Expose getting process dictionary value for specific key in Process.info/2

[michallepicki]

This functionality is present since OTP 26.2:
erlang/otp#7707

Discussed at #12857 (comment)

[sabiwara]

LGTM!

[sabiwara]

[nit] Maybe this one is too imprecise here? Since the key will always be an atom, keyword might be more accurate.

Or should we refer :erlang.process_info_result_item() which is a bit more precise? But dialyzer would generalize to atom anyway.

[josevalim]

I don't think this function can actually return individual dictionary keys? 🤔

[michallepicki]

[nit] Maybe this one is too imprecise here? Since the key will always be an atom, keyword might be more accurate.

I don't think this function can actually return individual dictionary keys? 🤔

Good points about this function clause, I will revert this change to info/1 spec

Or should we refer :erlang.process_info_result_item() which is a bit more precise? But dialyzer would generalize to atom anyway.

Unfortunately the type is not exported, Dialyzer says:

lib/process.ex:840:30: Unknown type erlang:process_info_result_item/0

[josevalim]

I would actually remove the specific clauses. Now that Erlang/OTP has good error messages, they will probably be more decent than function clause error?

Suggested change

	def info(pid, {:dictionary, key}) do
	nilify(:erlang.process_info(pid, {:dictionary, key}))
	end
	def info(pid, spec) when is_atom(spec) or is_list(spec) do
	nilify(:erlang.process_info(pid, spec))
	def info(pid, spec) do
	nilify(:erlang.process_info(pid, spec))

[michallepicki]

On Erlang 26.2.2 the error looks like:

iex(1)> Process.info(self(), :unknown)
** (ArgumentError) errors were found at the given arguments:

  * 2nd argument: invalid item or item list

    :erlang.process_info(#PID<0.114.0>, :unknown)
    (elixir 1.17.0-dev) lib/process.ex:864: Process.info/2
    iex:1: (file)

[michallepicki]

Suggested change

	@spec info(pid, process_info_item | [process_info_item]) ::
	process_info_result_item | [process_info_result_item] | nil
	@spec info(pid, process_info_item) :: process_info_result_item | nil
	@spec info(pid, [process_info_item]) :: [process_info_result_item] | nil

I think this could work as well and be better for documentation (althoguh maybe Dialyzer would unify these types internally?)

[michallepicki]

Pleasantly surprised to report that this improves the spec for Dialyzer as well:
for {:should_warn, _} = Process.info(self(), [:some_atom]):

The pattern
          {'should_warn', _} can never match the type
          'nil' | [{atom() | {'dictionary', _}, _}]

and for [{:should_warn, _}] = Process.info(self(), :some_atom)

The pattern
          [{'should_warn', _}] can never match the type
          'nil' | {atom() | {'dictionary', _}, _}

[josevalim]

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