Prefix/Suffix map keys proposal

[Tiago Botelho]

A common pattern that I've found myself using is having to prefix a few keys (namely `ids`) which I do with following approach:

```

%{}

|> Map.put(:prefix_key_1, map.key_1)

|> Map.put(:prefix_key_2, map.key_2)

=> %{prefix_key_1: :val_1, prefix_key_2: :val_2}

```

or 

```

%{prefix_key_1: map.key_1, prefix_key_2: map.key_2}

=> %{prefix_key_1: :val_1, prefix_key_2: :val_2}

```

It seems to me that we already have good functionality for taking specific keys out of a map by using Map.take/2 

(which is effectively the same thing we are doing in the examples above) but no good way of bulk updating specific key names.

My proposal would be to be able to pass a `prefix` as an option to `Map.take` which would look roughly like the following:

```

Map.take(map, [:key_1, :key_2, :key_3], prefix: :prefix)

=> %{prefix_key_1: :val_1, prefix_key_2: :val_2, prefix_key_3: :val_3}

```

Pros:

* A simpler interface to take keys from a map and "rename" them according to the context they will be used for

Cons:

* Using the `prefix` option would mean that all the keys would get prefixed, a mixed approach would still require the examples showed above.

* Might make Map.take do too much at once 

Since the Cons might be might seem to discourage this approach, another proposal would be to introduce Map.prefix/2 and Map.suffix/2 which would only

update the chosen keys in a map like so:

     

     ```

     map

     |> Map.take([:key_1, :key_2, :key_3, :key_4])

     |> Map.prefix([:key_3, :key_4], :prefix)

     => %{key_1: :val_1, key_2: val_2, prefix_key_3: :val_3, prefix_key_4: :val_4}

    ```

Interested in hearing your thoughts 😊

Thank you,

Tiago

[Alexei Sholik]

It's functionality that is very specific to a particular use case to be added to a standard library module. Plus, it either needs to create new atoms (which standard library will never do) or fail for some inputs (which is again not generally useful):

iex(1)> m = %{key_1: :val_1, key_2: :val_2}

iex(4)> m |> Map.take([:key_1, :key_2]) |> Map.new(fn {key, val} -> {String.to_existing_atom("prefix_#{key}"), val} end)
** (ArgumentError) argument error
    :erlang.binary_to_existing_atom("prefix_key_1", :utf8)
    (stdlib) erl_eval.erl:680: :erl_eval.do_apply/6
    (stdlib) erl_eval.erl:888: :erl_eval.expr_list/6
    (stdlib) erl_eval.erl:240: :erl_eval.expr/5
    (elixir) lib/map.ex:216: Map.new_transform/3
iex(4)> :prefix_key_1
:prefix_key_1
iex(5)> :prefix_key_2
:prefix_key_2
iex(6)> m |> Map.take([:key_1, :key_2]) |> Map.new(fn {key, val} -> {String.to_existing_atom("prefix_#{key}"), val} end)

%{prefix_key_1: :val_1, prefix_key_2: :val_2}

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[Tiago Botelho]

That is a great point, thank you for clarifying! 😊

[Allen Madsen]

One other note, not all keys are atoms. Any term can be used as a key.

Allen Madsen
http://www.allenmadsen.com

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