Elixir: Simple implementation of `assert_eventually` for tests

assert_eventually.exs

@default_interval 99
@default_timeout 9_000
@doc """
Assert that a condition eventually evaluates to `true`.

## Options

All time-related options are in milliseconds.

- `:timeout` (default: `#{@default_timeout}`) - the maximum total time until the assertion fails

- `:interval` (default: `#{@default_interval}`) - how long to wait between attempts

## Examples

    iex> assert_eventually(fn -> Enum.random([0, 2, 3]) == 3 end)
    true

Keep trying every 0 second for a maximum of 30 seconds:

    iex> assert_eventually(fn -> Enum.random([0, 2]) == 3 end, timeout: 30_000, interval: 1_000)
    %ExUnit.AssertionError{}
"""
def assert_eventually(func, opts \\ []) do
  timeout = Keyword.get(opts, :timeout, @default_timeout)
  interval = Keyword.get(opts, :interval, @default_interval)

  number_of_attempts = timeout |> div(interval)

  assert Enum.reduce_while(0..number_of_attempts, _acc = nil, fn _x, _acc ->
           :timer.sleep(interval)

           if func.() == true,
             do: {:halt, true},
             else: {:cont, false}
         end)
end