You could compose a function like this:
```elixir
def rename_keys(map, key_mapping) do
Enum.into(map, %{}, fn {key, value} ->
{Map.fetch(key_mapping, key), value}
end)
end
%{my_map: :is_cool}
|> rename_keys(%{my_name: :my_new_map})
# => %{my_new_map: :is_cool}
```
If you don’t want to put / drop. That way you don’t have to spell out the
99-key map, and it’s easy to pipe together with other functions.
It’s also more efficient than the proposed rename_keys would be as you can
rename multiple keys in one pass.
Adam
> On 12 Oct 2020, at 16:49, jonar...@gmail.com <jonarnet...@gmail.com> wrote:
>
> > Maybe if you want to keep the other keys as is, then "rename_key" can be
> > handy
>
> This is the use case I had in mind. Let's say I have a map with 100 keys, but
> only need to rename one of them. In that event, today I would write something
> like this:
>
> {val, new_map} = Map.pop(original_map, :original_name)
> Map.put(new_map, :new_name, val)
>
> Explicitly defining a new map with 99 keys matching exactly and only one
> changed would be a very large amount of effort, and brittle. With a
> Map.rename_key/2 function, I could write the following, which (to Bruce's
> point) would fit nicely in a pipe chain:
>
> Map.rename_key(original_map, :original_name, :new_name)
>
> > what happens if you add a new key to post? Does it automatically appear in
> > the new_map? Or should it not?
>
> The new key should appear in the new map. The only thing that
> Map.rename_key/3 (or Map.rename_keys/3) should do is exchange one key for
> another, without consideration for the rest of the map at all. I enjoy using
> maps where flexibility is more important than the guarantees that the
> strictness of structs. If I were converting one struct to another, I would
> explicitly map each key as in you example above
>
> new_thing = %Thing{
> user_id: post.author_id,
> content: post.body,
> ...
> }
>
> The application of Map.rename_key[s]/3 is more for a proxy application, where
> data is being shuttled from one place to another with a minor modification. I
> may or may not know (or care) what the entirety of the map looks like, I
> simply know that if a given key is present it needs to be replaced with a
> different one.
> On Monday, October 12, 2020 at 7:25:46 AM UTC-4 José Valim wrote:
> Hi everyone,
>
> I thought I had commented on this thread but apparently I have not, so
> apologies for the delay.
>
> I am not convinced about this functionality because I honestly do not find
> this:
> new_map =
> post
> |> Map.rename_key(:user_id, :author_id)
> |> Map.rename_key(:body, :content)
> clearer than this:
>
> new_map = %{
> user_id: post.author_id,
> content: post.body,
> ...
> }
>
> Maybe if you want to keep the other keys as is, then "rename_key" can be
> handy, but even then, what happens if you add a new key to post? Does it
> automatically appear in the new_map? Or should it not?
>
> Even if we say that "clearer" is personal, there are practical reasons for
> preferring the latter, such as the runtime can optimize it better (as all
> keys are literals and the map is not built dynamically), and it is easier to
> typecheck maps with known keys.
>
> So my $.02 here is that this is not something I would necessarily endorse
> and, if you really want to rename only certain keys inside a map, you can do
> it with a helper function or by using put+drop on the desired keys.
>
> Thanks!
>
>
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