Customer.__struct__/1 is undefined, cannot expand Customer

Can you post the exact contents of nested_dict.exs? Line 4 doesn’t match what’s in your post, and the ordering of Customer and BugReport in the file is important.

That’s a scope problem in iex and single file scripts. Since everything is compiled at once (not each file into .beam file inside _build directory) the code must work as custom modules are not defined at compile-time. Using them run-time is definitely not a problem. This behaviour is not best, but very reasonable.

This should do the job:

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

defmodule BugReport do
  defstruct owner: nil, details: "", severity: 1

  def new(data \\ %{}, customer \\ nil) do
    owner = case customer do
      struct when is_struct(struct, Customer) -> struct
      map_or_list when is_map(map_or_list) or is_list(map_or_list) -> struct(Customer, map_or_list)
      _customer -> %Customer{}
    end

    map = Map.new(data)
    struct(%__MODULE__{owner: owner}, map)
  end
end

This allows us to write code like:

iex> BugReport.new()
%BugReport{owner: %Customer{name: "", company: ""}, details: "", severity: 1}

iex> BugReport.new(details: "…")
%BugReport{owner: %Customer{name: "", company: ""}, details: "…", severity: 1}

iex> BugReport.new([], [name: "xyz"])
%BugReport{owner: %Customer{name: "xyz", company: ""}, details: "", severity: 1}

In both cases you are able to use Keyword lists as well as Map and in customer case also Customer struct (of course as long as call with %Customer{…} argument is in run-time).

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

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

report = %BugReport{
  owner: %Customer{name: "Dave", company: "PragProg"},  # this is line 10, error line  
  details: "broken"
}

  error: cannot access struct Customer, the struct was not yet defined or the struct is being accessed in the same context that defines it
    │
 10 │   owner: %Customer{name: "Dave", company: "PragProg"},
    │          ^
    │
    └─ nested_dict_api.exs:10:10

** (CompileError) nested_dict_api.exs: cannot compile file (errors have been logged)

NOTE, this is when i put all my modules in one file.

#Customer.exs
defmodule Customer do
  defstruct name: "", company: ""
end

#nested_dict_api.exs
Code.require_file("Customer.exs")

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

#nested_dict.exs
Code.require_file("nested_dict_api.exs")
Code.require_file("Customer.exs")

report = %BugReport{
  owner: %Customer{name: "Dave", company: "PragProg"},   #error is here on line, line 5 
  details: "broken"
}

IO.inspect(report)

error

    error: Customer.__struct__/1 is undefined, cannot expand struct Customer. Make sure the struct name is correct. If the struct name exists and is 
correct but it still cannot be found, you likely have cyclic module usage in your code
    │
  5 │   owner: %Customer{name: "Dave", company: "PragProg"},
    │          ^
    │
    └─ nested_dict.exs:5:10

** (CompileError) nested_dict.exs: cannot compile file (errors have been logged)

Both method WORK ON iex but not on elixir <filename>.exs command