"defstruct" diffrenet behaviors in iex and elixir

Questions / Help
, ,
1

Programming Elixir page 80 about “defstruct”

I first ask question and I am a quite beginner. I followed this book again to confirm each steps’ process.

Following book by using iex, it works. But implemented IO.puts into code and did “Elixir defstruct.exs” so that it didn’t work with Error.
please help me why the error occur and how to solve this.
code:

defmodule Subscriber do
  defstruct name: "", paid: false, over_18: true
end
s1 = %Subscriber{}
IO.puts inspect(s1)

Error:
** (CompileError) defstruct.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

2

You have your lines of:

s1 = %Subscriber{}
IO.puts inspect(s1)

Outside of a function, you should put them in a function.

You can also run it in iex directly as well:

iex1> defmodule Subscriber do
...1>   defstruct name: "", paid: false, over_18: true
...1> end
{:module, Subscriber,
 <<70, 79, 82, 49, 0, 0, 5, 124, 66, 69, 65, 77, 65, 116, 85, 56, 0, 0, 0, 146,
   0, 0, 0, 13, 17, 69, 108, 105, 120, 105, 114, 46, 83, 117, 98, 115, 99, 114,
   105, 98, 101, 114, 8, 95, 95, 105, 110, ...>>,
 %Subscriber{name: "", over_18: true, paid: false}}
iex2> s1 = %Subscriber{}
%Subscriber{name: "", over_18: true, paid: false}
iex3> IO.puts inspect(s1)
%Subscriber{name: "", over_18: true, paid: false}
:ok
3

Everything in elixir should be in a module. Try this in your file.

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

defmodule Run do
  s1 = %Subscriber{}
  IO.puts inspect(s1)
end
5

I do not think so!

Defining a struct and using it in the following script is essential in scripting with elixir.

2 Likes
6

Yes you are right indeed. My mistake. I don’t know why I had this impression.

7

Thanks for leading me. However, this issue comes out using “Elixir defstruct.exs” although using iex as you said works correctly.

In addition, I think “iex” and “elixir” packages are different in my environment. I might miss installing correctly. When I checked “mix deps.get”, errors happen and are still unresolved.

8

The issue is with the phases that the Elixir compiler goes through.

While this

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

end
s1 = %Subscriber{}
IO.puts inspect(s1)

results in an error, the following does not

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

  def new,
    do:  %Subscriber{}

end
s1 = Subscriber.new()
IO.puts inspect(s1)

During compilation the expansion phase happens before the evaluation phase. However the functionality of a module isn’t “defined” and available until it has been evaluated during the evaluation phase. So when the compiler comes across

s1 = %Subscriber{}

during the expansion phase it tries to expand %Subscriber{} - the problem is that %Subscriber{} is part of the Subscriber module which won’t be defined until later in the evaluation phase - so during the expansion phase there is no definition for %Subscriber{} (yet).

s1 = Subscriber.new()

doesn’t have anything to expand - it is simply a function call that is evaluated during the evaluation phase - by that point in time the Subscriber module has been fully evaluated and both %Subscriber{} and Subscriber.new() are defined and available.

See also

https://groups.google.com/d/msg/elixir-lang-talk/5Zj32zJssl8/wPJZioP9lUEJ

Main phases of Elixir compilation

5 Likes
9

peerreynders, Thank you so much.
I corrected my codes as what you said and it works. And I learned a lot from you.

modified code:

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

  def run do
    s1 = %Subscriber{}
    IO.puts inspect(s1)
    s2 = %Subscriber{name: "Dave"}
    IO.puts inspect(s2)
    s3 = %Subscriber{ name: "Mary", paid: true}
    IO.puts inspect(s3)
    IO.puts "#{s3.name}"
    %Subscriber{name: a_name} = s3
    IO.puts a_name
    s4 = %Subscriber{ s3 | name: "Marie"}
    IO.puts inspect(s4)
  end 
end

Subscriber.run()
1 Like