Map with fat arrow vs colon (Poison - json decode)

Question

ab = %{a: 1}
ac = %{"a" => 1}

What exactly is the difference? And why does Poison.decode! return format #2 (I need #1 to use with Ecto.Changeset.change).

Json response is taken from API and looks like

[{"a":3}]

Figural

Answer 1

ab = %{a: 1} # atom key you can access it like ab.a
ac = %{"a" => 1} # string key you can access it ac["a"]

Poison.decode! return format #2:

Data which comes from outside your application, broadly speaking, can’t be trusted. Given that atom allocation can lead to memory exhaustion in a long-running Erlang system, using atoms for external data opens up your app for a potential denial of service (DoS) attack.

This fact is reflected in many Elixir libraries, for example the popular JSON parser Poison. Well-behaved libraries will generally use strings as map keys when converting external data into an internal data structure

If you want to convert map keys from string to atom.You can do:

iex> maps = %{"foo" => "bar"}
%{"foo" => "bar"}
iex> for {key, val} <- maps, into: %{}, do: {String.to_atom(key), val}
%{foo: "bar"}

Mallett

Use String.to_existing_atom/1, to be a bit safer. Using String.to_atom/1 would open up exactly the vulnerability mentioned in this answer. – Theogony

Also, accessing a value by atom key, as ab.a in the example, can also be done with ab[:a]. The bracket syntax is the more general access syntax, while the dot syntax is sugar available for maps and structs, when the key is an atom. – Theogony