Is `@__struct__` an implementation detail or a public API?

[w...@resilia.nl]

Hi all!

I recently needed to check what fields for a struct were defined (if any) for a particular module, at compile-time (potentially in the body of that module, i.e. while the module was still 'open').

(for the curious, it's part of code that builds a runtime type-check from a typespec, see here)

There was no information in Elixir's documentation about this. Looking at the source of `defstruct` we however find that it defines a `@__struct__` module attribute (which is then later on used to implement the introspection method `__struct__/0`).

Now the question is: Is it OK for libraries to depend on `@__struct__` or should it be considered an implementation detail which is subject to change?

-If it's OK: Should we maybe add a sentence to the module documentation of `Module`? Or the documentation of `Kernel.SpecialForms.%/2`?

- If it's not: Is there a 'proper' way to obtain this information for an 'open' module instead?

Thank you all,

~Marten/Qqwy

[eksperimental]

Hi Marten,
You will find this PR interesting
https://github.com/elixir-lang/elixir/issues/10352

Cheers,
- Eks

On Fri, 19 Nov 2021 14:00:40 -0800 (PST)
"w...@resilia.nl" <w...@resilia.nl> wrote:

> Hi all!
>
> I recently needed to check what fields for a struct were defined (if
> any) for a particular module, at compile-time (potentially in the
> body of that module, i.e. while the module was still 'open').

> *(for the curious, it's part of code that builds a runtime type-check

> from a typespec, see here

> <https://github.com/Qqwy/elixir-type_check/blob/21d5de5a1979ebd10d0f30818b6e837062027269/lib/type_check/internals/pre_expander.ex#L265-L270>)*

>
> There was no information in Elixir's documentation about this.
> Looking at the source of `defstruct`

> <https://github.com/elixir-lang/elixir/blob/79cd891eb86ecb7654a7acdb63769cfdd950a5c0/lib/elixir/lib/kernel.ex#L5037-L5046>

[eksperimental]

Sorry, that was the issue.
This is the actual PR https://github.com/elixir-lang/elixir/pull/10354
On Fri, 19 Nov 2021 17:09:55 -0500
"'eksperimental' via elixir-lang-core"

[José Valim]

There is Macro.struct!(module, env), which will expand the struct for a given module accordingly and most likely what you would want to use.

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[w...@resilia.nl]

Thank you both!

`Macro.struct!/2` is perfect for my needs.

~Marten/Qqwy