CompileError while creating a struct

[Dzianis Dashkevich]

Hi everyone,

I'm trying to define a simple struct and create one in a single file:

defmodule Subscriber do
  defstruct name: "", paid: false, over_18: true
end

subscriber = %Subscriber{}

When I finally execute it with elixir subscriber.exs it fails:

** (CompileError) maps/subscriber.exs:5: cannot access struct Subscriber, the struct was not yet defined or the struct is being accessed in the same context that defines it

    (elixir) src/elixir_map.erl:44: :elixir_map.translate_struct/4

But it succeeds when I execute it without the last line in it. Am I missing something?

[Eric Meadows-Jönsson]

You cannot use the a struct in the same context it is defined. If you split the definition and usage into separate files it will work.

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Eric Meadows-Jönsson

[Dzianis Dashkevich]

Thanks, Erik!

[Onorio Catenacci]

On Saturday, November 15, 2014 12:13:19 PM UTC-5, Eric Meadows-Jönsson wrote:

You cannot use the a struct in the same context it is defined. If you split the definition and usage into separate files it will work.

Hi Eric,

I saw your response and I wanted to ask for a little further clarification.  I've got code analogous to this:

defmodule Test do

  defmodule Test.Struct do

    defstruct fields: [] , field_types: [], results: [] 

    @type t :: %Test.Struct{fields: List, field_types: List, results: List}

  end

  def testf do

    ts = %Test.Struct{

      :fields => [],

      :results => []

    }

  end

end    

And I can call testf just fine.  Does naming the module in which the struct is defined as Test.Struct qualify as a different context?  I'm assuming it does but I just wanted to insure I've got the right understanding of the situation.

--

Onorio

 

[Eric Meadows-Jönsson]

Not sure I understand the question. What you name the module has nothing to do with where you can use the struct.

To clarify. Where you can use the struct depends on when things are expanded by the compiler and when they are evaluated. Take this example:

defmodule Foo do
  defstruct []
end

%Foo{}

The above code won’t work because the compiler will try to expand %Foo{} before the module or the struct is defined. Expansion happens before evaluation and the struct will be defined when the call to defmodule is evaluated.

In your example you can use struct inside the function because macros in the function are expanded after the struct is defined.

​

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Eric Meadows-Jönsson

[Onorio Catenacci]

Thanks for the clarification Eric!

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Onorio