SyntaxError when defining module attribute with numeric-starting key

[vinibrsl]

Environment

• Elixir & Erlang/OTP versions (elixir --version): Elixir 1.11.3 (compiled with Erlang/OTP 23)
• Operating system: macOS 11.2.2

Current behavior

When declaring module attributes starting with numbers, a syntax error pops up.

defmodule Test do
  @5_days_in_seconds 5 * 24 * 60 * 60
end
** (SyntaxError) syntax error before: '_days_in_seconds'

Expected behavior

1. Is there any reasons not to accept numbers to start module attribute keys?
2. If there is, should this syntax error message be more detailed?

[wojtekmach]

My guess is module variables follow the same naming rules as regular variables:

iex(1)> 5_days_in_seconds = 5 * 24 * 60 * 60
** (SyntaxError) iex:1:2: syntax error before: '_days_in_seconds'

[josevalim]

Correct @wojtekmach! In fact, @ is a unary operator like plus, not a special token, so you could actually write this @ foo and it would work:

iex(1)> quote do: @foo
{:@, [context: Elixir, import: Kernel], [{:foo, [context: Elixir], Elixir}]}
iex(2)> quote do: @ foo
{:@, [context: Elixir, import: Kernel], [{:foo, [context: Elixir], Elixir}]}

For now, I think we can detect the case of identifiers starting with numbers in the tokenizer and raise accordingly.

[Eiji7]

Is there any reasons not to accept numbers to start module attribute keys?

@vnbrs As mentioned above module attribute have exactly same syntax as variables:

Source: https://github.com/itsgreggreg/elixir_quick_reference#variables

I can only guess why it's like that, but I would say that's for support syntax like: 0b1 (in this case it's an integer in a binary representation).

btw. Those examples are also worth to investigate:

iex(1)> 0b2
** (SyntaxError) iex:1:2: syntax error before: b2

iex(1)> 0o9
** (SyntaxError) iex:1:2: syntax error before: o9

iex(1)> 0xG
** (SyntaxError) iex:1:2: syntax error before: xG

[kipcole9]

Elixir is Unicode-centric and part of the decision @josevalim made is to follow the identifier syntax which is described in Unicode technical report 31 (with some incredibly minor differences). The core team also makes backward compatibility a commitment so your proposal is unlikely to get traction. If you feel passionately that the change has merit, then the recommended approach is to gather consensus in Elixir Forum or contribute a proposal to the Elixir Core mailing list.