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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