Doctests: defining a struct in your doctest

I am working on the MapDiff library, so it works better when structs are passed to it.

I wrote the following two doctests:

  iex> defmodule Foo do
  ...>  defstruct a: 1, b: 2, c: 3
  ...> end
  ...> MapDiff.diff(%Foo{}, %Foo{a: 3})
  %{changed: :map_change, struct_name: Foo,
    value: %{a: %{added: 3, changed: :primitive_change, removed: 1},
      b: %{changed: :equal, value: 2}, c: %{changed: :equal, value: 3}}}

and:

  iex> defmodule Bar do
  ...> defstruct a: 1, b: 2, c: 3
  ...> end
  ...> defmodule Baz do
  ...> defstruct a: "foo", b: "bar", z: "baz"
  ...> end
  ...> MapDiff.diff(%Bar{}, %Baz{})
  %{added: %Baz{a: "foo", b: "bar", z: "baz"}, changed: :primitive_change,
    removed: %Bar{a: 1, b: 2, c: 3}}

However, this will not compile. When running mix test, the following error is thrown:

Compiling 1 file (.ex)
** (CompileError) (for doctest at) lib/map_diff.ex:105: MapDiffTest.Foo.__struct__/1 is undefined, cannot expand struct MapDiffTest.Foo
    (stdlib) lists.erl:1354: :lists.mapfoldl/3
    test/map_diff_test.exs:3: anonymous fn/3 in :elixir_compiler_1.__MODULE__/1
    (elixir) lib/enum.ex:1755: Enum."-reduce/3-lists^foldl/2-0-"/3
    test/map_diff_test.exs:3: (module)

I think that this is the case because Elixir tries to evaluate the outcome value decoupled from the example, which means that Foo, Bar and Baz (and their respective __struct__/1 implementations) are not in scope.

Now, how can these doctests be rewritten so:

• they compile.
• it is clear to people that Foo, Bar and Baz are normal structs they could define themselves?