I’ve seen this referred to as `key_by` in a few other contexts, such as
lodash <https://lodash.com/docs/#keyBy>. When I was first learning Elixir
from a JS background, I recall looking for a comparable function in Enum a
few times, but I believe the `Map.new/2` approach is idiomatic.
On Sunday, May 8, 2022 at 7:39:57 AM UTC-7 José Valim wrote:
> I think a code snippet here would help but it does feel like Map.new does
> most of the job:
>
> Map.new(collection, &{&1.email, &1})
>
> I am not convinced about the name index_by. I understand the meaning of
> index here, but it is not what index means in Enum (i.e. an integer).
>
> On Sun, May 8, 2022 at 3:31 PM Adam Lancaster <[email protected]> wrote:
>
>> Map.new is the right approach in my humble opinion, i’m not sure what
>> index_by would add over that. Keen to hear other opinions though.
>>
>> The function proposed sounds more like “group_by_index” to me but that’s
>> just me.
>>
>> Best
>> Adz
>>
>> On Sun, 8 May 2022 at 14:24, Tobias Pfeiffer <[email protected]> wrote:
>>
>>> Hello everyone,
>>>
>>> I'd like to propose `Enum.index_by/2` (or potentially /3, see further
>>> down).
>>>
>>> Essentially, it is supposed to be same thing was `Enum.group_by/3` but
>>> without putting the values into a list - so one key, one item.
>>>
>>> It is inspired by the Rails Enumerable method of the same name [1].
>>>
>>> Now, why should we have this?
>>>
>>> I believe the use case, while not super common, is common enough and
>>> valid. Most often I see it when you have 2 different collections of data
>>> and you want to stitch them together. For instance, you have accounts of
>>> your users on another platform and you want to associate your users to
>>> those accounts - so you fetch your users, index them by their email as a
>>> key and then you can go through the other list effectively fitting them
>>> together with your user. Or take payment transactions in an external
>>> payment system and payment transactions as tracked in your application.
>>>
>>> I'm not aware of a "nice" way to do this in Elixir right now. The "best"
>>> would probably be `Map.new/2` although most often I see people fall back to
>>> `Enum.reduce/3` which I consider sub optimal for most cases (thankfully
>>> most common usages of `Enum.reduce/3` usually have a better equivalent in
>>> the language itself).
>>>
>>> While `Enum.group_by/2` does most of what I want here, transforming all
>>> the one element lists into just the element to easily work with it is
>>> somewhat annoying.
>>>
>>> Speaking of which, it might be fair to say that if we want to do this
>>> we'd want `Enum.index_by/3` to mirror `Enum.group_by/3` (with an optional
>>> value function) which sounds good to me.
>>>
>>> Happy to tackle the implementation myself.
>>>
>>> Thank you + cheers,
>>> Tobi
>>>
>>> [1]
>>> https://api.rubyonrails.org/classes/Enumerable.html#method-i-index_by
>>>
>>> PS: Have a bunny
>>>
>>> [image: IMG_20220130_140748_Bokeh.jpg]
>>>
>>>
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