I am just doing some poking around with module attribute and I created the following code:
defmodule ModuleAttribute do
Module.register_attribute(__MODULE__, :test_attribute, accumulate: true)
@test_attribute 1
@test_attribute 10
def increase(), do: IO.inspect(@test_attribute, label: "Value")
@test_attribute 20
end
When I run iex -S mix
i get the following values
iex(40)> ModuleAttribute.increase
Value: [10, 1]
If my understanding of elixir compilation is correct, should it be [1, 10, 20]? I am not sure why 20 didn’t get added in during compilation.
This is correct. Module attributes always have the value assigned to it in lines above the call. Basically you “hardcode” the parameter of IO.inspect
at the time line 5 is compiled.
1 Like
NobbZ
June 4, 2019, 12:29pm
3
20
isn’t in the list, because you only add it to the list after reading it. Each usage of a module attribute will only ever know about values that have been assigned before its usage, pretty much like regular variables.
1 Like
@LostKobrakai and obbZ thanks for your quick response. I understand now.
One quick question about Module.register_attribute/3
, is this function used to track the changes made to a module attribute?
For note, module attributes are a lot like normal bindings, just saving you from typing the unquote
part.
So this:
defmodule ModuleAttribute do
@thing 1
@thing 2
def blah(), do: @thing # returns 2
@thing 3
end
Is the same as this:
defmodule ModuleAttribute do
thing = 1
thing = 2
def blah(), do: unquote(thing) # returns 2
thing = 3
end
And thus this with an accumulated attribute:
defmodule ModuleAttribute do
Module.register_attribute(__MODULE__, :thing, accumulate: true)
@thing 1
@thing 2
def blah(), do: @thing # returns [2, 1]
@thing 3
end
Is the same as this:
defmodule ModuleAttribute do
thing = []
thing = [1 | thing]
thing = [2 | thing] # So this is now [2, 1]
def blah(), do: unquote(thing) # returns [2, 1]
thing = [3 | thing]
end
4 Likes