Assert that an Enum has no elements."

[] |> assert_empty    # true
%{} |> assert_empty   # true

If the value doesn't implement Enumerable produces an assertion exception.

The output is more friendly than a Protocol.UndefinedError. So, for example, the other assertions in this module start with this assertion.

defchain assert_empty(value_to_check) do
  assert_enumerable(value_to_check)
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  elaborate_assert(Enum.empty?(value_to_check),
    Messages.expected_no_element,
    left: value_to_check)
end

Assert that an Enum has only a single element.

[1] |> assert_singleton   # passes
[ ] |> assert_singleton   # fails

%{a: 1} |> assert_singleton       # passes
%{a: 1, b: 2} |> assert_singleton # fails

Returns the content element of what must be a single-element Enum.

''' [1] |> singleton_content # 1 [ ] |> singleton_content # fails %{a: 1} |> singleton_content # the tuple {:a, 1} 5 |> singleton_content # faila

Combines singleton_content/1 and FlowAssertions.StructA.assert_struct_named/2.

|> VM.Animal.lift(Schema.Animal)
|> singleton_content(VM.Animal)
|> assert_fields(...)

In addition to checking that the value is a singleton Enumerable, it checks that the content is a value of the named struct before returning it.