Hi!
I've recently discovered that get_and_update_in/3 and friends can access
multiple items at once via Access.all/0, Access.filter/1, and similar. It's
a game changer for accessing deeply nested structures.
However, there is no function in the standard library for accessing
multiple items of a map (similarly to items of a list). For maps where keys
are something item IDs and not predefined atoms, I think such accessors
would be very useful. In our codebase, for example, we often manipulate
deeply nested structures that include such maps and want to access all
values in such maps (e.g., when migrating data).
It's easy to write an accessor that accesses all values in a map (and
similarly for keys or both keys and values):
@doc """
Returns a function that accesses all values in a map.
The returned function is typically passed as an accessor to
`Kernel.get_in/2`,
`Kernel.get_and_update_in/3`, and friends.
## Examples
iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
iex> get_in(users, [AccessUtils.values(), :age])
[27, 23]
iex> update_in(users, [AccessUtils.values(), :age], fn age -> age + 1
end)
%{"john" => %{age: 28}, "meg" => %{age: 24}}
iex> put_in(users, [AccessUtils.values(), :phone], nil)
%{"john" => %{age: 27, phone: nil}, "meg" => %{age: 23, phone: nil}}
"""
@spec values() :: Access.access_fun(data :: map(), current_value :: list())
def values do
fn
:get, %{} = data, next ->
Enum.map(data, fn {_key, value} -> next.(value) end)
:get_and_update, %{} = data, next ->
{reverse_gets, updated_data} =
Enum.reduce(data, {[], %{}}, fn {key, value}, {gets, data_acc} ->
case next.(value) do
{get, update} -> {[get | gets], Map.put(data_acc, key, update)}
:pop -> {[value | gets], data_acc}
end
end)
{Enum.reverse(reverse_gets), updated_data}
end
end
Nevertheless, I think it might be useful to include it in the Access module
of the standard library. What do you think?
Note that something similar has already been discussed on Elixir Forum
<https://elixirforum.com/t/how-to-use-update-in-on-multiple-map-keys-at-once-re-access-all/63963>
.
Best regards,
David
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