>
> 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)
Where exactly is this documented?
On Sunday, July 17, 2016 at 5:04:43 PM UTC+3, José Valim wrote:
>
>
>
>> 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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