Chicago TDD for Erlang

Chicago School TDD applied to Erlang/OTP development.

Principles

1. •Tests First: Write failing test before implementation
2. •State-Based: Test observable states, not implementation details
3. •Real Processes: No mocks, use actual gen_server, processes
4. •AAA Pattern: Arrange, Act, Assert

EUnit Pattern

%% Arrange
State = #{count => 0},
Pid = start_link(State),

%% Act
{ok, 1} = increment(Pid),

%% Assert
#{count := 1} = get_state(Pid).

CT Pattern

happy_path_test(_) ->
    %% Arrange
    Config = init_config(),

    %% Act
    {ok, _} = my_server:start_link(Config),

    %% Assert
    {ok, State} = my_server:get_state(),
    ?assertMatch(#{status := ready}, State).

Proper Pattern

prop_invariant() ->
    ?FORALL({X, Y}, {int(), int()},
        begin
            State = my_module:process(X, Y),
            invariant(State)
        end
    end).

Quality Gates

• •Tests must fail before implementation (RED)
• •Tests must pass after implementation (GREEN)
• •No mocking of erlmcp processes
• •Real gen_server, supervisors in tests
• •Coverage >= 80%

References

• •apps/erlmcp_core/test/ for examples
• •CLAUDE.md for Chicago TDD rules