add field/2 for extracting field from struct

[lkarthee]

iex> s = Series.from_list([%{a: 1}, %{a: 2}])
iex> Series.field(s, "a")
#Explorer.Series<
  Polars[2]
  s64 [1, 2]
>
iex> Series.field(s, "b")
* (ArgumentError) field "b" not found in fields ["a"]
    (explorer 0.9.0-dev) lib/explorer/series.ex:6041: Explorer.Series.field/2
    iex:3: (file)

iex> df = DF.new([%{a: %{n: 1}}, %{a: %{n: 1}}])
iex> DF.mutate(df, n: field(a, "n"))
#Explorer.DataFrame<
  Polars[2 x 2]
  a struct[1] [%{"n" => 1}, %{"n" => 1}]
  n s64 [1, 1]
>
iex> DF.mutate(df, n: field(a, "m"))
** (ArgumentError) field "m" not found in fields ["n"]
    (explorer 0.9.0-dev) lib/explorer/series.ex:6041: Explorer.Series.field/2
    iex:3: (file)
    iex:3: (file)

[lkarthee]

Mistakenly clicked some button on mobile app. Will send pr again.

[josevalim]

I can reopen. Feel free to close this one if you prefer it closed. :D

[lkarthee]

I prefer a merge :D

I need few struct features for migrating to pure explorer workflow. I have a json column in database and I am using polars to extract the data.

• field
• json_decode
• json_path_match
• json_encode ??

One PR at a time..

[josevalim]

💚 💙 💜 💛 ❤️