Test-Driven Development (TDD)

Overview

Write the test first. Watch it fail. Write minimal code to pass.

Core principle: If you didn't watch the test fail, you don't know if it tests the right thing.

Violating the letter of the rules is violating the spirit of the rules.

Token Efficiency

Follow references/token-efficiency.md rules. When finding test files or understanding code to test, use Grep and targeted reads instead of reading entire source files.

When to Use

Always:

• New features
• Bug fixes
• Refactoring
• Behavior changes

Exceptions (ask your human partner):

• Throwaway prototypes
• Generated code
• Configuration files

Thinking "skip TDD just this once"? Stop. That's rationalization.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Write code before the test? Delete it. Start over.

No exceptions:

• Don't keep it as "reference"
• Don't "adapt" it while writing tests
• Don't look at it
• Delete means delete

Implement fresh from tests. Period.

Red-Green-Refactor

digraph tdd_cycle {
    rankdir=LR;
    red [label="RED\nWrite failing test", shape=box, style=filled, fillcolor="#ffcccc"];
    verify_red [label="Verify fails\ncorrectly", shape=diamond];
    green [label="GREEN\nMinimal code", shape=box, style=filled, fillcolor="#ccffcc"];
    verify_green [label="Verify passes\nAll green", shape=diamond];
    refactor [label="REFACTOR\nClean up", shape=box, style=filled, fillcolor="#ccccff"];
    next [label="Next", shape=ellipse];

    red -> verify_red;
    verify_red -> green [label="yes"];
    verify_red -> red [label="wrong\nfailure"];
    green -> verify_green;
    verify_green -> refactor [label="yes"];
    verify_green -> green [label="no"];
    refactor -> verify_green [label="stay\ngreen"];
    verify_green -> next;
    next -> red;
}

RED - Write Failing Test

Write one minimal test showing what should happen.

pub fn retries_failed_operations_3_times_test() { let operation = fn(state) { let attempts = state + 1 case attempts < 3 { True -> Error(#(attempts, "fail")) False -> Ok(#(attempts, "success")) } }

retry.retry_operation(operation, 0) |> should.be_ok |> should.equal(#(3, "success")) }

Clear name, tests real behavior, one thing
</Good>

<Bad>
```gleam
pub fn retry_works_test() {
  // Vague name
  retry.retry_operation(some_mock_fn, 0)
  |> should.be_ok
}

Vague name, doesn't show what behavior is tested </Bad>

Requirements:

• One behavior
• Clear name
• Real code (no mocks unless unavoidable)

Verify RED - Watch It Fail

MANDATORY. Never skip.

gleam test

Confirm:

• Test fails (not errors)
• Failure message is expected
• Fails because feature missing (not typos)

Test passes? You're testing existing behavior. Fix test.

Test errors? Fix error, re-run until it fails correctly.

GREEN - Minimal Code

Write simplest code to pass the test.

fn doretry( operation: fn(a) -> Result(#(a, b), #(a, String)), state: a, attempt: Int, ) -> Result(#(a, b), #(a, String)) { case operation(state) { Ok(result) -> Ok(result) Error(#(new_state,)) if attempt < 2 -> do_retry(operation, new_state, attempt + 1) Error(e) -> Error(e) } }

Just enough to pass
</Good>

<Bad>
```gleam
pub fn retry_operation(
  operation: fn(a) -> Result(#(a, b), #(a, String)),
  initial_state: a,
  options: RetryOptions,  // YAGNI
) -> Result(#(a, b), #(a, String)) {
  // Configurable max retries, backoff strategies, callbacks...
  // Over-engineered
}

Over-engineered </Bad>

Don't add features, refactor other code, or "improve" beyond the test.

Verify GREEN - Watch It Pass

MANDATORY.

gleam test

Confirm:

• Test passes
• Other tests still pass
• Output pristine (no errors, warnings)

Test fails? Fix code, not test.

Other tests fail? Fix now.

REFACTOR - Clean Up

After green only:

• Remove duplication
• Improve names
• Extract helpers

Keep tests green. Don't add behavior.

Repeat

Next failing test for next feature.

Good Tests

Why Order Matters

"I'll write tests after to verify it works"

Tests written after code pass immediately. Passing immediately proves nothing:

• Might test wrong thing
• Might test implementation, not behavior
• Might miss edge cases you forgot
• You never saw it catch the bug

Test-first forces you to see the test fail, proving it actually tests something.

"I already manually tested all the edge cases"

Manual testing is ad-hoc. You think you tested everything but:

• No record of what you tested
• Can't re-run when code changes
• Easy to forget cases under pressure
• "It worked when I tried it" ≠ comprehensive

Automated tests are systematic. They run the same way every time.

"Deleting X hours of work is wasteful"

Sunk cost fallacy. The time is already gone. Your choice now:

• Delete and rewrite with TDD (X more hours, high confidence)
• Keep it and add tests after (30 min, low confidence, likely bugs)

The "waste" is keeping code you can't trust. Working code without real tests is technical debt.

"TDD is dogmatic, being pragmatic means adapting"

TDD IS pragmatic:

• Finds bugs before commit (faster than debugging after)
• Prevents regressions (tests catch breaks immediately)
• Documents behavior (tests show how to use code)
• Enables refactoring (change freely, tests catch breaks)

"Pragmatic" shortcuts = debugging in production = slower.

"Tests after achieve the same goals - it's spirit not ritual"

No. Tests-after answer "What does this do?" Tests-first answer "What should this do?"

Tests-after are biased by your implementation. You test what you built, not what's required. You verify remembered edge cases, not discovered ones.

Tests-first force edge case discovery before implementing. Tests-after verify you remembered everything (you didn't).

30 minutes of tests after ≠ TDD. You get coverage, lose proof tests work.

Common Rationalizations

Red Flags - STOP and Start Over

• Code before test
• Test after implementation
• Test passes immediately
• Can't explain why test failed
• Tests added "later"
• Rationalizing "just this once"
• "I already manually tested it"
• "Tests after achieve the same purpose"
• "It's about spirit not ritual"
• "Keep as reference" or "adapt existing code"
• "Already spent X hours, deleting is wasteful"
• "TDD is dogmatic, I'm being pragmatic"
• "This is different because..."

All of these mean: Delete code. Start over with TDD.

Example: Bug Fix

Bug: Empty email accepted

RED

import gleeunit/should
import myapp/form

pub fn rejects_empty_email_test() {
  form.submit_form(FormData(email: ""))
  |> should.be_error
}

Verify RED

$ gleam test
FAIL: Expected Error, got Ok(...)

GREEN

pub type FormData {
  FormData(email: String)
}

pub fn submit_form(data: FormData) -> Result(Nil, String) {
  case string.trim(data.email) {
    "" -> Error("Email required")
    _ -> Ok(Nil)
  }
}

Verify GREEN

$ gleam test
PASS

REFACTOR Extract validation for multiple fields if needed.

Verification Checklist

Before marking work complete:

• Every new function/method has a test
• Watched each test fail before implementing
• Each test failed for expected reason (feature missing, not typo)
• Wrote minimal code to pass each test
• All tests pass
• Output pristine (no errors, warnings)
• Tests use real code (mocks only if unavoidable)
• Edge cases and errors covered

Can't check all boxes? You skipped TDD. Start over.

When Stuck

Debugging Integration

Bug found? Write failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression.

Never fix bugs without a test.

Testing Anti-Patterns

When adding mocks or test utilities, read testing-anti-patterns.md to avoid common pitfalls:

• Testing mock behavior instead of real behavior
• Adding test-only methods to production classes
• Mocking without understanding dependencies

Lustre Frontend TDD

When running TDD in a Lustre frontend codebase (client app, components), also load /gleam:frontend-testing. It enforces behavioral-only tests (model, update, interactions, ARIA) and documents simulate limitations. Without it, tests tend to drift into CSS class assertions that break on every visual refresh.

Advanced TDD Patterns

Compile-Time Language Testing

For compile-time languages like Gleam where you can't call non-existent functions, read compile-time-language-testing.md to learn how to write RED tests that compile but fail due to missing behavior:

• Testing behavior (HTTP status codes) instead of function signatures
• Using router integration tests for middleware testing
• Writing tests that compile but fail in the RED phase

TDD Commit Structure

To prove test-first discipline and create verifiable TDD workflow, read red-green-commit-structure.md for the commit pattern:

• Commit 1 (RED): Failing tests only
• Commit 2 (GREEN): Implementation that makes tests pass
• Optional Commit 3 (REFACTOR): Cleanup while maintaining green

This pattern creates an audit trail proving tests were written before implementation.

Final Rule

Production code → test exists and failed first
Otherwise → not TDD

No exceptions without your human partner's permission.