[Proposal] Module.attribute_registered?/3 or Add Module.has_attribute?/3

[Allen Madsen]

In working with the ExUnit code, there are at least two places where I think being able to introspect on module attributes would be useful.

One is here. In this code it would be useful to know if a module attribute has been set already. This code checks to see if it has a value. However, if a user set the value to `nil`, this code wouldn't be able to tell.

Another situation it would be useful, is in this PR: https://github.com/elixir-lang/elixir/pull/9181. Specifically, this check, where it would also be useful to know if the attribute is registered with accumulate or not.

So, the proposal is an API something like this:

defmodule X do

  Module.register_attribute(__MODULE__, :foo, accumulate: true)

  @bar true

 

  Module.attribute_registered?(__MODULE__, :foo) #=> true

  Module.attribute_registered?(__MODULE__, :foo, accumulate: true) #=> true

  Module.attribute_registered?(__MODULE__, :foo, persist: true) #=> false

  Module.attribute_registered?(__MODULE__, :bar) #=> true

  Module.attribute_registered?(__MODULE__, :bar, accumulate: true) #=> false

  Module.attribute_registered?(__MODULE__, :baz) #=> false

end

The third argument would be a subset match of the options. 

Thoughts?

[José Valim]

Thanks for the proposal.

I like has_attribute?. I prefer it over attribute_registered? because what would happen if you set an attribute without registered it? Should it return false for attribute_registetered? So has_attribute? is less ambiguous, although I would only support has_attribute?/2 for now.

Notice in order to implement this, you will have to change register_attribute to store something when called without the :persist or :accumulate. But it should be a matter of adding an else here: https://github.com/elixir-lang/elixir/blob/master/lib/elixir/lib/module.ex#L1308-L1311

Something like:

else
  :ets.insert_new(set, {attribute, nil, nil})
end

We need to document that once an attribute has been registered to accumulate or persist, it can't be done undone unless the attribute is deleted.

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Director of R&D

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[Allen Madsen]

> We need to document that once an attribute has been registered to accumulate or persist, it can't be done undone unless the attribute is deleted.

How is it undone today?

Allen Madsen
http://www.allenmadsen.com

To view this discussion on the web visit https://groups.google.com/d/msgid/elixir-lang-core/CAGnRm4%2Bb4afMwPpQP6Jv%3D_bCWJvRskNd%3DJJA_OHArJMH7621kg%40mail.gmail.com.

[José Valim]

It can’t be undone today either. This was just a general note. It probably belongs to a separate PR.

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