Updating a map's values that are lists

In functional languages that kind of access is ‘inverted’, you specify the default arguments on the calls instead of the object, thus:

iex(1)> m = %{}
%{}
iex(2)> Map.get(m, :nope, [])
[]
iex(3)> Map.update(m, :nope, [6.28], &[ 6.28 |&1])
%{nope: [6.28]}

Which of course you can wrap up as new calls in your own custom module.
Or do something generic like make a DefaultMap module that delegates to the normal Map module but sets an override default key in the map or something.
Lots of options.

A relevant tag would just be ‘elixir’ or so, but adding tags is a good way to be able to search for topics in the future.

Thanks for the response.

A relevant tag would just be ‘elixir’ or so,

There is no elixir tag shown in the drop down list. And, when I select a tag in error, and I want to remove the tag, I try to click on the the ‘x’ in the tag, but that doesn’t work (in Safari 11.1).

@AstonJ will need to look in to that, but do you get any error message in the javascript console or any error server responses in the network inspector?

Yeah, that doesn’t do what Ruby’s Hash.new does when you specify a block (or Perl’s autovivification). The idea is that you can treat the value returned by a non-existent key the same way you do an existing key. What’s lacking is that Map.udpate/4 does not pass the default value to the function, which I think is a terrible oversight:

If key is not present in map , initial is inserted as the value of key . The initial value will not be passed through the update function

Here’s what Map.update/4 does:

iex(5)> data = %{a: [1, 2], b: [3]}
%{a: [1, 2], b: [3]}

iex(6)> Map.update(data, :c, [], &([10|&1]))
%{a: [1, 2], b: [3], c: []}

iex(7)> Map.update(data, :a, [], &([10|1]))
%{a: [10, 1, 2], b: [3]}

I think Map.update/4 should create the key c: [10] on line 6.

Here is what you can do in Ruby:

2.4.0 :011 => h = Hash.new {|hash, key| hash[key] = []}
=> {}

2.4.0 :012> h[:a] = [1, 2]
=> [1, 2]

2.4.0 :013> h
=> {:a=>[1, 2]}

2.4.0 :014> h[:a] << 10
=> [1, 2, 10]

2.4.0 :015> h[:b] << 10
=> [10]

2.4.0 :016 >; h
=> {:a=>[1, 2, 10], :b=>10]}

See how convenient that is? Perl also allows you to do something similar.

I wish it did that as well, but you can always make your own function do it.

For note, I’ve barely touched Ruby, I find it a very difficult to reason about language due to everything being in flux and so very untyped, so I don’t know what the defaults do, so overall it is better to explain what you want it to do rather than just say ‘this other language thing’ where not everyone knows that other language. ^.^;

I agree, I’ve always found that to be really weird, but I’ve chalked up a lot of Elixir’s weirdness to Ruby, guess it is the opposite this time. ^.^

We generally don’t use ‘elixir’ as a tag since pretty much everything is related to Elixir on this forum (and so I’ve made it a staff only tag)

I’ve added maps and lists as tags as they seem most relevant

Something else to ponder:

iex(1)> data = Map.new()
%{}
iex(2)> Map.fetch(data, :some_key)
:error
iex(3)> some_key = Access.key(:some_key, [])
#Function<4.27061494/3 in Access.key/2>
iex(4)> some_cons = fn (data, item) ->
...(4)>   update_in(data, [some_key], &([item|&1]))
...(4)> end
#Function<12.127694169/2 in :erl_eval.expr/5>
iex(5)> data = data |> some_cons.(:some_data) |> some_cons.(:more_data)
%{some_key: [:more_data, :some_data]}
iex(6)> Map.fetch(data, :some_key)
{:ok, [:more_data, :some_data]}
iex(7)> 

Kernel.update_in/3
Access.key/2

Here are the javascript errors I get when I load the elixir forum page:

[Error] XMLHttpRequest cannot load https://forum.elixirforum.com/message-bus/28cbf0220fed43f297d5f0b8e70b0b09/poll due to access control checks.
	send (ember_jquery-27e777857b8c0730dacfe09cb11711365d21a5db4f9ee0b85d494e4259cf6cda.js:3:16695)
	ajax (ember_jquery-27e777857b8c0730dacfe09cb11711365d21a5db4f9ee0b85d494e4259cf6cda.js:3:14212)
	n (vendor-6e59f8d0190b766ab492d48c83983f67bb7a5ecbff9e213f79fa12e4c72a06e6.js:1:19242)
	(anonymous function) (vendor-6e59f8d0190b766ab492d48c83983f67bb7a5ecbff9e213f79fa12e4c72a06e6.js:1:18300)
[Error] XMLHttpRequest cannot load https://forum.elixirforum.com/message-bus/28cbf0220fed43f297d5f0b8e70b0b09/poll due to access control checks.
	send (ember_jquery-27e777857b8c0730dacfe09cb11711365d21a5db4f9ee0b85d494e4259cf6cda.js:3:16695)
	ajax (ember_jquery-27e777857b8c0730dacfe09cb11711365d21a5db4f9ee0b85d494e4259cf6cda.js:3:14212)
	n (vendor-6e59f8d0190b766ab492d48c83983f67bb7a5ecbff9e213f79fa12e4c72a06e6.js:1:19242)
	(anonymous function) (vendor-6e59f8d0190b766ab492d48c83983f67bb7a5ecbff9e213f79fa12e4c72a06e6.js:1:18300)
[Error] Failed to load resource: A server with the specified hostname could not be found. (analytics.js, line 0)

I don’t get any additional js errors when I try to “x out” a tag.

We generally don’t use ‘elixir’ as a tag since pretty much everything is related to Elixir on this forum

But the editor makes me choose at least one tag, and elixir is not a choice. So, should I choose the Ecto tag when my question has nothing to do with Ecto?

Ahh, okay that works!

I’m wondering if something more generic like ‘language implementation’ or ‘stdlib’ might be good?

The tags displayed change as you type and if the tag you want doesn’t exist, you can create it. So just tag a thread with what the thread is about

The language-implementation tag already exists and stdlib exists as standard-library