TIL: I knew about Enum.into but not Map.new.
On Sat, Jul 25, 2020 at 3:08 AM José Valim <jose.va...@dashbit.co> wrote:
> Hi Sabiwara,
>
> This is very close to Map.new/2 (and Enum.into/3):
>
> User |> Repo.all() |> Map.new(fn user -> {user.id, user} end)
>
> So my suggestion is to use Map.new/2 instead of adding a new function. :)
>
> Thanks for the proposal!
>
> On Sat, Jul 25, 2020 at 5:15 AM Sabiwara Yukichi <sabiw...@gmail.com>
> wrote:
>
>> Thank you, I'm glad to see I'm not the only one finding myself needing
>> this :)
>>
>> > I think that we should keep the first value found for each key
>> I really wasn't sure about which one would be more consistent with other
>> Elixir APIs, I actually also have a branch keeping the first occurrence
>> https://github.com/elixir-lang/elixir/compare/master...sabiwara:enum_key_by_first?expand=1
>>
>> Le samedi 25 juillet 2020 11:47:55 UTC+9, Zachary Daniel a écrit :
>>>
>>> I like it, I write this function by hand all the time. I like the name,
>>> and I think that we should keep the first value found for each key.
>>>
>>> On Friday, July 24, 2020 at 10:34:27 PM UTC-4 sabi...@gmail.com wrote:
>>>
>>>> Hi! I would like to propose introducing an `Enum.key_by/3` function
>>>> (name to be discussed) that would return a map where:
>>>> - each key is the result of the first callback applied to an item
>>>> - the value is the result of the second callback applied to the item
>>>> (identity by default)
>>>>
>>>> It would basically be a copy of Enum.group_by/3 keeping only the last
>>>> (or first?) value for each key instead of building a list.
>>>>
>>>> Rationale:
>>>>
>>>> While it could be achieved easily enough using Enum.map/2 and
>>>> Map.new/1, or Enum.into/3, having a function for it could help making the
>>>> code more explicit about the intent and therefore more readable. Like
>>>> Enum.group_by/3, I think it is a fairly common operation and it might be a
>>>> natural candidate for the standard library?
>>>>
>>>> Example use case: from a list of Ecto records, create a map of records
>>>> keyed by `id` for efficient lookups in future code:
>>>>
>>>> User |> Repo.all() |> Enum.key_by(fn user -> user.id end)
>>>>
>>>> If we want to build a map to lookup user names by their ids:
>>>> User |> Repo.all() |> Enum.key_by(fn user -> user.id end, fn user ->
>>>> user.name end)
>>>>
>>>>
>>>> Further considerations:
>>>>
>>>> The typical use cases I have in mind would rely on some kind of unique
>>>> key, so I'm not sure what would be the best API-wise when dealing with
>>>> duplicate keys and have no real opinion:
>>>> - keep the last value found for each key
>>>> - keep the first value found for each key
>>>> This behaviour could always be changed using `Enum.reverse/1` if needed.
>>>>
>>>> What do you think?
>>>>
>>>> I have a working branch for this here, happy to open a PR if you are
>>>> interested:
>>>> https://github.com/elixir-lang/elixir/compare/master...sabiwara:enum_key_by?expand=1
>>>>
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