |> fun e1 -> e1.SelectKeys (fun kvp -> fst kvp.Value.Value) // changes the keys to the ones f returned
      |> Series.mapValues snd // removes the keys from the values