Are module attributes "read" multiple times in same function?

No. The value is basically a static constant, and in fact won’t even take up memory in the process like a normal map, since it will be just a pointer to the shared constant pool.

Yes, the runtime memory is unaffected.
I’m wondering if there are 2 copies during compile time.

The same article says:

Therefore if you read the same attribute multiple times inside multiple functions, you may end-up making multiple copies of it. That’s usually not an issue, but if you are using functions to compute large module attributes, that can slow down compilation.

It is as if you hand typed that map in two places. This can affect compile times, but it generally requires very large constants to be an issue.

Perfect, thanks

To add onto the previous answer. The cost occurs when you make a change.

The following would be calculated twice. If we were doing expensive or numerous changes, it would affect the time taken to compile

@var1 1+1
@var1 1+2

@benwilson512 as a tangent here, could you please teach me how to verify this? I’d love to know how.

If you can read erlang you can use mix decompile to turn your compiled module into erlang source. And you will see that whenever an @attribute is used, the litteral value is present in the source.

Now if you cannot read erlang I guess you will recocgnize your own data enough to verify too.

the copies are inside your def. Note that it is not two copies only,
but 2 times the elements of Earth.list_humans()
if you don’t want to create copies, you may want want refactor your code:

@money %{amount: 0, currency: :DOGE}

def reset_global_debt do
  money = @money
  for human <- Earth.list_humans() do
    update_financial_record_for_human(human, %{
      "total_amount" => money.amount,
      "base_currency" => money.currency
    })
  end

you may even want to do

@money %{amount: 0, currency: :DOGE}

def reset_global_debt do
  amount = @money.amount
  currency = @money.currency

  for human <- Earth.list_humans() do
    update_financial_record_for_human(human, %{
      "total_amount" => amount,
      "base_currency" => currency
    })
  end

I don’t understand this at all, sorry.
How are the elements of Earth.list_humans() known at compile time?

Sorry, You are right. I was wrong about the part about 2 * number of Earth.list_humans(). It is just two copies.

The rest still applies. as you save from duplicating the content of @money, and access the keys of the map for every element in list_humans at run time.