shahryarjb
@before_compile with Module.delete_attribute do not delete @var
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
Most Liked
josevalim
Why do you want to delete them? They will disappear anyway after the module is compiled.
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.
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.
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.
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.








