@before_compile with Module.delete_attribute do not delete @var

shahryarjb

Hello, Based on typed_struct/lib/typed_struct.ex at main · ejpcmac/typed_struct · GitHub , I am practicing to learn more about macro in elixir, all the things are good but I can not delete the attr I register in module.

For example this is my macro

  @temporary_revaluation [
    :gs_fields,
    :gs_types,
    :gs_enforce_keys
  ]

defmacro guardedstruct(opts \\ [], do: block) do
    ast = register_struct(block, opts)
    quote do
        (fn -> unquote(ast) end).()
    end
end

def register_struct(block, opts) do
    quote do
        Enum.each(unquote(@temporary_revaluation), fn attr ->
        Module.register_attribute(__MODULE__, attr, accumulate: true)
        end)
    
        Module.put_attribute(__MODULE__, :gs_enforce?, unquote(!!opts[:enforce]))
        @before_compile {unquote(__MODULE__), :delete_temporary_revaluation}
        ...
    end
end

So the macro deletes attrs is:

  defmacro delete_temporary_revaluation(%Macro.Env{module: module}) do
    Enum.each(unquote(@temporary_revaluation), &Module.delete_attribute(module, &1))
  end

With these codes I try to delete module attributes. but inside the module I use this macro, I can still access to all the

  @temporary_revaluation [
    :gs_fields,
    :gs_types,
    :gs_enforce_keys
  ]

For example:

defmodule MishkaPub.ActivityStream.Type.Object do
  use GuardedStruct
  guardedstruct do
    field(:id, String.t())
    field(:type, String.t())
  end
  
  def list_attributes do
    @gs_fields
  end
end

As you see I can print @gs_fields. but inside the macro I tell delete all of them before compile!!

I want to handle this inside macro not the module

Please help me to find what is the problem?

Full code:

Thank you in advance

10 comments

#defmacro

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2023-07-01 20:28:48 UTC

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zachallaun

Module attributes are expanded during compilation, not runtime, so the @gs_fields in your function call isn’t actually referencing an attribute when you call it, it’s the literal value that was inserted into the AST.

Post #1

al2o3cr

@before_compile callbacks are called after the rest of the AST of the module has already been generated, so it will run after the list_attributes function is defined and @gs_fields is interpolated.

Post #2

shahryarjb

So if I do not call inside list_attributes, it will delete?, if not how can delete all the attr after creating struct?

Is there a way to see all global @var outside of the module? in iex for example??

Post #3

shahryarjb

As I understand, it exists because I call it? if I do not call it, it will be deleted?, and is there a way to prevent?

Another question, it can increase module size after compile? I just want to delete unnecessary things from my module

Post #4

D4no0

Attributes are present only before your module is compiled, as mentioned above, there is no way to get attributes at runtime as they don’t exist anymore in runtime context, their usage is replaced with their contents.

Post #5

shahryarjb

So why should I delete it in this example? it does not matter exists or not?

https://github.com/ejpcmac/typed_struct/blob/main/lib/typed_struct.ex#L214

Post #6

D4no0

This is obviously deleting the attributes to follow some internal logic, I am too tired now to read these macro abominations. The only reason you would delete attributes besides for a logic flow, would be to free memory when compiling, but I doubt this is the case here.

Post #7

shahryarjb

is there a way to delete these? after compile and in runtime? or we should create eval all the module again to prevent this? just for example I said

Post #8

josevalim

Creator of Elixir

Why do you want to delete them? They will disappear anyway after the module is compiled.

Post #9

shahryarjb

Thanks sir, I thought they will be there after compilation.

I asked it because I saw this link delete all the attrs of module. it did do any effect? or it is just extra code there without any effect
https://github.com/ejpcmac/typed_struct/blob/main/lib/typed_struct.ex#L214

Post #10

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