Elixir: Cannot access struct

Question

I'm trying to get structs to work but none of the documented examples on the Internet or printed books work.

This example on the web site (https://www.tutorialspoint.com/elixir/elixir_structs.htm) also shows the same problem:

defmodule User do
   defstruct name: "John", age: 27
end

john = %User{}

#To access name and age of John, 
IO.puts(john.name)
IO.puts(john.age)

I get the error cannot access struct User, the struct was not yet defined or the struct is being accessed in the same context that defines it.

Dogbert · Accepted Answer · 2016-09-19 15:01:44Z

36

You're probably trying to run this using elixir <filename.exs> while the book you may have seen similar code in was most likely typing the code into iex. (Edit: the code on the page you linked to has been lifted straight from the official tutorial (http://elixir-lang.org/getting-started/structs.html) which is typing that code into iex). This will work in iex but not in an exs script; this is a limitation of the way Elixir "scripts" are compiled and evaluated.

I usually wrap the code in another function (and possibly another module) and invoke that at the end when I have to create and use structs in exs scripts:

$ cat a.exs
defmodule User do
  defstruct name: "John", age: 27
end

defmodule Main do
  def main do
    john = %User{}
    IO.puts(john.name)
    IO.puts(john.age)
  end
end

Main.main
$ elixir a.exs
John
27

answered Sep 19, 2016 at 15:01

Dogbert

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3 Comments

Lukas Knuth Over a year ago

Can you clarify what exactly this "limitation" is? Does this only apply to .exs-files?

lyl Over a year ago

Why is import User not necessary in the module Main?

Jessie Koffi Over a year ago

import keyword is not for being able to use modules in this language, it is for this: hexdocs.pm/elixir/main/alias-require-and-import.html

Aleks Tkachenko · Accepted Answer · 2018-11-30 05:02:33Z

4

Wrapping struct creation and other related operations in a module should be sufficient.

defmodule Customer do
  defstruct name: "", company: ""
end

defmodule BugReport do
  defstruct owner: %Customer{}, details: "", severity: 1
end

defmodule Playground do
  report = %BugReport{owner: %Customer{name: "X", company: "X"}}
  IO.inspect report
end

$ elixir ./your_filename.ex

answered Nov 30, 2018 at 5:02

Aleks Tkachenko

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