> Regarding #2 - Without a clearer understanding of the formal
> specification of the language (ala #1) that tells me otherwise, I
> still think (your corrected) Version #1 should be legal Elixir but
> perhaps just requires a different mechanism for identifying when the
> Module/Struct is fully defined. A two pass system could absolutely
> still work this way.
Since this seems to be the contention point, let's try to understand why
this is the only possible way for this to work. Imagine you have this code:
variable = System.get_env("DEFINE_BAR")
defmodule Foo do
if variable in ~w(1 true) do
defmodule Bar do
end
end
end
If the module Bar was defined during compilation, Bar would be *always*
defined, because compilation does not know the value of the variable nor it
would evaluate the if (since it depends on runtime values). Code that is
expanded, is always expanded, even if inside a conditional. If the module
was defined during expansion then the module would always be defined.
In other words, a struct expansion that happens at compile time, cannot
depend on values that are in the same context and will be defined only at
runtime. That's the same reason why you can't invoke a function in the same
context you define it. The reason why multiple modules work is because we
first expand both modules and then we execute them.
While I personally think this behaviour is documented (because the two
passes compilation is documented as part of the macro system as well as the
fact that structs are expanded during compile time), I would agree it may
take a while to build the intuition around it and we could very likely
benefit from more examples and discussions.
*José Valim*
www.plataformatec.com.br
Skype: jv.ptec
Founder and Director of R&D
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