> 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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