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}}}


  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?

I guess the best would be to define those example structs somewhere like in test/support then just add a comment about their structure on your docs (indent the code four spaces so that it gets properly highlighted as elixir code) and then just import them for doctest:

@doc """
   For example suppose you define the following structs:

     defmodule Bar do
       defstruct a: 1, b: 2, c: 3

      defmodule Baz do
        defstruct a: "foo", b: "bar", z: "baz"

       iex> import Foo
       iex> import Bar
       iex> MapDiff.diff(%Foo{}, %Bar{})
       %{added: %Baz ....}

Yes! This solution works wonderfully :slight_smile: .

I ended up simply defining the structs in the tests/map_diff_test.exs file, inside the MapDiffTest module, before doctest MapDiff is being called, because it seemed the most clear (as long as the code does not become much longer.)

Thank you very much :blush:!

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